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The Opus Majus of Roger Bacon

THE OTUS MAJUS OF ROGER BACON

BACON'S STUDY AT OXFORD
From a Gough Print

THE
OPUS MAJUS
OF
ROGER BACON
*A Translation by
ROBERT BELLE BURKE
VOLUME II
NEW TORK
RUSSELL & RUSSELL INC
1962

FIRST PUBLISHED IN 1^8
REISSUED, 1962, BY RUSSELL & RUSSKM., INC, L. C. CATALOG GAUD NO! 63-10703
Printed in the United Statss of Am*rica

CONTENTS

VOLUME II
PART FIVE : Optical Science
Imagination, the common sense and memory Optic nerves and the
structure of the eye Perception Light, distance, rarity of the
medium, and the position, magnitude, and density of an object in relation to sight Errors of vision: the shape of the moon, the scintillation of the stars Reflection and refraction.

PAGE
4*9

PART Six : Experimental Science
Proof through experience External and internal experience The utility of experimental science : analysis of the rainbow, concept of an astrolabe, the possibilities of medical science Experimental science in relation to theology.

5^3

PART SKVEN : Moral Philosophy
The science of the salvation of man Man's duty to God, to his neighbor, and to himself The meaning of God Angels The im-
mortality of the soul Four hindrances to a knowledge of eternal life : sin, care, of the body, trammels of the sensible world* lack of revelation The worship of God Laws and statutes regulating
human relations Personal conduct Twelve virtues according to Aristotle Other definitions of virtue Discourses on sin Con-
tempt for prosperity and sensual pleasures Remedies for anger Endurance of adversity Thoughts from Seneca and Ovid Comparative religions Essence of the First Cause ia infinite Christ the perfect lawgiver Acceptance of the Sacrament.

635

ILLUSTRATIONS

VOLUME II

BACON'S STUDY AT OXFORD
From GougA Prints, Volume 37. Courtesy of the
Bodleian Library

Frontispiece

LOUIS IX
The Seurre Statue at Versailles

facing page 494

ROGER BACON A Miniature from MS. Sodl. ajri. Courtesy of the
Bodleian Library
DOCTORS OF THE UNIVERSITY OF PARIS . ,
From a Miniature in Chants Royawc, Bibliothique
Nationale, Paris

"

u

S88

"

74

THE OTUS MAJUS OF ROGER BACON

TAR? FIVE OF THIS TLEA
CONCERNING OPTICS, IN THREE PARTS
T/w firsi 'part treats of matters common to the other two; the second deals mainly with direct vision; the third with reflected and refracted vision. In Part One there are twelve distinctions,
FIRST DISTINCTION
This distinction deals with the properties of this science, with
the parts of the mind and brain and with the
organ of vision, in five chapters.
CHAPTER I
Concerning the properties of this science.
explained the fundamental principles of wis-
HAVING dom, both sacred and human, which are found in the tongues from which the sciences of the Latins have
been translated, and likewise in mathematics, I now wish to discuss some principles which belong to optics. If the consideration just mentioned is noble and pleasing, the one in hand is far nobler and more pleasing, since we take especial delight in vision, and light and color have an especial beauty beyond
the other things that are brought to our senses, and not only does beauty shine forth, but advantage and a greater necessity appear- For Aristotle says in the first book of the Metaphysics that vision alone reveals the differences of things; since by
means of it we search out experimental knowledge of all things that arc in the heavens and in the earth. For those things that are in the heavenly bodies are studied by visual instruments, as Ptolemy and the other astronomers teach. So also are those things that are generated in the air, like comets, rainbc.ws, and the like. For their altitude above the horizon, their size, form, number, and all things that are in them, are verified by the methods of viewing them with instruments. Our experience of things here in the earth we owe to vision, because a blind man can have no experience worthy of the name concerning this
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world. Hearing causes us to believe because we believe our teachers, but we cannot try out what we learn except through vision. If, moreover, we should adduce taste and touch and
smell, we assume a knowledge belonging to beasts* For brutes are busied with the things pertaining to taste and touch, and
exercise their sense of smell because of taste and touch, but the

things are of little value, few in number, and common to us
and to brutes concerning which these senses give verification,
and therefore they do not rise to the rank of human wisdom.

But because of necessity, utility, and difficulty, sciences are

formed, since art has to do with the difficult and with the good, as Aristotle says in the second book of the Ethics. For if what

is sought is easy, there is no need for the formation of a science. Likewise although a matter be difficult yet not useful, no science is developed concerning it, because the labor would be foolish and vain. Also unless a subject were very useful and

possessed many excellent truths, it does not require the forma-

tion of a separate science, but it suffices that this subject be

treated in some particular book or chapter along with other

matters in general science. But concerning vision alone is a

separate science formed among philosophers, namely, optics,

and not concerning any other sense. Wherefore there must be

a special utility in our knowledge through vision which is not
found in the other senses. What I have now touched upon in

general, I wish to show in particular by disclosing the basic

principles of this very beautiful science. It is possible that some

other science may be more useful, but no other science has so

much sweetness and beauty of utility. Therefore it is the flower

of the whole of philosophy and through it, and not without it,

We can the other sciences be known.

must note, moreover, that

Aristotle first treated this science, of which he says in the sec-

ond book of the Physics that the subject is placed under another
head. He also mentions it in his book on Sense and the Sensible,

and has proved Democritus in error, because he did not name

refractions and reflections of vision with reference to tho optic and concave visual nerves* This book has been translated into

Latin, After him Alhazen treats the subject more fully in a book which is extant. Alkindi also has arranged some <latu more fully, likewise authors of books on vision and mirrors,

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Optical Science

CHAPTER II

Concerning the internal faculties of the sensitive soul, which
arc imagination and the common sense (sensus communis).

SINCE the optic, that is, the concave nerves causing vision have their origin in the brain, and writers on optics ascribe to a distinct function through the medium of vision the formation

of judgments concerning twenty species of visible things, which will be considered later, and since it is not known

whether that distinct function is among the functions of the soul, the organs of which are distinct in the brain, and since

many other things to be treated of later suppose a definition of the functions of the sensitive soul, therefore we must begin

with the parts of the brain and the functions of the soul, in
order that we may discover those things that are necessary
for vision. Writers on optics give us a means to this end by
showing how the visual nerves descend from the membranes
of the brain and from the lining of the cranium, but no one

explains all things necessary in this matter. I say, then, as all

writers on the subjects of nature, all physicians and authorities
on optics agree, that the brain is enfolded by a double mem-

brane, one of which is called the pia mater, which enfolds the

brain

by

direct

contact ;

and

the

other,

the

dura

mater,

which

adheres to the concave side of the bone of the head called the

cranium. For this latter membrane is harder, so that it may

resist the bone, and the other is softer and tenderer owing to the softness of the brain, the substance of which is like marrow and

ointment, with phlegm as the chief constituent, and with three distinctions, which are called chambers, cells, parts, and divi-

sions* In the first cell there are two faculties: the one in the

anterior part is the common sense, as Avicenna states in the
first book on the Soul, which is like a fountain with respect to

the particular senses, and like the center with respect to the
lines extending from that same point to the circumference, according to Aristotle in the second book on the Soul This com-
mon sense judges concerning each particular sensation. For
the judgment is not completed in regard to what is seen before the form comes to the common sense, and the same is true in

regard to what is heard and to the other senses, as is clear from

Opus Majus
the end of the work on Sense and the Sensible, and in the second book on the Soul. This common sense forms a judgment concerning difference of impressions on the senses, as, for example, that in milk whiteness is different from sweetness, a distinction which sight cannot make, nor taste, because they do
not distinguish things in other categories, as Aristotle maintains in the second book on the Soul. It judges concerning the
operations of the particular senses, for vision does not perceive that it sees, nor hearing that it hears, but another faculty does,
namely, the common sense, as Aristotle maintains in the sec-
ond book on Sleep and Waking. But the final action of this faculty is to receive the forms coming from the particular senses and to complete a judgment concerning them. But it does not retain these impressions, owing to the excessive slipperiness of its own organ, according to the statement of Avicenna in the first book on the Soul. Therefore there must be
another faculty of the soul in the back part of the first cell, the function of which is to retain the forms coming from the particular senses, owing to its tempered moistness and dryness, which is called imagination and is the coffer and repository
of the common sense. Avicenna cites as an example a seal, the
image of which water readily receives, but does not retain
owing to its superabundant moistness; wax, however, retains the image very well, owing to its tempered moistness with dryness. Wherefore he says that it is one thing to receive and another to retain, as is clear from these examples. Such is the
case in the organ of the common sense and of imagination. The
whole faculty, however, composed of these two, namely, that which occupies the whole first cell, is called phantasia or the virtus phantastica. For according to the second book on the
Soul and that on Sleep and Waking and the book on Sense and the Sensible, it is evident that phantasia and the common sense are the same according to subject but differ according to being, as Aristotle says, and that phantasia and imagination are the same according to subject but differ according to being. Wherefore phantasia includes both faculties and does not differ from them except as the whole from the part. Therefore since the common sense receives the form, and imagination retains it, a complete judgment follows regarding the thing, a judgment formed by phantasia.
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Optical Science

CHAPTER III

Concerning the attributes that are apprehended by special senses, by
the common sense and by imagination.

WE must note that imagination and the common sense and any
particular sense form judgments of themselves only concerning

twenty-nine properties; as sight . concerning light and color; touch concerning heat and cold, moistness and dryness ; smell

concerning odors; taste concerning savor. These are the nine

special properties that belong to their own senses, as I have named them, of which no other particular sense can form a

judgment. There are, moreover, twenty other properties,

namely, distance, position, figure, magnitude, continuity, discreetness or separation, number, motion, rest, roughness,

smoothness, transparency, thickness, shadow, darkness, beauty,

ugliness, also similarity and difference in all these things and

in all things composed of them. Besides these qualities there are some that are placed under one or more of these qualities, as order under position, and writing and painting under figure and order. Further examples are straightness and crookedness, and concavity and convexity, which are placed under figure; multitude and fewness, which are placed under number;

equality, augmentation, and diminution, which are placed

under

similarity

and

difference ;

eagerness,

laughter,

and

sad-

ness, which are apprehended from the figure and form of the

face; lamenting, which is apprehended from the form of the

face together with the shedding of tears; and moistness and

dryness, which are placed under motion and rest, since from

the sense of vision moistness is not apprehended except from

the liquidity of the moist body and from the motion of one part

of it before another, and since dryness is apprehended from

the retention of the parts of the dry body and from the absence

of liquidity. Here, however, we must consider that Aristotle

maintains in the second book on Generation that moist and

dry are in one way primary attributes which naturally belong to the elements, and that through these attributes spring the non-elemental moistness and dryness, which are reduced to
the primary attributes and are caused by them. Concerning, then, these primary attributes it has been stated that they are

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Opus Majus

We proper objects of sense and perceptible by touch alone.

are

speaking, however, concerning the others. For primary moist-

ness is that which passes over easily into all forms, possessing

no definite bounds of its own, but conforming readily to the

limits of something else, as in the case of air particularly and

next in order in the case of water. Dry is the opposite, par-

ticularly in the case of earth, secondarily in the case of fire.
But here moistness is used in the sense of liquid and slippery, and dry in the sense of arid and coagulated. The same is true

concerning many other attributes, which are reduced to the

forms and principal modes enumerated above belonging to

visible things. All these matters are explained in the first book of Ptolemy on Optics and in the second book of Alhazen on

Aspects, and in other authors on optics. There are, moreover,

common qualities, some of which Aristotle defines in the sec-

ond book on the Soul and in the beginning of his work on Sense

and the Sensible, as, for example, magnitude, figure, motion,
rest, and number. These are not the only common qualities but

also all those mentioned before, although most writers on the

subjects of nature do not consider this fact, because they are
not expert in the science of optics. For common properties are not so called because they are perceived by the common sense,

but because they are determined by all the special senses or by

several of them, and particularly by sight and touch, since

Ptolemy states in his second book on Optics that touch and

sight participate in all these twenty. These twenty-nine, with

those that are reduced to them, are apprehended by the special
senses, and by the common sense, and by imagination, and these

faculties of the soul cannot judge of themselves concerning other qualities except by accident.

CHAPTER IV

Concerning the investigation of the estimative, memorative, and cogitative faculties.

BUT there are other sensibles per sef for animals use sense

alone, since they do not possess intellect. The sheep, even if

it

has

never

seen

a

wolf,

flees

from

it

at

once ;

and

every

animal

experiences fear at the roaring of a lion, although it has never

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heard a lion before nor seen one. The same is true in regard to

many things that are hurtful and contrary to the constitution or

complexion of animals. The same principle holds good as re-

gards what is useful and in conformity with their natures. For

although a lamb may never have seen another lamb before, it

runs to one and willingly remains with it, and the same is true

concerning other animals. Brutes, therefore, have some per-

ception in things advantageous and in things harmful. There

is then something sensibly in them besides the twenty-nine

properties mentioned above and besides those that are reduced

to them. For there must be something more active and produc-

tive of change in the sentient body than light and color, because

it not only causes apprehension but also a state of fear or love

or flight or delay. This is the property of the complexion be-

longing to each object by which it is assimilated to others in a

nature special or general. Through this quality things agree,

are strengthened and invigorated, or differ and oppose one

another and are mutually harmful. Wherefore not only do light

and color produce their forms and impressions, but to a far

greater degree do the properties of complexions, nay, the very

natures of things as regards their substance, agreeing or dis-

agreeing with one another, produce strong impressions, which

change greatly the sensitive soul, so that it is moved to states

of fear, horror, flight, or the opposites. These forms or im-

pressions coming from things, although they change and alter

special senses and the common sense and imagination, just as

they do the air through which they pass, yet no one of those

faculties of the soul judges concerning these impressions, but

of necessity a far nobler and more powerful faculty of the

sensitive soul does, which is called estimation or the estimative

faculty, as Avicenna states in the first book on the Soul, a fac-

ulty which he says perceives the insensible forms connected

with sensible matter. Sensible matter is spoken of here as that

which is apprehended by the special senses and by the common

We sense, as are the aforesaid twenty-nine.

call insensible form

that which is not taken cognizance of by those senses of them-

selves, since they are commonly called senses, although other

faculties of the sensitive soul may equally well be called senses,

should we wish so to name them, because they are parts of the

sensitive soul. For every part of the sensitive soul can be called

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Opus Majus
a sense, because it is in truth a sense, and a sensitive faculty. The
statement, therefore, that qualities belonging to complexions are not apprehended by a sense must be understood as apply-
ing to a special sense and the common sense and imagination ;
but they can readily be apprehended by estimation, which although not called a sense is, however, a part of the sensitive
soul.
But estimation does not retain a form, although it receives
it like the common sense, and it therefore requires another
faculty in the remotest part of the posterior cell to retain forms coming to the estimative faculty and to be its storehouse and
repository, just as imagination is the storehouse of the common
sense. This is the memorative faculty, as Avicenna states in the first book on the Soul. Cogitation or the cogitative faculty is in the middle cell and is the mistress of the sensitive faculties.
It takes the place of reason in brutes, and is therefore called
the logical, that is, the rational faculty, not because it em-
ploys reason, but because it is the ultimate perfection of brutes,
just as reason is in man, and because the rational soul in man
is united directly with it. By this faculty the spider weaves its
geometrical web, and the bee makes its hexagonal house, choosing one of the figures that fill out space, and the swallow its nest. The same is true of all the works of brutes that are simi-
lar to human art. Man by means of this faculty sees wonderful
things in dreams, and all the faculties both posterior and anterior of the sensitive soul serve and obey it, because they all exist on account of it. For the forms or species that are in the
imagination multiply themselves into the cogitative faculty, although they exist in the imagination according to their nature primarily because of phantasia, which uses those forms; but the cogitative faculty holds those forms in a nobler way,
and the forms of the estimative and memorative faculties exist
in the cogitative faculty in accordance with a nature nobler than that existing in those faculties, and therefore the cogitative faculty uses all the other faculties as its instruments. In
man there is in addition from without and from creation the
rational soul, which is united with the cogitative faculty pri-
marily and immediately, and uses this faculty chiefly as its own special instrument. Species are formed in the rational soul by this faculty. Wherefore when this faculty is impaired the judg-
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Optical Science
ment of reason is especially perverted, and when it is in a healthy condition the intellect functions in a sound and rational
way.

CHAPTER V

An exposition of authorities at variance regarding
the faculties mentioned.

