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KANSAS CITY, MO PUBLIC LIBRARY

RADIATION, LIGF;: AND
ILLUMINATION
A SERIES OF ENGINEERING LECTURES DELIVERED AT UNION COLLEGE
BY
CHARLES PROTEUS STEINMETZ, A.M., PH.D.
COMPILED AND EDITED BY
JOSEPH LaROY HAYDEN
THIRD EDITION
McGRAW-HILL BOOK COMPANY, INC.
239 WEST 39TH STREET. NEW YORK
LONDON: HILL PUBLISHING CO., LTD.
6 & 8 BOUVERIE ST.. E. C.
1918

COPYRIGHT, 1909, 1918, BY THE
McGRAW-HILL BOOK COMPANY, INC.

AUTHOR'S PREFACE.
THE following lectures were given as a course of instruction to
the senior students in electrical engineering at Union University. They are however intended not merely as a text-book of
illuminating engineering, nor as a text-book on the physics of light and radiation, but rather as an exposition, to some extent, from the engineering point of view, of that knowledge of light
and radiation which every educated man should possess, the
engineer as well as the physician or the user of light. For this purpose they are given in such form as to require no special knowledge of mathematics or of engineering, but mathematical formalism has been avoided and the phenomena have been de-
X scribed in plain language, with the exception of Lectures and
XI, which by their nature are somewhat mathematical, and are intended more particularly for the illuminating engineer, but
which the general reader may safely omit or merely peruse the
text.
The lectures have been revised to date before publication, and the important results of the work of the National Bureau of
Standards, contained in its recent bulletins, fully utilized.
CHARLES PROTEUS STEINMETZ.
SOHBNBCTADY, September, 1909.

COMPILER'S PREFACE.

A SERIES of eight experimental lectures on "Light and Radia-

tion" were delivered by Dr. Steinmetz in the winter of 1907-8

before the Brooklyn Polytechnic Institute. Unfortunately no

stenographer was present and no manuscript prepared by the

A lecturer.

far more extended course of experimental lectures

was however given by Dr. Steinmetz at Union University in the

winter of 1908-9, on "Radiation, Light, Illumination and Illu-

" minating Engineering,

and

has

been

compiled

and

edited

in

the following.
Two additional lectures have been added thereto by Dr. Stein-

metz to make the treatment of the subject complete even from
X the theoretical side of illuminating engineering: Lecture on
"Light Flux and Distribution" and Lecture XI on "Light

Intensity and Illumination." These two lectures give the

element^ of the mathematical theory of illuminating engineering,

With the exception of the latter two lectures the following

book contains practically no mathematics, but discusses the

subjects in plain and generally understood language. The subject matter of Lecture XII on "Illumination anc

Illuminating Engineering" has been given in a paper before th<

Illuminating Engineering Society; the other lectures are ne^

in their form and, as I believe, to a considerable extent also ir

their contents.

In describing the experiments, numerical and dimensiona data on the apparatus have been given, and the illustration! drawn to scale, as far as possible, so as to make the repetitioi of the experiments convenient for the reader or lecturer.
Great thanks are due to the technical staff of the McGraw-Hil
Book Company, which has spared no effort to produce the boo! in as perfect a manner as possible.

JOSEPH L. R. HAYDEN.

SCHENECTADY, September, 1909.

CONTENTS.

I. NATURE AND DIFFERENT FORMS OF RADIATION.

1. Radiation as energy.

1

2. Measurement of the velocity of light.

2

3. Nature of light.

4

4. Difference of wave length with differences of color. Meas-

urement of wave length and of frequency. Iridescence.

The ether.

6

5. Polarization proving light a transversal vibration. Double

refraction.

7

6. The visible octave of radiation. Ultra-red and ultra-violet

radiation.

9

7. The electric waves.

15

8. The spectrum of radiation covering 60 octaves.

16

VECTURE II. RELATION OF BODIES TO RADIATION.

9. Electric waves of single frequency, light waves of mixed

frequency.

20

10. Resolving mixed waves into spectrum. Refraction.

21

11. Relation of refractive index to permeability and dielectric

constant.

24

12. Spectrum.

25

13. Continuous spectrum. Line spectrum. Band spectrum.

Combination spectra.