BUT the Latin text of Aristotle does not disclose to us this divi-

sion, for there is no mention expressly made except in regard to the common sense, imagination, and memory. But since the

text of Aristotle owing to faulty translation cannot be under-

stood in this part of his work, and the same difficulty is found in other passages, and since Avicenna was the perfect imitator and expositor of Aristotle in nearly every respect, and was the

greatest philosopher subsequent to him, as the Commentator states in the chapter on the Rainbow, we must for this reason

hold to the opinion of Avicenna, which is clear and perfect,

although the translators of the works of Avicenna, as in his

book on the Soul, and in that on Animals, and in his books on

Medicine, have translated differently and have changed words,

so that Avicenna's meaning has not been translated to the same purport throughout, since in his book on Animals we find that

the estimative faculty takes the place of reason in brutes, and
so too we sometimes find elsewhere contradiction with respect

to

the

aforesaid

matters ;

but

the

fact

is

of

little

moment

that

different translators use different words, and sometimes in

part of a subject show some diversity. But we must hold to his

view as expressed in his book on the Soul, because he there dis-

cusses the faculties of the soul as his principal topic, but else-

where his mention of this subject is rather incidental. More-

over, that work has been translated far better than his other

books, a fact that is evident, since it has few or no words be-

longing to other tongues, while his other books have them in

countless numbers. If one really considers what has been said

above, he must assume that there are three faculties wholly

different, according to the three cells. For a diversity of ob-

jects shows us a diversity of faculties. For there are two kinds

of properties, one external, as the twenty-nine mentioned

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Opus Majus

above, the other internal hidden from external sense, as the

quality of a harmful or useful complexion, or rather the essential nature itself whether useful or harmful. There must then

for this reason be two kinds of senses, one containing the special
senses and the common sense and the imagination, which are influenced by the first class of qualities ; and the other containing the estimative faculty and memory, which are referred to the second class of qualities. But the cogitative faculty, owing
to the excellence of its functions as compared with other faculties, is distinguished from them. In the generally accepted translation of Aristotle the whole faculty is called memory, which has the power of retaining forms, and therefore not only the storehouse of the common sense, but also that of the estima-

tive faculty, is called memory. Therefore what is here called
imagination is comprehended under memory in the translation of Aristotle that is in common use. But without doubt memory
must be a double faculty with two quite distinct parts, so that
one shall be the storehouse of the common sense and the other

that of the estimative faculty; and these will differ in species, subject, organ, and operation. Moreover, although these fac-
ulties have been placed in the brain, we must understand that
the marrow-like substance of the brain is not itself sensitive, as Avicenna teaches in the tenth book on Animals, respectfully correcting Aristotle on this point.
For the marrow in other parts of the body is not sensitive, and therefore it is not so here. But it is the container and store-

house of the sensitive faculties, containing slender nerves in which sense and sensible forms are located. But in order that

all doubt may be removed, we must consider that the sensitive

soul has a twofold organ or subject; one is basic and the seat

of life, namely, the heart, according to Aristotle and Avicenna

in the books on Animals ; the other is that which is first changed by the forms of qualities, and in which the operations of the
senses are more manifest and distinguished, namely, the brain. For if the head is injured an evident injury is suffered by the
sensitive faculties, and an injury to the head is more evident

to us than one to the heart, and frequently occurs. For this
reason in accordance with the more obvious consideration we

place

the

sensitive

faculties

in

the

head ;

and

this

is

the

opinion

of physicians who do not think that the origin of the faculties

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Optical Science is in the heart But Avicenna in the first book on the Art of
Medicine says that, although the opinion of physicians is the more obvious to the sense, yet the opinion of the Philosopher is the truer one, since all the nerves and veins and all the faculties of the soul have their origin first of all in the heart, as Aristotle proves in the twelfth book on Animals, and Avicenna in the third book on the same subject.
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SECOND DISTINCTION
Containing three chapters. The first deals with the
nerves leading to the eye.
CHAPTER I
therefore, in order that there may be no
scruple of doubt in what follows, the structure of the
CLEARLY, eye must be studied, because without this nothing can
be known concerning the method of vision. But some writers say less, some more, and in certain things they are at variance. For Authors on perspective more commonly pass over these matters, assuming a knowledge of the natural philosophers and of the writers on medicine, as though every one who reads the science of perspective has already consulted the writers on medicine and on nature. Therefore their discussion is in itself obscure, nor can we understand it unless we have recourse to the fuller treatment of the subject by the writers on medicine and on natural philosophy. For this reason something more must be said here on this topic than is found in the writers on perspective. Although it may be too difficult to attest these matters, and to explain those that are attested, yet I hope that these facts can be made clear by certain authors. But that I may not draw upon sources of individual opinions to too great
an extent, I shall confine my description of the structure of the
eye mainly to three authorities Alhazen in his first book on Perspective, Constantinus in his book on the Eye, and Avicenna in his books; for these writers are sufficient and they
treat more definitely the matters in which we are interested.
I cannot, however, give the exact words of each, because they are sometimes at variance owing to faulty translation, but from them all I shall form a single statement of the truth. All, then, are agreed that there are two parts in the anterior cavity of the brain, which are called ventricles, or concavities, or cells.
These ventricles cannot be organs of the common sense and of the imagination, of which mention was made previously. For
those faculties are arranged anteriorly and posteriorly, while these ventricles are placed on the right and on the left, as Constantinus states. For the entire brain cell can be divided into an
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anterior part and a posterior part, as was stated above. There is, however, a division according to right and left. The anterior part of it, namely, the place of the common sense, has a right and left, where are the two ventricles in a measure distinct.
From the pia mater, which covers both ventricles, comes a
double nerve, one from the right ventricle, and the other from the left, and these are the optic nerves, that is, the concave ones, according to the authors mentioned. The hollow does not begin at the middle of the anterior part of the brain, because there is located the organ of smell. This is a single nerve with two small bits of flesh on the sides like the tips of the teats, as Avicenna teaches in the tenth book on Animals. But according to Avicenna, and the author* on Perspective, and Constantinus, these visual nerves of which we are speaking come out at the bottom of the ventricles of the anterior part. This is the explanation of Constantinus, who teaches us to examine the heads of large animals, when they are killed not in the summer or in the heat, and we shall find a small opening in the skull through which the nerve passes, and then we should examine the membrane of the pia mater cautiously lest it be broken, and we shall find that a nerve comes out at the bottom of the anterior part of the brain. But the two nerves, as we have stated, from the two directions right and left, meet, according to all authorities, and after meeting again are divided. It was, moreover, better that they should meet in the opening than before it or after it. For in either of these cases they would
make two openings in the bone of the head, but it is better that
the nerve should pass through one opening than through two.
The bone is firmer the less it is perforated. Therefore, since nature acts in the better way possible, the junction will be in
the opening of the skull, at which place they are again divided.
The nerve that comes from the right goes to the left eye and the one from the left to the right eye, so that there is a direct extension of the nerves from their origin to the eyes. For if the nerve coming from the right part of the anterior cell of the brain passed to the right eye, there would be an angle in the common nerve where they meet and the nerve would be bent and would not extend directly to the eye. But this would hinder
vision, because vision always selects straight lines as far as
* Alhazen.
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Opus Majus
possible. Moreover, since the bone of the eye is concave with an opening toward the head, the nerve enters the opening of the eye and spreads out in the hollow of the bone like the
utensil with which wine is put in jars. Let, then, abc be the skull, and d be the right
part of the anterior cavity of the brain,
and e the left, enveloped by the pia mater, from the bottom of which let two nerves come forth on the right and left, meeting in the opening in the skull, and afterwards dividing, so that the nerve coming from the right goes to the left eye, which is /, and the nerve from the left to the right eye, which is g, and let them enter the openings of the concave bones, so that they spread out in that hollow, as is shown in the figure. But we must understand that just as the two nerves are derived from the pia mater, so are they also derived from the dura mater, and similarly from the lining of the skull, in which externally it is wrapped, and these three nerves are concave and meet in the opening, and one nerve is formed with three nerve coats, and this nerve so formed goes to each eye, and each eye has naturally a position similar with respect to the meeting of these nerves in the opening and at an equal distance
from it, so that vision may be completed with greater certainty.
CHAPTER II
Concerning the coats of the eye formed from the three nerves mentioned.
THE eye, therefore, has three coats or membranes, and three
humors, and a web like that of a spider. Its first coat is formed from the interior coat of the nerve, which comes from the pia mater, according to all authorities, and spreads out from the end of the nerve where it enters the opening in the bone, and
is ramified like a concave net in its first part. It is therefore called rete or retina, according to Avicenna in the third book on Medicine and according to Constantinus, and is supplied
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Optical Science

with veins, arteries, and slender nerves. Its second part is

thicker, as Avicenna says, and spreads out spherically up to

the fore part of the eye with an opening in the middle of its
own fore part, in order that the impressions of light and color
and other visible things may be able to pass through the middle

of the eye to the nerve coming from the brain. For this opening

is placed directly opposite the extremity of the nerve at which

the retina expands, and therefore in this whole coat Alhazen

states there are two openings, one in front and the other be-

hind, which is the extremity of the concave nerve. This second

part is called the uvea, since it is similar to a grape [tmt],

owing to the fact that it has an opening in its anterior part

like that left in a grape when its stalk is removed, as Avicenna

says in the third book on Medicine. From the coat of the nerve

coming from the dura mater, the second coat of the eye is ex-

panded, according to all authorities. This coat has two parts.

For the first part is composed of veins, nerves, and arteries, and

is called secundina, because it is similar to the after-birth

[secundinae] ; and the second part spreads as far as the an-

terior of the eye, and there part of it is apparent, namely, the

portion of a sphere, which forms a circle above the extremity

of the uvea, and is like transparent horn and is therefore called

cornea. It is composed, as Avicenna says in the book mentioned,

of four thin rindlike coats, and they are like rinds, so that if

one of them should be stripped off, the others would not be

injured because of it. The result is that this coat is strong be-

cause of the injuries and attacks coming from the air, and yet

it is very transparent, so that the large number of its coats does

not hinder the passage of the impressions of visible things.

The third coat of the eye is formed from that third skin of

the nerve that comes from the membrane of the skull, and its

first part is joined to the bone of the eye, and is, in fact, hard

and solid, and for this reason is called sclerotic, but the re-

maining part extends to the cornea. For this coat is not a com-

plete one but lacks a portion of the sphere, and is filled with

fat and white flesh, as we see externally in eyes, and it is called

We consolidativa or conjunctiva.

must bear in mind that in one

way there are said to be only three coats, but in another way

six; and both views are true and rational. For if we consider

the coats as whole ones, there are only three. If, however, we

[4333

Opus Majus

consider the parts behind as distinct from the parts in the front both in name and fact, there are six. For three parts are posterior and three are anterior. Some also have maintained that

they are fewer, and that, too, with elaborate argument; but we should not pay attention to them, because their interpreta-
tion is forced and deviates from sound reason. Some also have

maintained

that

there

are

seven

coats ;

but

this

is

false,

because

they reckoned as a coat the spider-web structure, although it
is not. Those who say that there are three coats call the whole

first coat the uvea, and the whole second coat the cornea, and

the whole third one the consolidativa. Hence authors on per-

spective call the whole first coat the uvea, and so I wish chiefly

to employ the term in describing the method of vision. There-

fore Alhazen says that the uvea has two openings, one anterior,

and the other posterior, which is the opening of the nerve, at

which the expansion of the concavity of the uvea begins,

whence the extremity of the nerve with the whole cavity fol-

lowing as far as the anterior opening is the uvea in fact.

CHAPTER III
Concerning the humors of the eye and the weblike structure.
THIS coat contains in itself three humors and one web. For from the anterior part of that coat there begins a web small and fine like the web of a spider, and in this is contained a body that is called glaciale, crystallinum, or grandinosum [like ice, crystal, or hail], and this body is formed directly above the extremity of the nerve. But it has two parts ; one is interior at the extremity of the nerve and is similar to melted glass, and on this account is called the vitreous humor. The other part is anterior and is like ice and hail and crystal, is whiter than the vitreous humor, and is called anterior glacialis, not having any other name of its own in the author* on Perspective. But it is called in other authors the humorlike crystal, ice, or hail, because it is similar to them, and the whole body contained beneath the web is so called from this part. Then toward the anterior part of the eye outside of the web a humor like the white
* Alhazen.

Optical Science
of egg fills the anterior cavity of the uvea, and on one side touches the anterior part of the glacialis, and on another side* enters the opening of the uvea, and reaches to the cornea, so that the spherical convexity of this humor touches the concave figure of the cornea, and is called humor albugineus. There will then be the cornea, the humor albugineus, the humor glacialis, and the humor vitreus, and the extremity of the nerve, so that consequently impressions of things will pass through the
medium of them all to the brain. Avicenna says in the fourth
book on Animals that the retina conveys nourishment as a matter of fact to the parts of the eye, and contains the vitreous humor, as Constantinus states, and the author on Perspective agrees, maintaining that the lower part of the uvea contains the vitreous humor, and at the end of it carries blood in its veins and arteries well digested, with which the vitreous humor
is formed and nourished, in order that the vitreous may be
able to nourish the crystalline humor. For, as Avicenna says in the third book on Medicine, the vitreous humor is the nourishment of the crystalline humor. Constantinus also makes this statement. Hence, since the crystalline humor is quite white and clear, blood is not suitable for it as direct nourishment, but it needs a nourishment intermediate between blood and the crystalline humor, and of this kind is the vitreous humor, which is whiter than blood, and less white than the crystalline humor. Avicenna says that the humor albugineus is the overflow of the crystalline humor, and therefore is opposite in position as regards its nourishment, which is the vitreous humor, and for this reason the crystalline humor is between these. The vitreous
humor fills the whole cavity of the nerve as far as the common section, and is thicker than the glacial humor. Each, however, is transparent, so that the impressions of objects may pass through in them. The crystalline humor is called the pupil, and
in it is the visual power, just as in a subject that at first is
changed, although not radically, since the common nerve is the radical organ, and there vision is completed, as far as the visual power can, as what follows will show.
C4353

THIRD DISTINCTION
In three chapters. The -first is on tlie sphericity and central j>oint$ of the vitreous humor and of the glacial humor, of the cornea, of the humor albugineus, and of the uvea.
CHAPTER I
WE must consider next the form of the eye and of its parts, and the location of the centers of its coats and humors: for these facts are quite necessary; without them the method of vision is not understood. The whole eye, then, approaches the form of a sphere, and the coats likewise and the humors owing to the admirable qualities of the spherical form, because it is less subject to obstacles than a figure with angles, and is simpler than other figures and is more spacious than other isoperimetric figures, as Alhazen, the author on Perspective, states. But in what precedes these properties and others were touched upon. The anterior glacial humor is the portion of a sphere different from the sphere of which the vitreous humor is a portion ; for they are different bodies and of different transparency. Moreover, the anterior glacial humor is the portion of a larger sphere than is that of which the vitreous humor is a portion. Hence the bodies are not complete spheres, but are portions of different spheres, and therefore since these are intersecting spheres, they must have different centers, Since the cavity of the vitreous humor is toward the glacial humor, its center is beyond the center of the eye toward the anterior of the eye. Similarly the center of the anterior glacialis is in the depth of the eye. These centers are, however, on the same straight line that enters through the anterior opening of the uvea and through the opening that is at the extremity of the nerve, where the retina begins to expand. For these bodies are so arranged, according to the authors on Perspective, that from the opening of the bone, where the nerve enters, the nerve extends for some distance and is constantly more dilated, until it comes to the circumference of the whole glacialis, and is consolidated with its circumference. Then above the extremity of the nerve the whole glacialis is placed and is contained in the inferior part of the uvea, which

Optical Science
Alhazen calls the point of the cavity of the uvea, in the last part of which is the opening that is the extremity of the nerve, where the uvea begins. But the middle of the whole glacialis, namely, the vitreous humor, is in the orifice of the opening. For the extremity of the nerve contains the middle of the sphere of the whole glacialis, as Alhazen says, and this middle is the vitreous humor; and the uvea is consolidated with the circumference of the glacial sphere. The humor albugineus contained in the uvea touches the sphere of the anterior glacialis, and it fills the opening up to its contact with the cornea, not so as to touch the cornea in a point, but by the junction of surfaces, just as an interior sphere is contained by an exterior one. But since the convex surface of the cornea is continuous with the surface of the whole eye, and with the whole eye, as Alhazen states, they must have the same center. Since, moreover, the concave surface of the cornea is equidistant from the exterior convex surface, both surfaces of the cornea and the whole eye must have the same center, according to the book of Theodosius on Spheres: for equidistant spheres, one containing and the other contained, have the same center, for example, the spheres of the universe, as the starry heaven, and the sphere of fire, and similarly in the others; for the center of the universe is the center of them all. Since, moreover, the concave surface of the cornea and the convex surface of the humor albugineus, which is in the opening, are like an interior and an exterior sphere, the convex surface of the humor albugineus must have the same center with the aforesaid. But since the concave surface of the cornea does not touch the uvea in a
point, nor is united to it as an exterior sphere to an interior one, but is united to it on the circumference of its opening, the cornea must cut the uvea, and therefore they will have different centers. Since the cornea is a larger sphere than the uvea, because it is continuous with the surface of the whole eye, and the uvea is contained below the sphericity of the cornea, the center of the cornea, therefore, must be further in the depth of the eye, as is clear to the sense in the case of spherical bodies so united. This fact is shown by Theodosius on Spheres, and Alhazen so states it.
C4373