26

14. Reflection, absorption and transmission.

29

15. Conversion of absorbed radiation into heat and light.

30

16. Transmitted light.

31

17. Opaque colors and transparent colors.

32

18. Objective color and subjective color.

33

19. Effect of excess and of deficiency of certain wave length

of the illuminant on the opaque and the transparent

colors.

34

vii

viii

CONTENTS.

LECTURE III. PHYSIOLOGICAL EFFECTS OF RADIATION.

PAGE

Visibility.

20. The eye.

37

21. Dependence of sensitivity of the eye on the color. Mechan-

ical equivalent of light. Comparison of intensities of

different colors.

40

22. Sensitivity curves of eye for different intensities.

43

23. Change of shape of sensitivity curve with intensity.

45

24. Harmful effect of excessive radiation power.

48

25. Protective action of eye.

50

26. Specific high frequency effect beginning in blue.

51

27. Perception of ultra-violet light. Harmful effects of ultra-

violet.

52

28. Arcs as producers of ultra-violet rays.

55

Pathological and Therapeutic Effects of Radiation.

29. Power effect and specific high frequency effect.

57

30. Light as germicide and disinfectant.

59

LECTURE IV. CHEMICAL AND PHYSICAL EFFECTS OF RADIATION.

Chemical Effects.

31. Indirect chemical action by energy of radiation. , Direct

chemical action.

03

32. Chemical action of rod and yellow rays in supplying the

energy of plant life. Destructive notion of high frequency

on plant life.

64

Physical Effects.

33. Fluorescence and phosphorescence.

66

LECTUKE V. TEMPKHATUKK RAWATION.

34. Production of radiation by boat,

70

35. Increase of intensity and frequency with temperature.

73

36. Efficiency and temperature,

70

37. Carbon incandescent lamp.

78

38. Evaporation below boiling point. Allotropic modifications

of carbon.

HI

39. Normal temperature radiation,

M

40. Colored body radiation.

85

41. Measurement of temperatures by radiation,

89

42. Colored radiation and heat luminescence.

90

CONTENTS.

Ix

LECTURE VI. LUMINESCENCE.

PAGE

Fluorescence and Phorphorescenee.

43. Radioluminescence. Electroluminescence. Thermolumi-

nescence. Physical phosphorescence. Chemical phos-

phorescence. Biological phosphorescence.

94

44. Pyroluminescence. Chemical luminescence.

96

45. Electroluminescence of gases and vapors.

98

Disruptive Conduction.
46. Geissler tube and spark. Disruptive voltage. 47. Change from spark to Geissler glow.

"
101
105

Continuous Conduction.

48. Nature of continuous or arc conduction. 49. Distinction between arc and spark discharge. 50. Continuity at negative. 51. Rectification of alternating voltages by arcs, 52. Efficiency and color. 53. Most efficient light producer. 54. Electro-conduction from negative, long life, non-consuming
positive, limitation in the available materials.
55. Arc most efficient method of light production.

106 Ill 113 117 122 123
125 126

LECTURE VII. FLAMES AS ILLUMINANTS.

56. Hydrocarbon flames.

128

57. Effect of rapidity of combustion and of flame shape on

smokiness.

130

58. Effect of oxygen atom in the hydrocarbon molecule on

luminosity.

132

59. Mixture of hydrocarbon with air.

133

60. Chemical luminescence.

134

61. Flames with separate radiator.

135

LECTURE VIII. ARC LAMPS AND ARC LIGHTING.
Volt-Ampere Characteristics of the Arc, 62. Arc length and voltage. 63. General equations of the arc.
Stability Curves oj the Arc. 64. Instability on constant voltage. 65. Equations of the vapor arc.
Arc Length and Efficiency.
66. Maximum efficiency length of carbon arc, 67. Maximum efficiency length of luminous arc.

137 140
142 145
146 148

X

CONTENTS.

LECTURE VIII. ARC LAMPS AND ARC LIGHTING (Continued).
Arc Lamps.
68. The elements of the arc lamp.
69. Differential arc lamp. 70. Series arc lamp.
71. Luminous arc lamp.

PAGE
151 153 157 160

Arc Circuits.

72. Constant potential and constant current. The mercury

arc rectifier system. The arc machine.