Opus Majus
CHAPTER II
In which is explained a difficult doubt regarding the aforesaid.
BUT then there is serious doubt as to what fills the space be-
tween these bodies, where the smaller sphere deviates from the
larger, and many for this reason think that the humor albu-
gineus spreads itself below the cavity of the cornea, where the uvea is separated from it. But the objection is made that since the humor albugineus is concentric with the cornea, it will be contained in the cavity of the cornea, like a sphere joining it or equidistant; but it is not equidistant, since it touches it; therefore it will join this in its cavity, and will fill the space that is between the uvea and the cornea. But opposed to this statement is the fact that the author on Perspective does not so state it, but always says that it is within the uvea. The objections, therefore, are removed by the fact that the parts of the eye are not complete spheres, but portions of spheres, as is clear in regard to the parts of the glacialis and so, too, in regard to the others that precede these and serve them principally, as are the anterior of the cornea, and the humor albugineus in the opening of the uvea, and the anterior of the uvea.
Hence we must take into account here only the sphericity of the portions; and therefore when mention is made of the
sphericity of the cornea the statement has reference only to that portion which is necessary for vision, namely, that which is in the anterior part of the eye, but elsewhere is not spherical. Moreover, the uvea, although in the upper part spherical, is
not so in the lower part ; similarly the humor albugineus does
not have a sphericity concentric with the cornea, except in the opening of the uvea, where it is joined to the cornea, for below the uvea it has concentricity with the uvea. Moreover, since such is the case, the humor albugineus cannot flow between the cornea and the uvea. For if the bodies were of complete sphericity, this result would follow, but such is not the case. But where the cornea and uvea are apart the consolidativa spreads itself, and fills up whatever there is to be filled between the consolidativa and the cornea, or the cornea and uvea giving up their complete sphericity dilate, and externally unite or internally or in both ways, and fill all that should be filled.
Us* 3

Optical Science
Moreover, since the anterior glacialis in its convexity cuts the uvea, similarly its center must be different from that of the uvea and farther within the depth of the eye : and since the whole
eye, and the cornea, and the humor albugineus have a different center from the uvea and in the depth of the eye, just like the anterior glacialis, and these are required, that vision may take place in the glacialis, it is better, as Alhazen says, that the anterior glacialis should have the same center with them. Therefore the whole eye, the cornea, the humor albugineus, and anterior glacialis are concentric. But concerning the anterior glacialis it will be more definitely shown in what follows that it must have the same center as the cornea and the whole eye, when refraction in the vitreous humor is pointed out. In the meantime let what has been said suffice.
CHAPTER III
Concerning the center and sphericity of the consolidativa.
THE consolidativa is thought to have a different center from
all these farther back in the depth of the eye. But the author on Perspective does not say this, but merely states in regard to the uvea and the vitreous humor that neither mutually nor with the others have they the same center, and in arguing that the center of the cornea and the center of the uvea are not the same he says that the sphere of the uvea is not in the middle of the consolidativa, but extends forward to a part of the visible surface of the eye itself, and the visible surface of the eye is a part of a larger sphere than the sphere of the uvea, wherefore the center of this visible surface will be farther back in the depth of the eye than the center of the uvea. But the surface of the cornea and that of the visible eye, as he assumes here and later explains, are the same. Therefore the center of the uvea and
that of the cornea are not the same. From this it is argued by
some that the center of the concave surface of the consolidativa and that of the cornea are the same, because through the elevation of the uvea from the middle of the consolidativa it is shown that the uvea has a different center from that of the cornea and of the whole eye. But it is to be noted that the ex-
terior surface of the consolidativa is not concentric with the
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Opus Majus
interior surface, as is evident; nor, moreover, is the interior surface completely spherical, because it fills the space where the cornea and the uvea are separated, for which reason it loses its true sphericity, and goes farther into the eye in that part than elsewhere, and therefore the cornea and the interior consolidativa are not concentric. But if both spheres, namely, the cornea and the consolidativa, were complete, the cornea would lie in the cavity of the consolidativa, and the interior of the consolidativa and the exterior of the cornea would be con-
centric. Moreover, since the exterior and interior surfaces of
the consolidativa are not concentric, the exterior of the cornea and the exterior of the consolidativa will not be concentric.
Since also the consolidativa does not have perfect sphericity on the exterior, as Alhazen states, for it tends in its anterior toward pointedness and therefore does not strictly have a center from which all lines drawn to the circumference are
equal, for this reason neither on the outside nor on the inside is the body really spherical, and therefore a center is not assigned to it by Alhazen. It can, however, have a point similar to a center farther back in the depth of the eye than the center of
any other part, as will be shown in the figure. If, however, we wish to remove all contention, we can say that the exterior
surface of the consolidativa is not wholly spherical, just like that of the whole eye, because it is somewhat pointed in the anterior part, and thus the whole eye will not have the center of a sphere, nor will the exterior surface of the consolidativa. If the interior surface of the consolidativa were spherical, it would not fill the space that had to be filled between the cornea and the uvea. But either the cornea contracts itself at the sur-
face of the uvea, and is deepened by losing its true sphericity,
except in the anterior part that is opposite the opening, or the
uvea elevates itself and passes into a protuberant form, externally losing its true sphericity. But although the centers are different in the parts of the eye, yet all are in the same line, which is perpendicular to the whole eye and to all its parts. This line passes through the middle of the opening of the uvea, and through the middle of the opening in the extremity of the nerve, on which the eye is formed. This line is the axis of the eye, by which the eye sees with certainty and by which it passes
over the separate points of a visible object, so that it may make
[4403

Optical Science
certain of each in succession, although it grasps at the same time and at once with full certainty a visible object or a part of a visible object. For since this line is perpendicular to the eye and to all its parts, an impression coming along it is strongest, because approach along the perpendicular is the strongest, as was demonstrated in what was said on the multiplication of species. This arrangement is a necessary one, in order that the
eye may grasp most strongly and with the utmost certainty
what it should.
I shall draw, therefore, a figure in which all these matters
are made clear as far as is possible on a surface, but the full demonstration would require a body fashioned like the eye in all the particulars aforesaid. The eye of a cow, pig, and other
animals can be used for illustration, if any one wishes to experiment. I consider this figure better than the one that follows, although the following one is that of the ancients. For it is impossible that the center of the vitreous humor should be below
the sphere of the anterior glacialis, because in that case right
will appear left and the reverse, as will be shown below : nor yet on the surface of its body, because in that case an impression on the right would go too far to the right, and one on the left too far to the left, and they would never meet in the com-
mon nerve, wherefore the center will lie outside toward the
anterior part of the eye. Since, moreover, the opening of the uvea is small, and the anterior glacial opening is less, since it
is farther in, concentric with the humor albugineus in the opening, the vitreous must cut a very small portion from the glacialis, cfd in the figure, so that it can scarcely be drawn by the hand of man, for to this portion corresponds mo, the opening of the uvea, which is here larger than the opening of the uvea in the eye of any man of this day. The size of the opening is determined by the boundary lines of the visual pyramid, which is abl. For let al be the base of the pyramid, which is the visible
object, the impression of which penetrates the cornea under the pyramidal form and enters the opening, and which tends naturally to the center of the eye, and would go to it if it were not met first by a denser body by which it is bent, namely, the vitreous humor, chd. For this reason, therefore, I have so
placed the center of the vitreous humor and have so drawn its sphere ; and it is evident that its sphere is less than the sphere

Opus Majus
of the anterior glacialis. But in the eye there are not whole spheres, but only small portions of them, like the portion of the sphere of the anterior glacialis, cfd, and the portion of the vitreous humor, ckd; I have completed, however, the spheres in the figure to show the centers and the portions. The uvea, moreover, is drawn in a complete sphere, except in its opening, mo, and yet it is not in the eye completely spherical, as has been

FIG. 25.

stated, but is so in its anterior part, in which is the opening,

because sphericity is not required for vision except in that

place; elsewhere it is of irregular form, in order that a void

may

not

exist

between

the

uvea

and

the

cornea ;

and

the

lines

c and df which are drawn toward the interior of the eye, are in

the sides of the nerve of the uvea, and between these lines is

the opening of the uvea, which accordingly is toward the in-
terior of the eye. Above this opening the vitreous humor is

formed, as is apparent; for the aperture of these lines termi-

nates at the extremities of the portion of the vitreous humor,

Optical Science and its distance is between c and d, and this distance between the sides of the nerve is filled with the vitreous humor as far as the common nerve in the surface of the brain. This nerve, however, in which is this path of the vitreous humor, spreads and expands in the circuit of the humor vitreus, glacialis, and
albugineus, as far as the anterior opening of the uvea, mo,
which is opposite to its own interior opening, which is cd. Then follows the cornea, and then the consolidativa, as indicated in
the figure.
[4433

FOURTH DISTINCTION
In four cfwpters* The first charier is on the properties of the cornea, the humor albugineus, and the uvea.
CHAPTER I
coats and humors, according to Alhazen, have
THEtheir admirable qualities, from which follow the benefits of vision, as he himself shows. The first
function of the cornea is the closing of the opening in the uvea,
preventing the escape of the humor albugineus; it is, more-
over, transparent, so that the impressions [species] of light
and color may pass through it, as was verified before in the
multiplication of species. It has strength, moreover, so that it is not quickly destroyed. Since it is exposed to the air, and could easily be destroyed by smoke and dust and the like, it
has for this reason four coats, as explained above. The humor albugineus is transparent, so that impressions may pass through it and beyond. It possesses moisture, in order that it may always keep moist the humor glacialis and the spiderlike web, which is very thin and for this reason could be destroyed by excessive dryness. The uvea is usually black, in order that the humor albugineus and the glacialis may be obscured, so that feeble impressions of light and color may appear in it, since feeble light is very apparent in dark places, and is concealed in places full of light. It is somewhat strong, in order that it may retain the humor albugineus and prevent any of
it from exuding. It is also thick so as to be obscure. In the eyes
of human beings, however, it is sometimes found to be gray,
and frequently so in the eyes of horses. This is due to the fact that the natural heat was not able sufficiently to assimilate the substance of the uvea and of the humors, and for this reason they are somewhat white, because the weak action of the
assimilating heat in moisture is the cause of whiteness. Or it
sometimes happens from the complete assimilation of the moisture and the victory of the dryness, as is evident in the leaves of the trees in autumn. This grayness either can be due to the uvea, for if the uvea is gray, the eye is gray; if black, the eye
is black. Or the grayness can be caused by the humors, since
[444]

Optical Science
if the moistures are placed near the exterior, and the crystalline is of large size, and the albugineous is moderate-sized, the eye will be gray, unless the opposite happens from the coat. If the moistures are dark, and the crystalline lies toward the interior of the eye and is small, and the albugineous is large, so that it causes darkness, just like very deep water submerging and
covering things, the eye will be black. Aristotle held this view,
and Avicenna in the nineteenth book on Animals.
CHAPTER II
Concerning the properties of the anterior glacialis.
THE anterior glacialis has many properties. For the first and
chief of these is the fact that in it alone is the visual power, according to Alhazen and the other authorities. For if it is injured, even though the other parts are whole, vision is de-
stroyed, and if it is unharmed and injury happens to the others, provided they retain their transparent quality, vision is not destroyed; for while the transparent quality through the glacialis
remains continuous with the transparent quality of the air, vision is not destroyed, provided the anterior glacialis itself is un-
injured. Moreover, the anterior glacialis is moist, so that it may
more quickly respond to the impression of light and color, for very dry substances do not easily receive impressions; and it is of delicate structure, since such a structure in a body causes delicacy of perception. It is somewhat transparent, in order that it may receive the forms of light and color, and that they
may pass through it to the common nerve; and it is somewhat thick, so that it may retain in it impressions for some time, in order that they may become apparent to the visual power and
permit the exercise of judgment. For if the transparent quality were too great, the impressions would pass through, and would not remain so that a judgment could be formed. Hence it must
be somewhat thick, in order that it may experience a feeling from the impressions [species] that is a kind of pain. For we
observe that strong lights and color narrow vision and injure it, and inflict pain. But every action of light is of one nature and of color likewise, except that one is stronger, another weaker. Therefore vision always experiences a feeling that

Opus Majus
is a kind of pain, although it does not always perceive this, that is, when the impressions are moderate. But a painful feeling would not exist in a body unless it be fairly dense, because if it were of too great rarity the impression would not remain until it could cause the action of pain. Its surface forms part of a larger sphere than that of the vitreous, so that its surface
is equidistant from the anterior surface of vision, that they may
have the same center, which is the center of the whole eye and of the cornea and of the humor albugineus. These bodies aid it mainly in the act of vision, and more so than the uvea. The surface or portion of the anterior glacialis is les than half of its sphere, for otherwise it would not follow that its center is farther within the depth of the eye, as has been assumed.
CHAPTER III
On the properties of the vitreous, the weblike struc-
ture, the optic nerve, and the consolidativa.
THE vitreous humor is thicker than the anterior glacialis,
since the ray that is not perpendicular must be refracted between the perpendicular drawn from the point of refraction and the direct path. Concerning this refraction sufficient mention was made in a preceding part on the multiplication of light. Unless refraction took place thus, vision would never be completed, as will be explained below. The color of the vitreous is whiter than that of blood, and not so white as the anterior glacialis, because the vitreous is its nourishment, as explained
above. It is the portion of a smaller sphere, so that the center
of this sphere is different from that of the anterior glacialis, since this is necessary owing to the refraction noted above. Its noblest property is the fact that the perception that is in the anterior glacialis is continued in it through the whole optic nerve to the point of ultimate perception, which is in the anterior part of the brain, as Alhazen says. These two humors are wrapped around by a web, since humors unless retained would flow somewhere and would not retain one shape. This web is very thin, so as not to hide the ray, and is spherical, because it contains the portion of a sphere, although there are
other reasons for this, as for the whole eye and its parts. The
[446]

Optical Science
nerve, moreover, as Alhazen says, on which the eye is formed
is entirely the optic, so that the impression may be transmitted
in it to the brain, and the impression of the visible object and
the natural heat due it and the force of first perception may
come freely to the eye, and therefore the term optic is the same as concave. The consolidativa is exterior, so as to assemble and preserve all the parts; and is somewhat moist, in order that
it may adapt itself better to the positions of the coats in it, be-
cause more quickly and easily they assume the form of their position in it owing to its softness than if it were hard; and in
addition it is moist, so that dryness may not quickly happen in
the coats. It is somewhat firm and strong, so as to preserve the
positions and forms of the coats, in order that they may not be
altered quickly; and it is white, that by its means the form of
the face may be beautiful.
CHAPTER IV
Concerning tJie eyelids, the lashes, and the whole eye.
THE eyelids are to protect the eye during sleep and to enable the eye to rest when fatigued by a strong impression. And al-
though the impressions are moderate even, the eye needs the
means of closing furnished by the eyelids, in order that it may not work continuously. In a similar way smoke and dust and other things harm the vision, for which reason it needs a means
of closing, and therefore the lids have a quick motion, in order
that they may swiftly close over the eye when harmful things approach. The lashes are to moderate the light when vision takes place, and for this reason when looking one unites them
and draws them together, so as to look from a narrow opening when the strong light would harm him. Creative goodness has supplied two eyes, so that if accident or injury should happen to one of them, the second might remain. Another purpose is to add beauty to the form of the face. Both eyes, moreover, are alike in their arrangements, in their coats, and in the forms of their coats, and in the position of each coat with respect to the whole eye; and both have a similar position with respect to the common nerve and the brain. Moreover, although the general reasons for the roundness of the eyes and of their parts

Opus Majus

were given above in the properties of the sphere, yet it was especially necessary that they should be round for two rea-
sons, namely, owing to the swift motion of this form, enabling

the vision to pass at our will from one visible object to another,

and from one part of the same visible object to another part,

in order that anything may be perceived with full certainty by

a swift motion of this kind. But of all figures the sphere is best

adapted to motion. It was necessary also that the eyes should

be round and the parts likewise, for if the surface of the eye

were plane, the impression [species] of an object larger than

the eye could not fall perpendicularly on it, because perpen-

dicular lines to a plane tend to different points and at right

angles in each case, as

shown in the figure. For

the lines can fall per-

pendicularly on the eye

fff that come from the

visible object cdt which is

FIG. 26.

equal to the eye, but from

the points a and b the ray

cannot come perpendicularly, but at oblique angles. But sen-

sible action, and such as is required in seeing, does not take

place except when the ray falls perpendicularly on the vision.

Since, therefore, the eye sees large bodies, as almost a fourth

part of the heavens in one view, it is evident that the eye can-

not be of a plane figure nor of any other figure than that of a

sphere, since on a small sphere perpendiculars infinite in num-

ber coming from a large body can fall, and they tend to the

center of the sphere, and thus a large body can be seen by a

small eye, as is shown in the figure.

[4483

FIFTH DISTINCTION
In three chapters. The first chapter shows that impressions [species] of light and color are required for perception.

CHAPTER I

now understand those things that need explanation
Aweon account of the mode of our vision, we must pass on to a consideration of the mode itself and how vision

takes place. First we must determine what is required for rectilinear vision. The first requisite here considered is, that vision

needs the impression of a visible object, for without this there can be no vision. Accordingly Aristotle says in the second book

on the Soul that in every case the sense receives the impressions

of sensible things, so that there may be an act of sensation.

Moreover, the passive must resemble the active through the

active ;

but

vision

is

a

passive

force,

and

for

this

reason

must

resemble the active, which is the visible object But there is

no similitude of the active except the impression, as all know.
Moreover, an object makes an impression [species] everywhere along all diameters. But when there is an obstacle between the impression of the object and vision, vision does not take place. But when every impediment is removed, so that the impression comes to the eye, the object is seen. Wherefore vision must happen by means of an impression; but especially by means of the impression of light and color. For it is evident that colors have their effect on vision from the fact that when

one views a thick growth of vegetation on which the light of
the sun falls, and continues gazing at it, if he then removes his vision and turns it to a dark place, he will find in that dark
place the form of that light tinged from the green of the
vegetation. And if under this condition he looks at visible
objects that are white in the shadow, and in a place where the light is weak, he will find the colors of the objects mixed with green. If, moreover, he closes his eye and looks in it, he will find in his own eye the form of the light with green. In the same way he will find a change of this kind if he looks at the color blue or purple, or at any other strong color, as any one can prove by experiment. Therefore color must have an effect

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on vision. But light more so, since in accordance with the di-

versity of light falling on the vision and on visible objects is the

diversity of the image in individual objects. For a very bright

light hides other visible objects, and even injures and oppresses vision and weakens the action of sight. But a very weak

light does not change vision as it should, nor does it reveal

A objects;

moderate light greatly strengthens vision in its

action and reveals objects sufficiently. Wherefore the species
of light is especially essential for vision. And again we see

that in accordance with the diversity of the fall of the same

light on the same object the aspect is changed, and the color

appears different to the vision, as in the case of the dove's neck

when it turns the neck in different positions to the light, and

so too in the case of the peacock's tail. In the same manner

many things, as, for example, the scales of fish, or decayed oak,

and certain worms, and the bird that is called noctiluca [shining by night] ; when the light shines on them, their light is hidden and a color is seen ; but when they are in darkness, their

light is apparent. It is clear, then, that the species of light is
especially essential in vision. Without contradiction we find

by

experiment

that

without

light

nothing

is

seen ;

for

the

ex-

trinsic light of the sun, or of the stars, or of a fire must be

present in the air, or the light belonging to the eye and multi-

plied by it, as is the case in. the eye of the cat. Wherefore the

species of light must always be necessary for vision.

CHAPTER II
Showing th&t vision is not completed in the eye, but in the common nerve.
WE must understand that vision is not completed in the eyes,
according to the teaching of the authors on Perspective* For two different species come to the eyes, and a difference in the species causes a difference in the judgment, since by the differ-
A ent species will one object be judged to be two. similar re-
sult follows through a difference in the one judging. For in two eyes there are different judgments. Therefore one object as viewed will be considered as quite different. Therefore there must be something sentient besides the eyes, in which vision is
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Optical Science
completed and of which the eyes are the instruments that give
it the visible species. This is the common nerve in the surface
of the brain, where the two nerves coming from the two parts of the anterior brain meet, and after meeting are divided and extend to the eyes. Therefore the visual faculty is located there, as in a fountain, and since in this case the fontal faculty
is a single one, to which the faculties of the eyes are continued
through the medium of optic nerves, an object can appear as
single, as far as concerns this cause. But it is, moreover, necessary that the two species coming from the eyes should meet
at one place in the common nerve, and that one of these should be more intense and fuller than the other. For naturally the
two forms of the same species mingle in the same matter and in the same place, and therefore are not distinguished, but become one form after they come to one place, and then since the judging faculty is single and the species single, a single
A judgment is made regarding one object. proof of this is the
fact that when the species do not come from the two eyes to one place in the common nerve, one object is seen as two. This is evident when the natural position of the eyes is changed, as
happens if the finger is placed below one of the eyes or if the eye is twisted somewhat from its place, both species do not
then come to one place in the common nerve, and one object is seen as two. This happens in the case of him who does not
have the position of his eyes similar with respect to the com-
mon nerve, and for this reason the species of his eyes, unless
he takes diligent care and rectifies their position, will come to different places in the common nerve, and therefore one thing frequently appears to be two. But naturally eyes that are well formed and healthy have a position similar with respect to
the common nerve, and therefore the two species come to the
same place in it and become one, so that thus through one species and one perception one judgment is formed regarding a single object. Experience teaches that when the common nerve is injured vision is destroyed in the eyes, and that the faculty is not destroyed in the common nerve owing to an in-
jury to the eyes. When the common nerve is cured, vision is
restored in healthy eyes; and when the eyes become well, vision is again restored, because the faculty in the common nerve was preserved, and is not destroyed in the common nerve.
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Opus Majus

CHAPTER III

On the ultimate perception.

BUT since Alhazen says that this ultimate perception is in the

anterior part of the brain, some one might think that it is the common sense and the imagination or phantasy which are in the anterior brain, as was stated above: especially as it was

there said that a judgment concerning any sensible object is

not completed before the impression [species] reaches the
common sense. But it is necessary to state that the ultimate per-

ception can be the origin of all the senses, and in this signification we are not speaking here of the ultimate perception : for this is the common sense in the anterior part of the brain. The

ultimate perception is taken otherwise in the case of the special

sensations of seeing, hearing, smelling, or the others, when

some

particular

sense

is

mentioned ;

and

thus

the

ultimate

per-

ception is the common nerve in vision with respect to the two

eyes, which are the instruments that are first affected by a visible object The same is true of the small pieces of flesh

similar to the nipples of the breasts; for they are the instru-

ments that are first affected by an odor, and the nerve to which

they are continued in the anterior part of the brain is the radi-

cal and original instrument of the sense of smell. As to his

statement that the ultimate perception is in the anterior part
of the brain, we must note that this anterior part of the brain

is not taken as the first cell of the brain, but as the place near

it, namely, in the opening in the skull where the nerves meet;

for since it is before and near the brain it is called the anterior

part of the brain. The same is true regarding the olfactory

process; for it begins at the middle of the anterior brain and

extends between the two nipples [olfactory bulbs] near the brain, and closer than the visual nerve, because it is very necessary for an animal that the brain be strengthened by odor. This

is especially true in the case of man, because he has a larger brain in comparison with his body than any other animal, as Aristotle says in the book on Animals.

Thus it is manifest that the eyes not only judge concerning a visible object, but that the judgment is begun in them and is

completed by the ultimate perception, which is the visual fac~

Optical Science ulty with its source in the common nerve. In a similar way it is evident that the eyes perceive and not only the common nerve. But since the eyes are adjusted to the radical faculty, and from it flow the forces to the eyes, and the sensitive force is continued through the whole nerve from the common nerve to the eyes, as Alhazen says, therefore the visual act is a single and undivided one, which is performed by the eyes and the
common nerve. And although he says that the eye is the in-
strument of the ultimate perception and is the medium between it and the visible object, yet the eye of necessity has judgment and the power of sight, although its judgment is incomplete. For the angle by which the size of an object is known does not exceed the glacialis, and the arrangement of the object seen is made to accord with the object itself on the surface of the glacialis. It is by this arrangement that the object is distinguished and known. This is the statement that is
forthwith made.
C4533

SIXTH DISTINCTION
On the removal of difficulties in tfw theory of vision, in four chapters,

CHAPTER I

In which is removed chiefly the difficulty tfiat seems to arise from the smallness of the

WE must consider, then, as was verified in what precedes, that the natural action is completed by a pyramid whose vertex is in the patient [object receiving

the action] and whose base is the surface of the agent. For thus does the force come from the whole agent opposite to the pa-

tient, as has been stated before, so that the action may be

vigorous and complete; and therefore in vision there is the requirement that the impression [species] should come from
the whole surface of the agent. But although in the natural

change produced in the objects acted upon it is required that separate pyramids come to the individual parts of the body acted upon, because each point of this body must be affected, yet in the change produced the chief requirement is that one pyramid come from the agent, and the vertex fall on the eye, This pyramid falls perpendicularly on the eye, so that all of its lines are perpendicular to the eye. For the chief requirement

is that vision should perceive distinctly the object itself both

with certainty and sufficiency, and this can be done by means

of one pyramid in which there are as many lines as there arc

parts or points in the body seen. Along these lines the separate

impressions come from the individual parts to the anterior

And glacialis, in which is the visual force.

those lines will ter-

minate at the individual parts of the glacialis, so that the im-

pressions of the parts of the thing seen are arranged on the
surface of the perceiving member just as the parts are arranged

in the thing itself, so that the judgment in regard to the sepa-

rate parts may be distinct and not confused. Those lines, more-

over, are perpendicular to the eye, in order that stronger

impressions may come, so that it may be possible to see with

perfect vision and judge concerning the thing as it really is,

For the eye either does not judge at all, or forms an imperfect

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Optical Science

judgment by means of those lines alone that are not perpendicular, owing to the weakness of the impression coming by

their means, although those lines converging with the perpendiculars will act more fully in securing a perception of a
visible object, as will be shown below. The requirement of perpendicularity in lines and species for effective action has

been expained at sufficient length in what precedes on the mul-

tiplication of species.
But we must now verify the fact that on the surface of the

glacialis, although it be small, the distinction of any visible
object whatsoever can be made by means of the arrangement

of the species coming from such objects, since the species of a thing, whatever be its size, can be arranged in order in a very

small space, because there are as many parts in a very small body as there are in a very large one, since every body and

every quantity is infinitely divisible, as all philosophy proclaims. Aristotle proves in the sixth book of the Physics that

there is no division of a quantity into indi-

visibles, nor is a quantity composed of indi-

visibles, and therefore there are as many parts

in a grain of millet as in the diameter of the
world, as is shown in the figure. If a triangle or pyramid has a large base, abc, and a very

short line, edf subtends its vertex, it is evident

that from any point of the line ab a line can be

FIG. 27.

drawn to c, likewise from its other points and

from all its parts, because an infinite number of lines can ter-

minate at a point. This fact is well established. If, therefore, all those lines extend to c, then they pass through the points of the

line de; since, therefore, they do not meet be-

fore cf they will pass through different points in the line def because if all or some of these

lines passed through the same point, the lines would meet before c, but it has been assumed

that they do not meet. For if there were a

meeting of all the lines or of some of them in

FIG. 28.

some point of the line de, then without doubt

after the meeting they would separate one from another to in-

finity, and would never meet in c, as is evident to the sense in this shorter pyramid, fgh. Since, therefore, the species of an

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Opus Majus
object seen, of whatever size it may be, can be arranged on the
surface of the glacialis owing to the divisibility of quantity
which continues to infinity, and which assumes as many parts in a small body as in a large one, no confusion happens when a
large species comes to the small surface of the glacialis.

CHAPTER II

In which there is removed a second, difficulty due to the meeting of oblique rays with the perpendicular ones.

BUT we must remove another difficulty that can be imagined

from another source. For from any part of an object seen come

forth species in infinite number, as explained in the laws of

multiplications. Therefore to any part of the glacialis there

comes the species from the whole object, and separate pyra-

mids, the vertices of which are in any point of the eye and of

the cornea and of the opening of the uvea, and the base of all

these pyramids is the object seen. Therefore any point of the

cornea and of the opening of the uvea will have species of all

the parts confused in it; wherefore the judgment will become

We confused.

must not say that any point of the eye is infi-

nitely divisible, in order that we may fall into the sophistry given above, since we here accept a point of the pupil or a

part as the least sensible portion in the division of the parts of
the perceiving member in accordance with the division of the parts of the thing seen, And therefore, excluding this sophistry, we can say that although in reality the vertex of one pyramid

from the whole thing comes to every point of the eye and the

cornea, and that although the species of all parts are there

mingled, yet to one point of the eye or cornea and opening of the uvea a species does not come perpendicularly except from

one point of the thing seen, although to the same point an infinite number of species come falling obliquely at unequal

angles. Therefore since the body of the eye is denser than the

air, according to the laws of refraction determined above, all

lines falling obliquely are refracted at the surface of the cor-
nea. And since the species is weakened by falling at oblique
angles and likewise by refraction, and since the species falling

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Optical Science

perpendicularly is the strong one, therefore the perpendicular species conceals all the oblique ones, just as a larger and

stronger light hides many weak lights, as, for example, the
light of the sun hides the countless lights of the stars, whence

to the point b there comes a perpen-

dicular from c itself, and to the same b

comes ab not perpendicular, since it

does not fall to the center of the eye,

and therefore the species of a itself is

concealed, although from the point b

the species of a itself may be able to

come to the glacialis by the refracted

line bd; and therefore the judgment is

in accordance with the perpendiculars.
And since perpendicular species are

FIG. zg.

distinct and arranged on the surface of the vision, therefore a distinction takes place. The pyramid,

therefore, coming perpendicularly produces both a strong vi-

sion, so that by reason of its strength forms falling obliquely are

hidden, and nevertheless a distinct vision, so that it excludes the

confusion that seems to happen from an infinite number of rays

falling obliquely which meet each point of the pupil. This pyramid is called the visual and the radiant one, by which vision

is chiefly caused.
I say this because rays coming from a point of an object

from which a perpendicular ray comes to a point of the eye, al-

though they do not fall to that point directly, but to other

points, yet can extend from the other points to which they fall,

by refraction in the coats of the eye, to the same place in the

glacialis and the common nerve to which the perpendicular

species comes from the same point in the object, from which

those

oblique

rays

come ;

so

that

there

is

a

fuller

vision

of

any

part of a visible object, since it is seen by means of its own

direct rays and by means of the refracted ones. But concerning

this matter mention will be made in what is to be said in re-

gard to refraction. Moreover, for another reason I have said that vision takes place chiefly by means of the radiant pyramid ; for since this pyramid alone is perpendicular to the eye, and
falls to the opening of the uvea, and is directly opposite to the center of the eye, it produces for this reason a good vision and

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Opus Majus

the principal one; and yet species can come outside of this pyramid to the eye which will fall not perpendicularly on the

cornea, and will all be refracted, so that in this way vision is

produced

by

them ;

but

the

vision

will

be

weak,

because

things

seen not perpendicularly are not clearly apparent to the eye.

Therefore we can here consider two pyramids: namely, the

principal one, which falls to the opening of the uvea, or the

greater one, composed of this pyramid and of the rays coming

from either side of the opening on the cornea. This whole pyra-

mid so formed is not called the visual pyramid, or the radiant

pyramid, although the eye sees by means of it, but it sees principally and clearly all that falls under the visible pyramid, and "other things obliquely and weakly, namely, those things

that fall outside this pyramid. Hence a thing can be so large

that some part of it will fall in the visual pyramid and be well

seen, and other parts on the sides will fall without the pyramid
on the eye, and will be seen imperfectly. Or it may happen that

a thing of moderate size falls in the visual pyramid and other
different things will be seen on the sides. Or it may happen

that several small objects will fall in the visible pyramid and

others fall in like fashion at the side. But that which falls in

the visible pyramid and nothing else will always be seen principally and clearly. But with greatest clearness will that be seen at which the axis of the visual pyramid terminates. For that axis is perpendicular to all the coats and humors, and passes through all the centers; and therefore the species which comes along it is strongest and fullest, and causes certainty of vision. But a discussion of this matter will be given below.

CHAPTER III
In which a third difficulty due to the mixing of species in the air is removed.
BUT up to the present time there is no small doubt in regard to
the removal of the third difficulty, which requires full discussion in the treatise on the generation, multiplication, corruption, and action of species. Without this treatise perspective cannot be understood. But, however, this doubt ought here to
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Optical Science

be set forth owing to the requirement of vision somewhat more

briefly, that it may not increase too greatly our confusion in the mode of our vision. For as a matter of fact the species of

colors mingle at every point of the medium, since from extreme

colors an intermediate is formed, and from two species of the

same specific nature one is formed. For opposites, that is, ex-

tremes, according to the statement of Aristotle in the tenth

book of the Metaphysics, produce an intermediate, as, for ex-

ample,

white

and

black ;

and

two

white

colors

blend

into

one

when they are in the same subject; for in the same place and

subject they cannot be distinguished, but become one. But what

is true of colors is also true of the species of colors, for the

species is of the same nature as that which produces it, and

therefore any species of color is of some genus of color, since

the species of white cannot be derived from substance or from

any predicable [of substance] other than quality ; nor of this,

from

any

genus

other

than

color ;

nor

of

this,

from

any

species,

however specialized, other than white, not from black, say, or

from green. Hence it remains true of white [as formerly of

color] that any particular species of it, being like white, will be an individual case within the special class [of colors] to be

called white. From which follows that just as in the same sub-

ject white is commixed with black, so species of white with
species of black. And if this is true, then a mixed species comes
perpendicularly from every point of the air to the eye, and the whole radiant pyramid will be mixed from its point of min-

gling in the air, and this is a necessary conclusion.
Many philosophers hold that in this view there is difficulty, and say that species have a spiritual existence in the medium

and in the sense, and Aristotle and Averroes maintain this in
the second book on the Soul. And because they have a spiritual

existence and not a material one they therefore do not observe

the laws of material forms, and for this reason do not mix,

because due to a material existence material forms mix.