160

73. The constant current transformer. The constant current

reactance.

163

LECTURE IX. MEASUREMENT OF LIGHT AND RADIATION.

74. Measurement of radiation as power.

360

75. Light a physiological quantity.

107

76. Physiological feature involved in all photometric methods. 109

77. Zero method photometers.

170

78. Comparison of lights,

.172

79. Flicker photometer.

178

80. The luminometer.

175

81. Primary standards of light.

177

82. Proposed primary standards.

178

83. Illumination and total flux of light. Incandescent lamp

photometry.

179

84. Arc lamp photometry.

182

85. Discussion. Mean spherical, horizontal, downwards, maxi-

mum, hemispherical candle power.

184

LECTURE X. LIGHT FLUX AND DISTRIBUTION.

86. Light flux, light flux density, light intensity.

180

87. Symmetrical and approximately symmetrical distribution. 187

cum 88. Calculation of light flux from meridian

of Asymmetri-

cal radiator.

188

Distribution Curves of Radiation.
89. Calculation of distribution curves. Point or sphere of uniform brilliancy.
90. Straight line or cylindrical radiator, 91. Circular line or cylinder.
92. Single loop filament incandescent lamp as illustration*

190
tOfi
197 200

CONTENTS.

xi

LECTURE X. LIGHT FLUX AND DISTRIBUTION (Continued).

PAGE

Shadows.

93. Circular shade opposite and symmetrical to circular radia-
tor.
94. Calculation of the meridian curves of a circular radiator, for different sizes of a symmetrical circular shade, and for different distances of it.
95. Circular shade concentric with end of linear radiator.

202
206 210

Reflection.
96. Irregular reflection. 97. Regular reflection.
98. Reflector with regular and irregular reflection.

212 215 218

Diffraction, Diffusion and Refraction.

99. Purpose of reducing tho brilliancy of the illuminant. 100. Effect of the shape of the diffusing globe on the distribu-
tion curve.
101. Prismatic refraction and reflection.

221
223 224

LECTURE XI. LIGHT INTENSITY AND ILLUMINATION.

Intensity Curves for Uniform Illumination.

102. Calculation of intensity distribution of illuminant for
uniform total, horizontal and vertical illumination. .103. Uniform illumination of limited area.

226 229

Street Illumination by Arcs,
104. DincuHsiou of problem.
105. Combined effect of successive lamps.

234 238

Room Illumination by Incandescent Lamps.

106. Distribution curve of lamp. Calculation of resultant total intensity of direct light.
107. Reflection from walls and ceiling. 108. Total directed and diffused illumination.

242 246 251

Horizontal Table Illumination by Incandescent Lamps.

109. Location of lamps,

253

xii

CONTENTS.

LECTURE XIL ILLUMINATION AND ILLUMINATING ENGINEERING,

110. Physical and physiological considerations.

111. Light flux density. Illumination. Brilliancy.
112. Physical problems. Ceilings and walls. Reflectors, diffus-

ing globes, diffracting shades, etc.

t

113. Objective illumination. Subjective illumination. Con-

traction of pupil. Intrinsic brilliancy. Direct and in-

direct lighting.

114. Fatigue.
115. Differences in intensity and in color. Control of color differences. Shadows and their control. Directed and

256 259 260
201 203

diffused light. 116. Direction of shadows.

205 207

117. Color sensitivity in relation to required intensity of illu-

mination,

20t)

118. Domestic lighting. 119. The twofold problem of domestic lighting: daylight mid
artificial light. 120. Street lighting.
121. Defects of present Htreet lighting.
122. Tower lighting.

270
271 272
27't 27<1

LECTURE XIII. PHWIOMXWM^ PHOUUOMB OF IU,UMINATW<
KNGINHBHING,

123. Physical Hide of illuminating engineering, 1'hysiologieal problems,
32*1. Physiological difleroneo between <Uffuno<l ntul (tireded
light.
125 % IndofmitonoHH of diffused light. HlmdowH cant by dlffuwtl
daylight. Equivalent dilTiiHion nrar light our< 4n of large extent.
wim% 120. Equivalent <Kffuioii by using wwrnl light
127. Unociual diffusion in different <lin*ofionH. (lomplex

277 27K
270 2H1

128, Phymologioftl light <iiHlrilmtion.
12ft. Phyniologicuilly, light not a vector quantity, 130, Resultant effect of several light wmrmm,

2HH
2K<t
287

KADIATION, LIGHT, AND
ILLUMINATION.