Therefore they maintain that different species of light in the
medium and countless sources of light are found at the same

point of the air and are distinguished, so too the species of color and all similar species of things, and for this reason

vision is able to see things distinctly. This error is a very
serious one, for it contains many false and absurd things, and

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arises from the fact that they believe that they must maintain the distinction of vision, which they think cannot exist unless

the species are wholly distinct in the air. First, then, I shall
preserve the distinction of vision, so that it may be seen that it is not necessary to err in this way. Then I shall remove the error more easily, and I shall explain the authors who seem to
be at variance.

I say, therefore, that species have a material and natural

existence in the medium and in the sense; and that opposite

species mingle in a real mixture, as, for example, the species

of white and black and of intermediate colors, and that the

species of two white colors and of two lights become one, and

the same is true of the other species of the same categorical
A species. mixed species will come to the eye from the place

where the species mix, and the whole pyramid will be mixed.

But the species of a visible object has a principal and primary

multiplication, but the others have an accidental Moreover,

the principal or primary multiplication is straight, refracted,

and reflected, and comes from the agent, as we have shown

above. But the accidental or secondary does not come from the

agent, but from the principal species; just as is the case with

light which comes to the corners of a house from a ray of

the

sun

through

a

window ;

and

this

is

so

weak

that

it

has

no

comparison with the principal one, nor does it direct the eye

to the object from which the multiplication comes. Hence a
man in the corner of the house with the secondary species of

the solar light in his eye does not see the sun but the ray fall-

ing through the window. If, however, he applies his eye to

the principal ray., he will see the sun distinctly. I say, therefore,

that just as the perpendicular ray hides all the oblique rays that

terminate with it, so the principal ray hides all the accidental

rays. Hence at the point d there is a real mixture of the white,

black, and red, and from this point comes a mixed species to

the eye along the line de. But in the line de there is no prin-

cipal multiplication except from the visible point b itself, nor is

there one from a or c, but merely an accidental and secondary

one ;

because

a

multiplication

similar

to

a

and

c

does

not

come

except from their species, not from the points themselves. But

the principal multiplication hides all accidental ones, just as the perpendicular ray hides all oblique ones conterminous with

Optical Science

it. Thus the whole pyramid is mixed everywhere, but no mix-

ture as regards the principal multiplication comes to the eye.
This conclusion is confirmed by the fact that when differ-

ent colors have the same p* rinciApal multiplication, a mixed color ap-

pears to the eye; as when glass or crystal or some other transparent

colored body is placed before the

vision, and another dense body is

behind the transparent one in a

direct line with it and the vision, the

species of both bodies enter the

sight in the same place as regards

the principal multiplication, and

therefore a mixed color appears.

Therefore, on the contrary, a single

color will appear, when one color

multiplies itself along the principal

line, and another accidentally, al-

though in the same place. If, there-

fore, philosophers should consider

this distinction of vision, they would

never maintain that species do not
mix in a medium, because owing to the distinction of vision, which they do not know how to preserve, they fall into that
error. And if it should be said that just as those species mingle

in a mixture of principal multiplications at any point in the

air, so do they also in the eye ; therefore there will then be com-

plete confusion, because a mixture will take place there real in

character and principal as regards multiplications; the reply

to all this must be that a real and principal mixture can take

place at any point of the eye ; but only one species at one point

will be perpendicular, which falls to the center of the eye, and

all

others

will

be

oblique,

as

are

ah t

eg,

and

therefore

will

be

hidden, and a judgment will not be formed concerning these but

concerning the perpendicular one, which is be, and by means of

it concerning the thing itself, which is b.

Opus Majus
CHAPTER IV
In which a real mixture of species is proved at any 'point of the medium, with the removal of cavilings to the contrary.
BUT when they say that species has a spiritual existence in a medium, this use of the word spiritual is not in accordance with its proper and primary signification, from spirit, as we say that God and angel and soul are spiritual things; because it
is plain that the species of corporeal things are not thus spiritual. Therefore of necessity they will have a corporeal existence,
because body and soul are opposed without an intermediate.
And if they have a corporeal existence, they also have a mate-
rial one, and therefore they must obey the laws of material and corporeal things, and therefore they must mix when they are contrary, and become one when they are of the same categorical
species. And this is again apparent, since species is the simili-
tude of a corporeal thing and not of a spiritual; therefore it will have a corporeal existence. Likewise it is in a corporeal and material medium, and everything that is received in another is modified by the condition of the recipient, as the book on Causes states, also Boetius in the fifth book on Consolation. Therefore it must have a corporeal existence in a corporeal medium. Moreover, it produces a corporeal result, as, for ex-
ample, the species of heat warms bodies, and dries them out, and causes them to putrefy, and the same is true of other
species. Therefore since this produces heat, properly speaking,
and through the medium of heat produces other results, species
must be a corporeal thing, because properly speaking a spirit-
ual thing does not cause a corporeal action. And in particular
there is the additional reason that the species is of the same essence as the complete effect of the agent, and it becomes that when the agent affects strongly the patient [thing acted on]. Since at first, when wood becomes warm, while it still remains wood it has the species of fire, and later the action becomes stronger, and the species is changed into complete fire, when the fire has destroyed the specific nature of the wood, and flame is produced and charcoal. The species does not, therefore, differ from the charcoal and the flame, except as the incomplete differs from the complete, the embryo from the child, and the

Optical Science

child from the man. But it is agreed that the complete is mate-
rial; therefore also the incomplete, wheresoever we observe
these things, because the incomplete becomes complete. It is
manifest, therefore, that the species of corporeal and material things will have always a material and corporeal existence, whence it is madness to think otherwise.

When, therefore, Aristotle and Averroes say that the species has a spiritual existence in the medium and in the sense, it is
evident that spiritual is not taken from spirit nor is the word
used in its proper sense. Therefore it is used equivocally and improperly, and this is the truth. For it is taken in the sense of

imperceptible ; for since everything really spiritual, as, for ex-

ample, God, angel, and soul, is imperceptible and does not fall

under the sense, we therefore convert the terms and call that

which is imperceptible spiritual. But this is homonymous, and

outside the true and proper meaning of a spiritual thing. Hence

the species of things do not fall per se under the strong and dis-

tinguishing

sense ;

for

since

nothing

is

visible

per

se

unless

it

be dense, because this alone can terminate vision, light or the

species of color in transparent air is not visible per se, but becomes so per accident, because no doubt there is something

denser than the air, at which vision terminates, and thus per-

ceives that there is a transparent quality in the medium, at

which vision is not terminated, and consequently the clearness

of light is apparent in it. And similarly when a ray falls

through a window, it is seen per accidens owing to the fixed

shape of the window, by which the light is shaped, and owing

to the opaque places everywhere, so that thus an opposite

placed beside its contrary becomes more easily apparent. Similarly when a solar ray passes through glass or through

a highly colored cloth, the species of the color is apparent on

an opaque body. But this takes place doubly per accidens,

both because of the excessive clearness of light with respect

to color and with respect to an opaque body, which intercepts
the light. And in the bodies of the stars the species of the

solar light is seen, not because of this light, but owing to

the density of the body of the star, a density that terminates

vision ;

and

a

dense

body

is

the

cause

of

illumination,

as

was

noted above. In these cases, therefore, the species is seen per accidens, and likewise at times because of excessive weakness

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Opus Majus
of vision and because of negligence in looking, as in certain
cases will be explained below. And since it is only per accidens
(as from defect of vision or negligence in looking) that species can sometimes, as it were, by chance be perceived in a measure, they are not for this reason said to be visible and perceptible
in the simple and absolute use of the word. The same principle applies to the species of objects perceived by taste, smell, and the other senses; perception of them is gained neither per se nor per accidens, and therefore the species are imperceptible. And because they are imperceptible they are called spiritual;
but this spirituality does not contradict corporeality nor materiality in material and corporeal things.
That, moreover, species do unite into one, and that one species is really formed from several, is shown by Alhazen the author on Perspective and by Ptolemy, who makes these statements, and by what has been said concerning the first part of the act of perception. For in it the two species coming from the eyes must
become one, so that the object seen may appear as one and not as two. Alhazen says in the first book that lights mix in the me-
dium ; and Ptolemy in his third book clearly teaches a mixing of the species. But as to the fact that Alhazen wishes to prove by experiment that lights do not mix in the air, when three candles are placed opposite an opening ; for then the lights appear distinct beyond the opening, and therefore also in the opening, as he seems to say ; we must state that in one way there is understood to be a real mixing, and in another way there is said to be a distinction. For in fact they do mix in the opening: but since light travels along a straight path, while it is being multiplied in the same medium, therefore the light of each candle, just as it passed along different straight lines before the opening, so must it continue to do beyond the opening as regards the principal multiplication, and therefore the primary and principal paths are divided beyond the opening just as they were before it. But an accidental multiplication of the two candles occurs with the principal multiplication of the third candle, and thus there is a mixing beyond the opening. But since an accidental multiplication is not taken into account with a principal one, nor does vision form a judgment in regard to the former, because it is hidden by the principal one, therefore no confusion is apparent to us, nor is there a mixing in the places
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Optical Science
where the lights of the candles fall. There is then a mixing in the case of the lights, namely, of the accidental light with tHe principal ; and the author [Alhazen] says that there is no appearance of mixing, because it is hidden, and he says that there is no mixing of the principal multiplications in the places where they fall; and I grant this; and yet there is a mixing there, which I mentioned in regard to the opening. I say that light considered without restriction as it is in the opening must mingle in a natural mixing and become one undivided light, and he does not deny this. But if we consider the lights as regards their relation to the straight principal paths divided after the opening, just as they were before it, they are thus said to be divided and not to mix. Hence speaking without restriction they do mix, but with respect to the different principal paths,
into which it is stated that they must be now distributed in the
opening, they are divided and distinct But distinct existence
in this way is not understood in the ordinary sense of the term,
nor is it opposed to a real mixing without restriction, because
this has to do with effect and not with mode of existence for ;
they are said to be distinct in the opening solely because they
make after the opening distinct paths, just as the sun is said to be hot because it produces heat, not because it is formed by
heat. Whenever, therefore, any of the sacred writers, or of the philosophers, or of the men of science in the past say that the species of light and color or other species at the same time are distinct in a medium, this statement must not be understood without qualification, but it is made because the species make
distinct principal paths beyond the place of mixing, just as they did before it.

SEVENTH DISTINCTION
In four chapters. First there is removed the error of vision which would result if the vitreous and the lens [glacialis \ were of the same nature.

CHAPTER I

removed the difficulty in seeing, we must now

HAVING show how other inconsistencies may be avoided. For if the rays of the visual pyramid meet at the center

of the anterior glacialis, they must be mutually divided, and

what was right would become left and the reverse, and what

is above would be below, and thus the whole arrangement of

the visible object will be changed, as is easily apparent in the

figure: and thus the species of the right part of the object will

not come to its place, but to the opposite side, and the same is

OCXTRUM

SINISTRUM

true with regard to the left, and with

,

.

.

i*/r

r

.

regard to other differences of positions.

In order, therefore, that this error may

be avoided and the species of the right
part may pass to its own side, and the
left to its side, and so too of other positions, there must be something else between the anterior of the glacialis and its center to prevent a meeting of this kind. Therefore nature has contrived to

FIG. 31.

place the vitreous humor before the center of the glacialis, which has a different transparency and a different center, so

that refraction takes place in it, in order
that the rays of the pyramid may be diverted from meeting in
the center of the anterior glacialis. Since, therefore, all rays
of the radiant pyramid except the axis, which passes through all the centers, are falling at oblique angles on the vitreous humor, which is of a different transparency, all those rays must be refracted on its surface, as was shown above in refractions. Since, moreover, the vitreous humor is denser than the anterior

glacialis, it follows, therefore, that refraction takes place be-
tween the straight path and the perpendicular drawn at the

Optical Science

point of refraction, as has been shown in the multiplication of species. Wherefore of necessity the ray mq, when it reaches the point q on the surface of the vitreous humor, which is gdf, does not pass by the straight path to a, center of the anterior glacialis, which is ghf, but will be refracted at the point q, between the straight path, which is qa, and the perpendicular drawn from the point of refraction, which is q, into the vitreous
humor. This perpendicular is bl; for bl goes to the center of
the vitreous humor, which is b. Thus the right species will always go according to its own direc-
tion until it comes to a point of the
common nerve, which is c, and will

not go to the left. In the same way the ray pu will not pass to a, the cen-
ter of the anterior glacialis, but will be refracted between the straight path, which is ua, and the perpen-
dicular bs drawn from the point of refraction, which is #, and thus the
ray pu will pass to a point of the
common nerve at c, and will always be to the left. The same is true of the

species coming from all other parts, because they will always travel along the paths that are due to them, and through the location they should
have, so that no error may happen.
Since the nerve is filled by a similar vitreous humor as far as the common

FIG. 32.

nerve, there is, therefore, no other refraction, but the species
travels uniformly without refraction, nor does it change its straight path in any manner except in accordance with the tor-
tuosity of the nerve. And in this fact we must wonder at the
power of the soul's force, in that it causes the species to follow the tortuosity of the nerve, so that it flows along a tortuous line, not along a straight one, as it does in the inanimate bodies of the world. For while it is in an inanimate medium it always

travels along straight paths, as stated above : but owing to the necessity and nobility of the processes of the soul, species in
an animated medium keeps to the path of the medium and

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disregards the common laws of natural multiplications, rejoicing in the special privilege of the soul. Thus, therefore, we
must consider that the species of the thing seen is necessary to
vision, and we must also consider how it falls upon the vision
and all its parts.
CHAPTER II
In which it is shown that the species or force of the eye radiates
to the visible object because of the act of seeing.
WE must now consider whether the species of vision is neces-
sary for the act of seeing. It is clear, moreover, that a species is produced by vision just as by other things, because accidental qualities and substance inferior to vision are able to produce
their own forces; much more therefore has vision this power.
This fact is evident also for this reason, because the eye is seen
by itself, as by means of a mirror, and can be seen by another. But nothing is seen except by means of a species coming from the object seen. But whether this species, or force of vision, or visual rays come from the eye to the thing seen has always been a doubtful matter with scientists. But Aristotle settles this ques-
tion on his own authority in the ninth book of the De Ani-
malibus, saying that vision is nothing else than the visual force touching the object seen. Ptolemy also in his book on Optics, that is, on vision or in his theory of perspective, previous to the teaching of Alhazen, which Alhazen expounded as re-
ceived from Ptolemy, maintains throughout his whole work that visual rays come from the eye to the object seen, Tideus also in his book on Aspects affirms this, and states that sight never determines the distance between itself and the object seen, nor the size of the object seen, nor the position and situation of it, unless the visual rays pass to the object seen and rest upon it, and grasp its surface and contain its extremities, This
likewise Jacobus Alkindi asserts in his Science of Optics, also
Euclid and all other authorities. If we wish, moreover, to confirm this by the sacred writers, we shall state that they agree
in this view, Augustine in particular; for he maintains in the sixth book on Music that the species of vision comes and is propagated in the air to the object. Hence just as an inanimate
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object produces its own inanimate species, so does an animate
thing produce a species that has in a measure the force of the soul \anima\. For just as an inanimate thing has a relationship to its species, which is similar to it, so is an animate thing
A related to a species similar to it. medium, however, which is
inanimate will not because of this fact be animate, but will be
made like an animate one through its likeness now received.
CHAPTER III
In which objections are removed.
IF, moreover, Alhazen, and Avicenna in the third book on the Soul, and Averroes in his work on Sense and the Sensible are cited as opposed to this view, I reply that they are not opposed to the generation of the species of vision, nor to the part it plays in producing sight; but they are opposed to those who have maintained that some material substance as a visible or similar species is extended from the sight to the object, in order
that vision may perceive the object itself, and that it may seize
upon the species of the object seen and carry it back to the sight. For this was the opinion of some of the ancients in this matter, who did not yet possess definite knowledge of vision.
We must state, then, that the philosophers mentioned above,
Alhazen, Avicenna, and Averroes, are opposed only to this view, as is evident from their text. But nevertheless the majority is imbued with the contrary notion, owing to a passage of Aristotle in the Topics, because what one hears from his youth he accepts as a matter of habit, so that he is unwilling to receive anything else. For Aristotle in the book of the Topics, since he is giving the art of proof for every problem, states examples, which are the arguments of philosophers on matters concerning which there was doubt and which are
discussed in common among them, as is evident from that
book. Therefore that famous instance, that vision is the result of internal reception and not of external transmission, he cites in accordance with well-known opinions. For the Stoics held this view, as is shown by Boetius in the fifth book on Consola-
tion in that verse, "Quondam porticus attulit obscures nimium
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senes, etc." And in the book of the Priores he says that we do
not always cite examples because they are true, but that he
may assent who learns. Aristotle, therefore, does not assert
that vision is not the result of external transmission, but makes
the statement in accordance with the general opinion, and as an example, not as a truth. As to the fact, also, that in the second book on the Soul he strives to show that in general sensation belongs to the class of passive faculties and does not teach that sensation is active, we must state that this was neces-
sary owing to the position of his master Plato and of many Platonists. For it was the common belief among them that
vision was only active, and that it emitted a visible species for viewing all visible objects, whence according to them vision sends suddenly to the stars a visible species which views them
and returns to the sight their species. Therefore Aristotle, who
wished to verify all things as far as the possibilities of his age permitted, rejects both opinions regarding vision; namely,
that of the Stoics, who maintained that it is passive only, and that of the Platonists, who held, and erroneously so, that it
was only or principally active. But one of these views, namely, that of the Stoics, he refutes in his book on Animals, and the other, that of the Platonists, in his book on the Soul, just as it suited his purpose. But those versed in the philosophy of Aristotle and particularly in perspective think that vision is active and passive. For it receives the species of the thing seen,
and exerts its own force in the medium as far as the visible
object. Since the multiplication of species is instantaneous for every distance, as most people reckon, or rather it does require time, but an insensible amount, this time escapes perception
owing to its brevity.

CHAPTER IV

'
*

In which the true theory is given.

THE reason for this position is that everything in nature com-
pletes its action through its own force and species alone, as, for example, the sun and the other celestial bodies through
their forces sent to the things of the world cause the generation and corruption of things; and in a similar manner inferior

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things, as, for example, fire by its own force dries and consumes and does many things. Therefore vision must perform the act of seeing by its own force. But the act of seeing is the perception of a visible object at a distance, and therefore vision perceives what is visible by its own force multiplied to the
object. Moreover, the species of the things of the world are not fitted by nature to effect the complete act of vision at once because of its nobleness. Hence these must be aided and excited
by the species of the eye, which travels in the locality of the visual pyramid, and changes the medium and ennobles it, and renders it analogous to vision, and so prepares the passage of
the species itself of the visible object, and, moreover, ennobles it, so that it is quite similar and analogous to the nobility of the animate body, which is the eye. But since this theory is
doubtful to many, therefore besides the verifications now given
I shall adduce diverse true and certain experiences, as they shall appear in different places below in regard to other conclusions, which necessarily accompany this view of the sub-
ject. Concerning the multiplication of this species, moreover,
we are to understand that it lies in the same place as the species
of the thing seen between the sight and the thing seen, and takes place along the pyramid whose vertex is in the eye and
base in the thing seen. And as the species of an object in the same medium travels in a straight path and is refracted in different ways when it meets a medium of another transparency, and is reflected when it meets the obstacle of a dense
body; so is it also true of the species of vision that it travels altogether along the path of the species itself of the visible
object. And although the species of the eye lies in the form of
a pyramid, whose vertex is in the eye and whose base rests on. all parts of the object seen, yet from the surface of the glacialis there are pyramids in an infinite number, all of which have one base, and the vertices of these pyramids fall upon
the separate points of the thing seen, so that thus all parts of the visible object are seen with such intensity as is possible.
Nevertheless one pyramid is the principal one, namely, that one whose axis is the line passing through the center of all parts of the eye, which is the axis of the whole eye ; for that pyramid attests all things, as was stated above and will be explained more fully.
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Opus Majus

Although the species of visible things, as those of light and of color, mix in a medium, that is, several lights unite to form one, and several colors mix, as has been stated, and this species of an object and the species of vision lie in the same undivided

place ; yet is there no confusion of these species, or mixing, nor is one thing formed out of them, since they are not of the same

species or of the same genus; because the pupil does not have color, nor do color and light have the force of the soul. More-

over, the species of the eye is the species of an animate substance, in which the force of the soul holds sway, and therefore

it bears no comparison to the species of an inanimate thing, so

that one thing should result from them, just as there is no such

result

from

whiteness

and

sweetness

in

milk ;

and

much

less

so

in the case in point, because there is a much greater distinction

between the animate and the inanimate, than between the two

inanimate.

EIGHTH DISTINCTION
In which we are shown that besides species there are nine conditions required for vision. The distinction contains three chapters. In the first it is shown that light and a
proper distance are required for vision.
CHAPTER I
these things we must consider that besides species
ATERnine things are required for vision, as the authors on Perspective show. One of these is light, because nothing is seen without light. For light in the first place is the visible thing, then color and the rest of the twenty things which I enumerated before, and all other things are seen with the participation of these, but no one of them is seen unless it is bathed in light. The reason for this is thought to be found in several ways, namely, either because color does not have a real existence in darkness, according to Avicenna in the third book on
the Soul, or if it has, it is not able to produce its species in darkness, according to Alhazen, or if it can, it will not affect vision, nor will it change it, so that an act of vision takes place,
according to the same Alhazen. The first supposition Ptolemy proves false in the second book on Perspective as follows : For if such were the case, any two things having the same position with respect to light and vision would seem of like color, the reverse of which we see in different things quite generally, and in the same thing at different times, as in the chameleon, which
changes its color in accordance with the difference of those
things that approach it, and in the case of him who turns red from shame and becomes pale from fear; although the thing always has the same position with respect to the light. The second supposition is shown to be false by comparison with every
other active thing that produces species in darkness and in light. The third supposition is the true one, and with reason, because the first and principal visible thing is light, and therefore nothing can be seen without its help; just as in the other
senses we smell nothing without the aid of the sense of smell, nor do we touch without the aid of the first four qualities,
which are hot, cold, moist, dry.
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Opus Majus

The second thing that is required for vision is distance. For

in general a sensible object placed on the organ of sense is not

perceived, as Aristotle states in the second book on the Soul.

The reason for this is that every sense acts by external trans-

mission, that is, by emitting from itself its own force into the

medium, so that the sensible species is returned more fitted to

the sense, and receives a nobler essence from the species of the

sense, so that it may be more conformed to the sense. More-

over, we find this in all the senses. For we have shown this

above

in

regard

to

vision ;

and

the

like

is

true

in

the

other

two

senses that have an extrinsic medium, namely, the sense of

smell and hearing, since Aristotle says in the nineteenth book on Animals that forces are produced by the sense of smell and by hearing, just as water from canals. Similarly also regarding the senses that do not have an extrinsic medium, but an intrinsic one, namely, touch and taste. For concerning touch
Aristotle says in the second book on Animals that its medium
is flesh and its instrument is nerve. But in the twelfth book on

Animals he maintains that the flesh perceives in touch, just as the eye in vision. Avicenna, moreover, in the first, second, and third books on Animals maintains that the skin and the flesh

perceive. Therefore the sensitive force which is in the nerve scatters its force in the medium of touch, which is the flesh and the skin. But taste is a kind of touch, as Aristotle says in the second book on the Soul, and has an intrinsic medium, as

touch has. Therefore the force of taste, which is in the nerve,

emits its species into the flesh and skin of the tongue, nay,

what is more, into the palate and other parts of the mouth, so

that in a measure those parts seem to perceive the savor.

To what distance we can see on a plain of the earth's surface

and on mountains the author on Twilights shows, saying that

we

see

on

a

terrestrial

plain

about

three

miles ;

and

on

a

very

high mountain, the greatest height of which is eight miles, we

shall see even on a plain of the earth's surface only about 250

miles; and the gibbosity of the earth restricting vision causes

this.

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CHAPTER II

Concerning the third condition which requires that the visible object confront the eye.

THE third condition is that the visible object confront the eye.
For this is required in vision effected along straight lines, the kind of vision here under consideration, although by reflection and refraction a thing can be seen without confronting the eye. But this is a remarkable fact, since we hear and smell in all directions, and feel in front of us the heat of a fire placed behind us, if it is large and strong; and vision, which is a nobler sense, does not act in this way. Moreover, the cause of this thing is quite hidden and still strange and unperceived by scientists. For Aristotle in his book of Problems should have

informed us in regard to this matter, for he touches upon it
there among his other secret problems. But either a bad trans-
lation or error in the Greek edition, or some other reason hin-

ders us in this particular.

It is certain, however, that we hear in every direction sounds

without refraction and without reflection, as, for example, one

speaking hears his own voice; but it is impossible that this hap-

pens by an accidental multiplication, because an accidental

species does not cause us to perceive an object, as has been ex-

plained above. Nor does it take place by refraction, since there

is

only

one

medium ;

nor

is

there

present

a

dense

body,

from

which reflection takes place; wherefore the action must be

along straight lines produced to the ear, therefore there must

be a real sound and not merely the species of sound opposite the

ear, and there must be a multiplication into it; but the first sound cannot strike the ear. Therefore a real sound must be

produced

opposite

the

ear ;

and

the

way

in

which

this

is

true

I

shall describe. For sound is- produced because parts of the

object struck go out of their natural position, where there fol-

lows a trembling of the parts in every direction along with

some rarefaction, because the motion of rarefaction is from the

center to the circumference, and just as there is generated the first sound with the first tremor, so is there a second sound with

the second tremor in a second portion of the air, and a third sound with the third tremor in a third portion of the air, and

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Opus Majus
so on. Moreover, because these tremors are violent and especially so in the air, which is easily moved, and when it is put in violent motion retains the impression of the motion, therefore the second sound and successive ones for a good distance are not merely the species of sound, but a real sound possessing at any rate more than the species. Moreover, since this is a fact, for this reason a sound similar to the voice multiplied from the mouth to the surface of the ear can be generated by tremors of this kind in the air produced in every direction to the ear, not by an accidental generation but by a principal one, since the reason for this is found in the tremor mentioned, which causes the air to vibrate to the ear, and produces a real sound in it, and in every direction. Moreover, a sound made in the ear or near it, and in a straight line with it, is not the species of a species, but is sound itself produced by the tremor. Moreover,
a proof that sound in the parts of the medium of the air from
the first place of its production is not merely the species of the sound, but has more of the nature of the object, is the fact that sound is more violent than other sense perception, because
sound suddenly confuses the hearing and destroys it, when it
is very loud.
Odor not only produces a species, but from the odorous body a vapor goes forth, which is a subtle body diffusing itself everywhere in the air, and when it comes opposite to the nostrils it multiplies its species to the organ of smell, and therefore that vapor has real odor, just like the first odorous body : and for this reason not only the species, but the real odor is found here in the opening of the nostrils, not, however, the first odor, but the second, which is in the vapor.
Now the four 'qualities, as stated above in the laws of mul-
tiplications, are able to complete their species like the four ele-
ments, because of the necessity of generation. For we see that fire not only generates its species, but also a real and perfect fire, in flame and charcoal; and thus the heat of fire, having
more than does mere species, is able to generate real heat, and this can take place everywhere in the air. Therefore it can reach to the opposite part of an animal which is not exposed to the fire, namely, when it comes outside the shadow of the fire, which the FIG. 33. exposed object makes as far as the point a, and
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Optical Science
then is able to produce a species on the surface of the object not exposed to the fire by the line ab. But cases of this kind do not happen in those things ordinarily seen, and therefore an object must confront the sight.
CHAPTER III
Concerning the sensible magnitude of the visible object.
THE fourth condition required for vision is that the object be
of a magnitude perceptible by the sense. For the object can be so small that it will not be visible. The reason for this is that the species coming from the parts of the visible object must be arranged distinctly on the surface of the lens, and in addition sensibly as regards the sentient faculty. But when the object is too small, the species coming from the individual parts of the object to the parts of the sentient organ, although distinguished according to the position of its magnitude, as its surface is infinitely divisible, are not, however, distinguished as regards sensation, but are confused because of too great proximity on the small part of the sentient organ which the visual pyramid occupies.
Joined, moreover, to this consideration is that of the maxi-
mum magnitude that we can see by means of the visual pyramid. This question is one of great doubt, namely, How much
in extreme magnitude can be seen by the eye? Men versed in
the problems of perspective estimate that the eye on the surface
of the earth, as we now see, cannot see a fourth part of the heavens by means of the radiant pyramid. But if the eye were
at the center of the earth, it would see a fourth part of the heavens under that pyramid; since they maintain that the pyramid contains in the eye a right angle, because by that angle is subtended the side of a square that can be described in the sphere of the uvea, namely, because the portion of the uvea in which the opening is can take the side of a square, and to the side of a square a right angle corresponds. Since this is a fact according to the opinion of those men, they think that in the portion seen of the heavens the side of a square can be contained, and for this reason the whole fourth part would be
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seen if the eye were at the center of the earth. But if this be a

fact, then without doubt a fourth part cannot be seen by an eye

that is on the surface of the earth, by the twentieth proposition

of the Elements of Euclid. For according to

this proposition, if from the extremities of

the base of a triangle two lines are drawn be-

neath the triangle they will contain a larger

angle, as is evident in the figure. Therefore

the vertex of a pyramid coming on a fourth

part of the heavens to an eye on the surface

of the earth would contain an obtuse angle.

FIG. 34.

Since, therefore, the angle of the pyramid

according to these men is a right angle, the

pyramid itself will not have a quarter of the heavens as a base,

since the eye is on the surface of the earth, but will have less

than a fourth.

But the supposition that the radiant pyramid contains a right angle has neither authority nor experience nor proof up to the present time, and therefore is rejected with the same facility with which it is proved. Moreover, the reason for this
assigned in the side of a square described in the sphere of the uvea cannot stand; because the sphere of the uvea and the anterior glacialis are not concentric, and therefore a right angle in the sphere of the glacialis will not correspond to the side of the square in the sphere of the uvea, nor will the portions of the glacialis and the uvea be similar, and therefore the portion of the sky will not be similar to the portion of the uvea
in which the side of the square is drawn; which, however,
would be necessary. Moreover, I am here calling portions simi-
lar which have portions proportional with respect to their
spheres when they are cut by the same diameters of the earth, as is evident in the figure. For let the sphere of the heavens be aci, the sphere of the glacialis concentric with it do, and let the sphere of the uvea, whose center is different toward the an-
terior of the eye, namely t, be efg. It is evident, therefore, that ac9 a fourth of the heavens, and do, a fourth of the glacialis, are similar, and will have the same right angle, namely, ohd; but eg, the fourth of the uvea, does not have that angle, since it faces the angle ghe, which is part of the right angle ohd, but the portion is larger than that fourth is, namely, ef. There-

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fore the side of the square described in a quarter of the uvea cannot face the right angle of the pyramid and of the glacialis, and therefore the angle of the pyramid cannot be closed by that side. But if it should be so closed, it will be acute, not right, as is evident to the sense in the angle ghe, since it is part

FIG. 35.

of the right angle fhd and ohdf which are identical. But

it would be better to say that the portion of the anterior gla-

cialis is a fourth of the sphere ; for thus the angle of the pyra-
mid would be a right angle, because that whole portion, how-

ever small it may be, is occupied by the angle of the pyramid,

because it as a whole is the organ of vision, and not merely

some part of it. Then, moreover, the side of the square de-

scribed in the glacialis is subtended by the right angle of the

pyramid ; and similarly in the portion of the albugineous humor,

which

is

in

the

opening

of

the

uvea ;

and

in

the

same

way

in

the portion of the cornea, for they will then be fourths of their

spheres, because they are concentric with the glacialis. More-

over, since the heavens would be concentric with them, if the

center of the eye were at the center of the earth, the fourth

part of the heavens would then correspond to them, together

with the side of the square described in it, and thus a fourth

part of the heavens would be seen. But there has been no

U79]

Opus Majus

verification of the statement that the portion of the anterior
glacialis is a fourth of its sphere, and therefore we have no proof that an eye at the center of the earth would see a fourth of the heavens, nor by this method or proof can we be certain
that an eye on the surface of the earth will see less than a
fourth, nor how much less.
But experiment proves that the eye cannot see a fourth part of the heavens on the surface of the earth. For if one views a

star that is above his head, and stands in a level space, he will

not be

able

to

see

as

far

as

the

earth,

however

much

he

tries ;

but a fourth part of the heavens extends from the zenith to the earth. Therefore he will not see a fourth part. But he will see,

however, a little less, since if on fixing his eye he bends his head a little he will see the earth, and he will not see the star.

There is no other reason for this than the arrangement of the

eye, because the pupil is so situated, and the opening of the uvea so arranged, that he cannot see more. This is manifest,
because one person owing to the ar-

rangement of his eye will see more of ' the quarter and another will see less.
For he who has the opening of the

uvea small, and the pupil deep set will

see less of this fourth, and where the

arrangement is the opposite, one will

see more, as is shown in the figure ; so * that if the pupil is at the point a and

* the chord of the opening is be, one will

see less than if the pupil is at d, be-

FIG. 36.

cause the lines db and dc diverge more

than ab and ac. Similarly, if the open-

ing is greater with nearness of pupil more will be seen, as is evident by the lines df and dg, which separate more widely ;

and this is the definite reason in this case. Therefore those sup-

positions given above have no basis in fact.

C480]

NINTH DISTINCTION
In four chapters. The first deals witA the density and rarity of the object.

CHAPTER I

fifth condition is that the visible object, which we
THEcommonly call the object of vision, must exceed the density of the air and of the heavens, as Alhazen shows. For this reason we see clearly water, because it is denser than air, and vapors, and clouds, and glasses and transparent objects of this kind, which have little density as compared
with those things that have perfect density. But, however, we should know that Ptolemy says in the second book of the Perspective that we see the air or the celestial transparent body
far off and within a considerable distance although not within a distance near at hand; for much of a transparent body is accumulated in a great distance and has the same effect on vision as that which is perfectly dense within a small distance. Much, therefore, of a transparent object accumulated in a great distance becomes shaded, just as we see in the case of deep water, through the medium of which we cannot see the ground as we can through water not too deep. For the parts of deep water cast forward a shadow on those that succeed them, and

a darkness is produced that absorbs the quality of the rarity,
so that in this way the whole body of water appears like some dense body, and the same is true of the air or celestial transparent medium at a distance, for which reason it is rendered

visible, but not so at close range. But there is also another reason, namely, that vision is ter-
minated at a distant transparent object. For as Alhazen says in the seventh book, the rarity of bodies in the world is limited, and for this reason all of them have some density and rarity, although not very perceptible especially when near; for the species of vision is emitted from the sight to the visible object, and is weakened in distance, so that although on account of its

strength it penetrates the air near at hand, but does not pene-

trate,

however,

the

transparent

celestial

medium

at

a

distance ;

and therefore vision is terminated at it. But Avicenna says in

1:4*0

Opus Majus

the third book on the Soul that that is really visible which

terminates vision, and therefore this transparent celestial

medium is really visible at a distance. I offer an example in

the case of air filled with vapor and cloud in winter, which is

seen at a distance but not near at hand, and when the air filled

with vapor is uniform in density and rarity. But from a great

distance it is seen because it can terminate the species of vision

and resist it; and thus it becomes visible, a visibility it cannot

have near at hand owing to the strength of the species of vision. Nor is there deception of vision, because that air filled with

vapor becomes an object of vision. The explanation of this ac-

cording to Avicenna lies in the fact that it is able to terminate

vision. Likewise I say here that air, or the sphere of fire, or

the heavens, near and remote, is of similar rarity as far as

perception is concerned; but it has, however, some density of

its own nature, and this density is able in a great distance to

terminate the species of vision, which it cannot do in a short

distance, and therefore it will be quite visible at a distance, but

not near at hand. Alhazen's statement, therefore, that a visible

object must exceed the density of the air, and the fact that he

calls air wholly transparent as far as the stars, are to be under-

stood in the case of the usual visible objects which can be seen

Why within the required distance and strength of vision.

a

color appears approaching black, namely, blue, is explained in

the same way as in the case of deep water, where in a like

manner that color appears owing to shadows projected by

particles. Darkness is caused by these shadows, which is simi-

lar to blackness. This is what takes place in the air or medium

between us and the last heaven. If some celestial body were a dense body, we could then say that that body would be visible

chiefly because it terminates vision, and that the whole trans-

parent medium between the object and the eye would not be

perceived, except for the fact that vision perceives that it is

not terminated before its species reaches that dense body. More-

over, it is thought by learned astronomers, who are ignorant

of theology, that the starry heaven is everywhere dense. But

there is another view, namely, that that which is seen beyond

is the heaven of water. For it has a color like the water of the

sea, and we see that heaven through the medium of all the

eight heavens that are on this side of it. For there is no doubt

[4823

Optical Science

in the minds of theologians and those philosophizing in ac-

cordance

with

theology

that

the

ninth

heaven

is

of

water ;

and

then beyond it is the tenth. But concerning these the discussion

belongs elsewhere.

But an objection is raised in regard to the ray or luminous air

falling through a window that it is quite visible, and yet that

portion of the air is rare, and rarer than that outside the path

of the light, because light rarefies air as it generates heat; for

heat is not generated except from rarefaction. Here, moreover,

some have attempted to show that density is not required in

luminous bodies, whence they judge that the stars are not dense,

nor are the spheres. But owing to the excessive brilliancy of the

light they say that vision cannot penetrate them, but is beaten

back or fails from the excess of splendor. But the example of

the ray falling through the window is clearly explained, as

stated above. For it is not of itself a visible object nor does it

terminate vision, but the dense body surrounding it terminates

vision, and not the ray ; for if there were not a dense body be-

yond it, it would not be seen, just as the luminous air outside is

not seen, unless it be terminated by the density of some portion

of the heavens. The air in the window is rendered more per-

ceptible, because its shape is determined by the parts of the

window and by the ground on which it falls. It is not, therefore,

a visible object except in a secondary and not a primary sense,

nor is this of which we are speaking here a visible object, which,

namely, has the power of itself by means of the necessary dis-

tance to terminate vision. Moreover, just as the wrong under-

standing of this example is now removed, so must we now have

the right view regarding that which they attempt to prove by

the example just quoted in regard to luminous bodies. For after

the antecedent is shown to be false, we need not believe the con-

We sequent.

have stated above in the sections on mathematics

that the spheres are not luminous, but that a star is so solely on

account of its density, as Averroes says, and as was stated in

that place. Moreover, although too great brilliancy confuses
vision, as is evident when we gaze on the sun in its full strength,

yet this does not exclude density in the sun, because density is
the cause of illumination, as Averroes states, and the moon and

the fixed stars when viewed do not confuse vision from exces-

483:1

Opus Majus
sive splendor. Therefore vision would penetrate them, unless
their bodies possessed density.
Now from this chapter it is clear to him who gives it due
consideration that vision must take place by means of its species emitted to the visible object. For if a single continuous transparent medium in extreme distance terminates vision, and does not terminate it owing to a perfect density, which independ-
ently is perceptible in every distance, but terminates it because of the weakness of the species of vision, which fails at a too great distance, then vision must be produced by sending
forth its species; that is, by emitting its species owing to the
act of seeing.

CHAPTER II

Concerning the rarity of the medium,

THE sixth condition required for vision is rarity of the medium.

For if a dense body is placed between the sight and the visible

object, the species cannot pass through from one side to the

other, and thus of necessity vision is cut off. But some, possess-

ing an excellent understanding of many things in the science of

Perspective, raise an objection in regard to a flame placed be-

tween

the

sight

and

a

visible

object

impeding

vision ;

and

the

flame, as they say, is a very rare body, because Alhazen says in

the third book that the rarity of flame deviates from the mean.

Therefore a rare body hinders vision more than a dense one.

This, however, is a great error, for of necessity flame is denser

than air, because Aristotle says in the second book on Genera-

tion that flame is a glowing terrestrial vapor; and therefore the

deviation of rarity from the mean can be understood, either

through a change to an extreme deficiency, or to an extreme

excess, because the mean is midway between the two, just as liberality is between prodigality and avarice. And therefore just as avarice deviates from the mean of liberality through a

lack of this quality, so does flame deviate from the mean of

rarity by a lack of it, not by an excess. This, moreover, is Al-

hazen's meaning, for he so explains himself in what follows. If

then an objection be raised regarding the lynx, which sees

through the middle of a wall, as Boetius in the third book on

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Consolation imputes to Aristotle, we reply that although this be true of the sight of the lynx, it is not true, however, of human
vision with which the science of Perspective deals. For we are

here discussing the latter vision.

But if we assume that there is a vacuum between heaven and

earth, there would be neither a dense body nor a rare one. And

yet Democritus thought that an eye on the earth could see an

ant in the heavens, as Aristotle states in the second book on

the Soul. Moreover, vacuum does not possess some nature by which it should impede species, or resist it, because no form of

nature exists in a vacuum, as Aristotle says in the fourth book

of the Physics. Therefore species will not pass through from heaven to the eye, and thus we should see the stars without a

rare medium and without a dense one. But we must here state

that we should not see anything if there were a vacuum. But

this would not be due to some nature hindering species, and re-

sisting it, but because of the lack of a nature suitable for the

multiplication of species; for species is a natural thing, and therefore needs a natural medium but in a vacuum nature does
;

not exist. For vacuum rightly conceived of is merely a mathe-

matical quantity extended in the three dimensions, existing per se without heat and cold, soft and hard, rare and dense, and

without any natural quality, merely occupying space, as the

philosophers maintained before Aristotle, not only within the

heavens, but beyond.

The seventh condition required for vision is a sensible time.

For Aristotle says in his book on Memory and Reminiscence

that

our

whole

intellect

is

concerned

with

continuity

and

time ;

much more so, therefore, is sensation. For the action of the in-

tellect is spiritual, and the act of sensation is corporeal. But if

a thing is brought suddenly before the eyes it is not seen dis-

tinctly and perfectly, and therefore a sensible time is required

for vision in which the judgment of vision may be made.

Whence Ptolemy in the second book of the Optics says that

things which pass through the visual pyramid are thought to

move swiftly, like the embers of fire and things passing through

holes and narrow places, to which the vision penetrates. For

since they pass through the pyramid in a short time, they are

thought to move swiftly. But such is not the case.

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CHAPTER III

Shelving that the species of vision and of the visible are
produced- in time.

BUT here a very great doubt arises in regard to the species of

sight and of the visible object, whether they are produced sud-

denly

and

instantaneously,

or

in

time ;

and

if

in

time,

whether

they require a sensible and perceptible time or not. Alkindus,
moreover, tries to show in his book De Aspectibus that the ray

passes through in a wholly indivisible instant, and cites in support of his view quite a curious and probable reason, when he says, "If the species, as for example the light of the sun when

it rises, is produced in a particle of time in the first section of

the air, then if the time is doubled in the second section of the
air, and tripled in the third, and so when the light reaches the west, there would result a time made up of many parts which

would be large in comparison with the first particle of time; and although the first particle would be imperceptible, yet the

whole time owing to its magnitude incomparable, as it were,

with respect to the first particle, will be perceptible." Aristotle,

moreover, says in the second book on the Soul that although in

a small distance the multiplication of light might be able to

escape our sense, yet it would not be able to do so in so great

a distance as that which separates east and west. Therefore, if

the species required time this would be perceptible to the sense.

Therefore the species will not be produced in time but instan-
taneously. Aristotle says in the book De Sensu et Sensato that

the explanation in regard to light differs from that in regard to

other things of perception, and in regard to these others he

teaches that their multiplications take place in time. Therefore

the multiplication of light is instantaneous.

All authorities make this statement except Alhazen, who at-

tempts to prove this view false in the second book, arguing as

follows: "Let us take the last instant at which the light is at

the terminus a quo and the first at which it is at the terminus

ad quern'' Since therefore the instants are different, as he

strives to prove by experiment, there will be a time between

them ;

and

he

says

that

every

change

takes

place

in

time,

but

the medium and the eye are changed by species. But these rea-

C486]

Optical Science

sons of Alhazen do not have any weight, because the first is explained elsewhere. For it is not necessary to give always a last instant of an existing thing at the terminus a quo, just as universally happens in the generation of permanent things, but it is necessary to give the first instant of the terminus ad quern, as Aristotle teaches in the eighth book of the Physics. Whence when Socrates becomes white from non-white, it cannot be said
now at last is he not white, taking now as an instant, but now
first is he white ; for he is not white during the whole time measuring the change, and he becomes white at the end of this time,
namely, at the instant which is its terminus, as Aristotle teaches, and as is certain, although it is too difficult to understand
unless well explained ; but this is required elsewhere. His second reason has no weight; for all holding the opposite view say that the multiplication of light is not a successive and temporal

change.
A sound argument, however, for the view of Alhazen can be
drawn from the statements which he makes in the seventh

book. For he there teaches that from the same terminus the

perpendicular ray reaches more quickly the terminus of the space than the ray that is not perpendicular. But quicker and slower belong only to time, as Aristotle says in the fourth and
in the sixth book of the Physics. And this is demonstrated with-
out possible contradiction. For no finite force acts instantaneously, as Aristotle says in the sixth book of the Physics ; and he proves this, because in that case a greater force would act in less than instantaneous time, which is impossible. But the force
of the eye, and of its species, and of everything created is finite.
Therefore no force can act instantaneously. Moreover, in the

eighth book of Physics at the end he maintains that a finite force and an infinite one cannot act in the same and equal period of time, since in that case they could have equal results, and thus in turn the forces themselves would be equal. But it is the prop-
erty of an infinite force to act instantaneously. Therefore a finite force cannot produce any result in an instant, wherefore it must require time. Moreover, an instant has the same relation

to time as a point to a line. Therefore, interchanging terms,

an

instant

has

the

same

relation

to

a

point

as

time

has

to

a

line ;

but the passage through a point is in an instant. Therefore the

passage through the whole line is in time. Therefore species

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Opus Majus
passing through linear space, however small, will pass through in time. Moreover, before and after in space are the cause of before and after in making translation over space and in duration of time, as Aristotle says in the fourth book of the Physics. Therefore since space through which species is carried has a before and after, passage of the ray must have a before and after both in itself and in duration; but before and after in dura-
tion exist only in time, since this condition cannot exist in an instant. If, moreover, it be said that this is true of those things that have a corporeal existence in a medium, not concerning those things that have a spiritual existence, as it is assumed here, evidently the objection has no weight, owing to what has already been said. Again if it be said that this is true in regard to those things that are measured by parts of space, but that species is not so measured, as they assume, this objection also
has no weight, because that second statement is not made except
on account of a spiritual existence. Since, therefore, the species of a corporeal thing has a really corporeal existence in a me-
dium, and is a real corporeal thing, as was previously shown, it must of necessity be dimensional, and therefore fitted to the dimensions of the medium,
But if at the same instant it were throughout the whole medium, it would then be at the terminus a quo, and at the middle point of the space, and at the terminus ad quern* namely, at one
and the same time. But this is in many ways impossible. For, in
the first place, it follows from this that a created thing would be at one and the same time in several places, and by this reasoning if in several places, then also in an infinite number, as has already been shown in the chapter on matter* It would therefore have an infinite power and would be God, or equal to God. In the second place, the argument is drawn from this that while the thing is at the terminus a quo it is wholly quiescent,
nor does it suffer change in any way ; and when it is at the
terminus ad quern the change has already been effected, and the change takes place between these termini. Therefore at the same instant the species would be quiescent before its passage, and the change would be effected, and would take place actually through the whole space. Therefore at the same time it would be changed, and would not be changed, which are contradictory,

Optical Science
as Aristotle argues reducing the matter to an impossibility in another case in the sixth book of the Physics.
But there is still another reason for this, namely, since the multiplication of light does not depend on some other motion,
we may assume, therefore, that the heavens are at rest and that
there is no motion, for in a stationary heavens the multiplication of light can take place excellently, and will be accomplished at the end of the world, if the heavens shall be stationary, as is assumed. If, therefore, the multiplication of light is instantaneous, and not in time, there will be an instant without time; because time does not exist without motion. But it is impossible that there should be an instant without time, just as there cannot be a point without a line. It remains, then, that light is multiplied in time, and likewise all species of a visible thing and of vision. But nevertheless the multiplication does not occupy a sensible time and one perceptible by vision, but an imperceptible one, since any one has experience that he himself does not perceive the time in which light travels from east to west.
CHAPTER IV
Concerning the removal of objections to the truth.
MOREOVER, to this statement made by Jacobus Alkindi we must
reply that, just as the first period of the time is imperceptible, so also is the double of it, and the triple, and the thousandth
multiple : whence the whole time is imperceptible, although it
has many parts which taken together make an imperceptible
whole, for this motion of the species is of such great velocity that it can traverse in an imperceptible time a very great distance. Aristotle's statement is true according to his understanding of it, for he is arguing in that place against Empedocles,
who maintained that light is a body and the flow of a body, just as water flows from a spring; and it is not possible that a body
should change its position wholly from east to west, so that it would not be perceived owing to the great distance. But the species is not a body, nor is it changed as regards itself as a whole from one place to another, but that which is produced in the first part of the air is not separated from that part, since form cannot be separated from the matter in which it is, unless
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it be soul, but the species forms a likeness to itself in the second
position of the air, and so on. Therefore it is not a motion as regards place, but is a propagation multiplied through the different parts of the medium; nor is it a body which is there generated, but a corporeal form, without, however, dimensions per se, but it is produced subject to the dimensions of the air; and it is not produced by a flow from a luminous body, but by a re-
newing from the potency of the matter of the air, as we stated above when the question was discussed in regard to the genera-
tion of species. Moreover, if we inquire still more carefully why
we do not perceive this generation of light take place succes-
sively in the particles of the air, the answer can be given that light in the air is not an object, but a species with a weak and in a manner imperceptible existence as regards itself, and its sub-
ject between the east and the west is imperceptible, namely, the air itself, and for this reason the sense is not able to perceive a

successive generation of this kind. Moreover, as regards Aristotle's statement that there is a dif-
ference between the transmission of light and that of the other

sensory impressions, we must say that many are deceived in this
particular; for his statement is true, but this difference is not to be understood as consisting in the fact that light is trans-

mitted instantaneously and the other impressions require time ;

nay, we must understand that, although light has a succession
in its transit, it does not, however, have so great a one as sound

and odor, of which he is speaking in this place. For sound has

the motion of the displacement of the parts of the body struck

from its natural position, and the motion of the following tremor, and the motion of rarefaction in every direction, as was

stated before, and as is evident from the second book on the

Soul ;

and

these

are

the

three

local

motions

of

the

particles

of

the air, as well as of the body struck, no one of which motions

takes place because of the multiplication of light. For although

the air must be rarefied for light to produce heat, yet rarefaction

is not necessary because of the multiplication of light itself,

since in celestial spaces light is multiplied, where rarefaction

and the generation of heat are not possible. Since, therefore,

there is no succession on the part of light except the succession

itself in multiplication, but in the multiplication of sound a

threefold temporal succession takes place, no one of which is

490]

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present in the multiplication of light, for this reason there is a great difference between light and sound. However, the multiplication of both as regards itself is successive and requires

time.

Likewise, in the case of odor the transmission is quite different from that of light, and yet the species of both will require
time for transmission, for in odor there is a minute evaporation of vapor, which is, in fact, a body diffused in the air to the sense besides the species, which is similarly produced. There is also a strong attraction of the nostrils for this kind of vapor and
of species, so that sensation may be produced, as Avicenna shows in the third book on the Soul and as we know by experi-

ence. This results in the removal of the covering that is over the organ of smell, according to Aristotle in the second book on the Soul. Therefore in the sense of smell there is a twofold

local motion, one from the resolution of the vapor, and the

other from its attraction, besides a succession in the multiplica-

tion of the species. But in vision nothing is found except a suc-

cession of the multiplication. The fact that there is a difference

in the transmission of light, sound, and odor can be set forth in

another way, for light travels far more quickly in the air than

We the other two.

note in the case of one at a distance striking

with a hammer or a staff that we see the stroke delivered before

we hear the sound produced. For we perceive with our vision
a second stroke, before the sound of the first stroke reaches the
hearing. The same is true of a flash of lightning, which we see before we hear the sound of the thunder, although the sound is

produced in the cloud before the flash, because the flash is produced in the cloud from the bursting of the cloud by the kindled
vapor. Therefore his statement that there is a difference in the transmission of light and of the other sensory impressions can be understood, because this difference is not one of instantaneousness and time, but of less time and more time. For all au-
thors, whether sacred or others, who state that light is multi-
plied instantaneously, are to be understood in regard to a divisible instant, which is imperceptible time, and not in regard
to a real instant, which is the indivisible terminus of time, just
as the point is of the line. If, therefore, the vision of the species
and the visible multiplication take place in imperceptible time,
how was it said above that vision will take place in a perceptible

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time? It is evident that besides the multiplication there is a judgment of vision in regard to the object seen, and this judgment must be known to the sense ; wherefore the judgment must be made in a perceptible time.
The eighth condition required for vision is a healthy state of the eye together with its natural arrangement; for an eye that is torn out, or blind, or much injured, or clouded from some flowing humor, or from a resolution of vapors confusing the pupil cannot judge regarding objects, as is evident, and therefore need not be considered here. For if anything need be said further on this point, it will be explained in what follows. Concerning position, which is the final condition in vision, an ex-
planation cannot be given here, because it coincides with other
matters of which mention must be made later.
492:1

TENTH DISTINCTION
In three chapters. The first shows with greater precision than above what things are perceived per se and
'what per accidens.

CHAPTER I

stated these eight conditions without which
HAVING vision cannot function, we must consider of what things vision takes cognizance, and in what ways they
are perceived and certified to, and how and why vision errs in

taking cognizance of visible objects, although vision has been

We produced by direct rays.

must note, then, that when these

nine conditions are present in just degree, that is, they neither
exceed nor fall below what is required, a reliable vision is as-

sured. When, therefore, they differ from the due measure either

by excess or deficiency, error is present in vision. Moreover, the

twenty-two qualities enumerated above, as light, color, remoteness, etc., are determined by vision ; and besides this, vision per-
ceives man and horse and the other things of this world. For
by light and color these twenty things are recognized. Then through the medium of light and color and those twenty quali-

ties other things are known, as is possible for the sense, for the

particular senses are not able to certify regarding all things ; but
those twenty-two the particular sense and the common sense

and imagination are able to comprehend without error, pro-

vided the eight conditions previously mentioned are present

in proper measure. Those qualities, moreover, are called the

sensibles per se, certainty in regard to which can be secured

from

the

senses ;

but

the

sensibles

per

accidens

are

those

that

cannot be apprehended in this way.
Although we previously touched upon these sensibles per se and per accidens, yet in order that no mistake may be made they
must be explained more fully. I say, then, that the sensibles per accidens are twofold : certain can be determined by other fac-

ulties of the sensitive soul, as, for example, by the estimative and memorative faculties, as previously stated; certain, how-

ever, are called sensibles per accidens with respect to particular
senses and the common sense and imagination, since such senses

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do not perceive things of this kind independently, but because
they are found in the same things together with their own
sensibles per se. As, for example, since enmity with respect to a lamb is present at the same time with the form and color of a wolf, the lamb in looking at a wolf saw something hostile and colored, but the eye per se forms no judgment concerning the hostile object, but judges it such, because it is found in connection with the colored object. Moreover, since a particular sense
and the common sense are commonly called senses, for this rea-
son the sensibles in regard to which they certify are called sensibles per se, and those concerning which they do not certify are called sensibles per accident, although some of them can be recognized by other inner faculties of the soul For the estimative, cogitative, and memorative faculties are not called senses in the ordinary meaning of the term, although they are parts of the sensitive soul, and therefore the sensibles determined by them
are called sensibles per accidents, because the sensibles are re-
ferred to the particular senses and the common sense.
But there are other sensibles besides those that are recognized
by the faculties of the sensitive soul. As, for example, when I see a strange man I cannot perceive by the sense whose son he is, nor at what hour or in what place he was born, or what name he bears, Peter or Robert; and there are countless things of this kind that are accidental to each person, in regard to which no faculty of the sensitive soul can inform us, nor can one know
the truth except through information given. And yet on looking at that man vision falls upon all his qualities. For if he be the
son of Robert, and a Frenchman, and born in Paris during the first hour of the night, in looking at him the eye sees Peter, a Parisian, born in the first hour of the night, son of Robert, because these things are coincident with his color, and form, and other visible characteristics. In like manner the natural sub-
stances of things, both in things animate and inanimate, are not perceptible by some faculty of the sensitive soul except per accidens. Those things are excepted that are harmful or useful, which the estimative faculty grasps, and sense, however, falls
upon this per accidens. Hence when I see a man I see substance
and an animated object, and therefore vision falls in a measure
on his natural substance, and on his soul also, which is a spirit-
ual thing; but this is surely per accidens. Moreover, the sensi-
494:1

LOUIS IX
From the Seurre Statue at Versailles

Optical Science

bles belonging to one sense are sensibles per accidens of other senses, whence hot and cold, moist and dry, odor, sound, savor,

are sensibles per accidens with respect to vision, and thus the

sensibles belonging to each sense are sensibles per accidens with

respect to other senses. Sensibles, then, per se, as has been said,

are twofold : certain are proper, namely, nine, and certain are

common, namely, twenty, because they can be perceived in com-
mon by several senses, especially by sight and touch. For as

Ptolemy says in the second book all things that vision perceives

touch

discerns,

except

light

and

color ;

and

all

things

of

which

touch certifies can be certified to by vision, except four proper

sensibles, namely, hot, cold, moist, and dry.

CHAPTER II
Concerning those things that produce species in vision.
MOREOVER, that we may know how sensibles of this kind are recognized per se, we must know in the first place whether all things propagate their species to the sense. The determination
of this is difficult, but Ptolemy in the second book of the Optics settles this question with the statement that only light and color propagate their species to the eye. This view Alhazen maintains in the fourth book; whence other things are not active
upon the sense or upon the medium. The reason why they are
not active is that they are all magnitudes or properties of magnitudes, as is evident, and magnitude is not active, because it is a quality of matter which is not active but passive, as Aristotle states in the first book on Generation, and Avicenna agrees with him in the second book on the Soul. For the medium or instru-
ment that takes up the sound is soundless, and the instrument that takes up color is colorless, as he states. Therefore his opinion is that the medium and the senses do not have to have the
natures of the sensibles whose species they must take up, in
order that they may judge concerning the sensibles perceived by means of these species. Whence the humor glacialis does not
possess some nature of light or color to the degree possessed by objects outside it. For although the eye has light, this is in re-
spect to the perception of color, not in respect to light, because
the object acted upon does not possess in actuality but in po-
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tentiality the ability to become like the agent. Nor does the anterior glacialis possess some degree of color, by which it may be made really like colored objects outside itself, of which it has to form judgments, although the eye has in its humors and coats some weak substance of color, by means of which imaginary colors sometimes appear, as will be explained when we consider the iris. But the eye does possess form and magnitude and corporeality, and other common sensibles that are appropriate to it, and therefore it is not fitted naturally to receive the species of these things, nor are they themselves active. And although
Aristotle says in the second book on the Soul that the final per-
fection of every sense consists in its organ being the medium of
sensibles, yet that degree of mediumship does not exist in things pertaining to the senses; since if it did exist and its species were produced on the sense, the sense would not judge concerning that mediumship, and therefore vision would not be able to receive the species of things so as to judge by means of these species concerning things whose natures are similar in vision. When, therefore, the objection is made that these are

sensibles per se, that they therefore act on the sense, just like

proper

ones ;

we

must

reply

that

they

are

not

called

sensibles

per se because of their action on the sense, but because the sense
is able to certify concerning them. And if the objection should
be made that vision will not certify concerning them, since they do not propagate their species to vision, we must reply that such is the case; for its own species is not required in all things, but
the species of vision is sufficient along with the species of light
and color and with certain other considerations. When these

have all been combined certainty can be secured, as will be ex-
plained later. If it be said that authors on perspective state frequently that shape and quality and the like of the thing seen are arranged on the surface of the sentient member, and that they
cannot reach there except through their species, we must reply
that the statement is not their own, or has not been correctly
translated; for they do not mean to say more than that from the whole magnitude and form the species of light and color come which are arranged on the surface of the sentient member, and this suffices.

When it was stated above that we see the air or the heavens

at a distance, and that it is certain that in the water of a river

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or in some other body of water that has great width the heavens are seen by reflection, just like an object in a mirror; and an object in a mirror is seen by this means, because its species is reflected from the mirror to the vision; some one might then
say that the celestial transparency of air at a distance produces
a species by means of which it may be seen in water. But we
must state that no species of the thing seen exists there, but the species of vision, which in the air without water is multiplied along a straight line to the celestial transparency at a distance, and apprehends that transparency with the aid of the species of light illuminating that transparency ; and when it is seen by means of water reflexively, the species of vision is reflected from the water to the air seen at a distance, and the species of the thing is not reflected. Hence not everything that
is seen by reflection is seen by means of its own species. But this
happens as in the case of several familiar visible objects, but this must not be understood generally, because I can see water by means of a mirror, but the species of water is not propagated there, but the species of vision is, and this is the case here.
CHAPTER III
Distinguishing three universal modes of perception through vision.
IN the next place, we should know that besides the particular
modes of perceiving the sensibles per se, there are three universal modes, according to the authors of Perspective. But in particular Alhazen explains these modes and he does so adequately, except that sometimes an inept translation of words occurs. There is, then, first an impression by the sense alone without any faculty of the soul, and it is thus that light and color in general are perceived. For vision is able to decide that there is color or light without error when it beholds an object, provided the eight aforesaid qualities are in their proper proportion ; and therefore these two are perceived by the intuition alone of the sense. But the species and modes of color and of light cannot be so easily recognized ; for if a strange color appears which we have not seen before, we will not know what species of color it is. Likewise if we see some bright object in the air, which has a light different from other objects with
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which we are familiar, as, for example, a star with a train or some other object, and we have not seen it before, we cannot decide by the vision what that light is. Likewise if we have seen some color before, and later have forgotten it, then when it again appears to the sight we can judge it a color, but we will not perceive what color it is. Likewise when in infancy we saw the full moon we did not perceive whether it was the light of the sun or of the moon until we became accustomed to it and
the fact became fixed in our minds that uch a light belongs to the moon, not to the sun. This is also true of the stars, because
many men at some time or other see Jupiter, or Venus, or Mercury, and because of the beauty of these stars they look at them with pleasure, and they are then told by astronomers that the
light of one particular star is of this kind, and the light of still another is of that kind, and they are made sensible of the difference. But after time has passed, when on another occasion they see one of those stars, they do not distinguish whether the light is that of Mercury or of Venus or of some other star, because they have forgotten the impression of the particular light of each of those stars seen formerly, but they recognize the lights of these stars by means of a second perception which is by similitude, according to Alhazen. This mode of cognition concerns not only color and light, but all things in which we distinguish the universal from the particular, and particulars from
one another. For example, when I see a man whom I have seen
before, if I have the impression that I have seen him before, I recognize then not only a man in general, but that particular in-
dividual whom I distinguish from others by means of this kind
of cognition. If, however, I have forgotten, I see a man, but I
do not know who he is.
We must bear in mind, however, that when the statement was
made that vision perceives color or light as a universal by the sense alone, and not as a particular, the particular is excluded
which is lower in the line of predication, as, for example, the species of color. Again, particulars designated by the species of something are excluded, as the light of the sun, and the light of the moon, because perchance light does not have species but modes, because all lights of stars have their origin in the rising sun. But the indefinite particular is not excluded, for it is as
general as its own universal and is convertible with it, as some
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color, some light, some man, some ox. Therefore the recognition
of universals from one another and from particulars, and particulars from one another by a comparison of a thing seen with the same thing seen before, by recollecting that it was seen before and known to the beholder, produces here a second mode

of comprehension through vision.
But there is still a third perception, which cannot take place by the sense alone, and does not depend on a comparison with previous vision, but without limitation considers the thing present. For its perception several things are required, and the
process is like a kind of reasoning. As, for example, when one holds in his hand a transparent stone, and does not perceive its transparency, but if he exposes it to the air, and if there is some

opaque body behind it at the required distance and if the light

is sufficient, he will see the light and the opaque body beyond

the stone ; and then, since he is not able through the medium of

the stone to see the object that is behind it, unless the stone be

transparent, he reasons that it is clear and transparent. But in

ordinary things we employ this perception instantaneously, and we do not perceive that we are reasoning, although we are

doing so. For a man reasons naturally without difficulty and

labor ;

as,

for

example,

when

a

boy

is

offered

two

apples,

one

of

which is finer in appearance than the other, he chooses the finer

one, but for no other reason than because it looks finer to him, and for this reason should be chosen rather than the other.

Whence he follows this reasoning; that which is finer in appearance in so far is better, and what is better is rather to be

chosen, therefore the finer in appearance should be chosen ; and yet he does not perceive that he is reasoning, because of the

swiftness of the reasoning process innate in man, as Alhazen

teaches.

And now in metaphysical and logical terms I have estab-

lished the view of Alhazen, and I have proved that our knowl-

edge of the science of reasoning, which is logic, is derived from nature, but we are ignorant of the proper terms at the begin-

ning, and these through their zeal for discovery the first authors

on logic have found, but we learn them by study. Because of

these terms the formal subject of logic exists, not because of the

potency of the science itself, because this is innate in every one,

as Alhazen here maintains and I have proved elsewhere. Thus

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are comprehended the twenty common sensibles, with their
species, for they could not be determined except through this perception, and it is evident that the sense alone is not able to determine in these two modes; and this is called the sense alone
when vision is in the pupil and in the common nerve reaching to the common sense. For unless imagination and the memory of
the previous sight of an object be present, there will be no com-
prehension in the second mode. But imagination and memory are beyond the common sense. The third mode is still further removed from the sense pure and simple, because in this mode more things are taken into consideration than in the second
mode, and there is more recourse to reason because of the process of ratiocination. But these modes do not have names properly translated. The first Alhazen calls perception by the sense pure and simple. The second he chiefly calls perception by science. The third he calls perception by syllogism, because of the mode of reasoning. But these names are not proper, because the faculties of the sensitive soul have these cognitions, for which neither science nor syllogism is needed as they are generally accepted. But concerning this we shall learn more definitely
when we inquire what are the faculties of the soul which form
these judgments through the mediumship of vision. This cannot be accomplished before it becomes more apparent, in the case of examples from different kinds of visibles, how they are
determined.