LECTURE I.
NATURE AND DIFFERENT FORMS OF RADIATION.

1. Radiation is a form of energy, and, as such, can be produced from other forms of energy and converted into other forms of

energy.

The most convenient form of energy for the production of radiation is heat energy, and radiation when destroyed by being

intercepted by an opaque body, usually is converted into heat. Thus in an incandescent lamp, the heat energy produced by the

electric current in the resistance of the filament, is converted

my into radiation. If I hold

hand near the lamp, the radiation

intercepted by the hand is destroyed, that is, converted into heat,
and is felt as such. On the way from the lamp to the hand, how-

ever, the. energy is not heat but radiation, and a body which is

transparent to the radiation may be interposed between the

lamp and the hand and remains perfectly cold. The terms

"heat

radiation''

and

"radiant

7
heat/

which

are

occasionally

used, therefore are wrong: the so-called radiant heat is not heat

but radiation energy, and becomes heat only when, intercepted

by an opaque body, it ceases to be radiation; the same, however,

applies to any radiation. If we do not feel the radiation of a

mercury lamp or that of the moon as heat, while we feel that of a

coal fire, it is merely because the total energy of the latter is very
much greater; a sufficiently sensitive heat-measuring instrument,

as a bolometer, shows the heat produced by the interception of

the rays of the mercury lamp or the rays of the moon.

The most conspicuous form of radiation is light, and, therefore,

it was in connection with this form that the laws of radiation

were first studied.

1

2

RADIATION, LIGHT, AND ILLUMINATION.

2. The first calculations of the velocity of light were made by

A astronomers in the middle of the eighteenth century, from the

observations of the eclipses of the moons of Jupiter.

number

of moons revolve around the planet Jupiter, some of them so close

that seen from the earth they pass behind Jupiter and so are

eclipsed at every revolution. As the orbits of Jupiter's moons
M were calculated from their observations by the law of gravita-
tion, the time at which the moon should disappear from sight,

'M

x--rx

FIG. 1.

when seen from the earth E, by passing behind Jupiter, 7 (Tig. ]), could be exactly calculated. It was found, however, that sometimes the moon disappeared earlier, sometimes later than calculated, and the difference between earliest and ktost disappearance amounts to about 17 min. It was also found that tho disappearance of the moon behind Jupiter omiirol earlior when the earth was at the same wide of the sun as Jupiter, at A, while
the latest disappearance occurred when the (Uirt.li wan on the opposite siclo of the sun from Jupiter, at B. Now, in tho latter case, the earth is further distant from Jupilor by Uio diameter
ASB of the orbit of the earth around tho sun tf, or by about
195,000,000 miles and tho delay of 17} min. thus must bo due to the time taken by the light to travenso tho additional distance of 195,000,000 miles. Seventeen and one-third min. nre 10>10 sec. and 195,000,000 mikvs in 3 (MO sec. thus givas a velocity of

~ .. , ,195,000,000

light of

i

or

, 00 AAA 188,000

mil-ios

per sec.

IIMU

Later, the velocity of light was measured directly in a number

D of different ways. For instance, let, in Fig. 2, be a clwk per-

A forated with holes at its periphery,

lamp L wends its light

H through a hole

in tho dink to a mirror Af located at a con-

siderable distance, for instance 5 miles; there the light is reflected

NATURE AND DIFFERENT FORMS OF RADIATION. 3
H and the mirror is adjusted so that the reflected beam of light
passes through another hole i of the disk into the telescope T. If the disk is turned half the pitch of the holes the light is blotted out as a tooth stands in front of both the lamp and the telescope. Again turning the disk half the pitch of the holes in the same

I
U-=--r rznnii
f

5j*ILJ?

FIG. 2.

direction the light reappears. If the disk is slowly revolved, alter-
nate light and darkness will be observed, but when the speed increases so that more than from 10 to 20 holes pass per second, the

eye is no longer able to distinguish the in-lividual flashes of light

but sees a steady and uniform light; then increasing the speed

still more the light grows fainter and finally entirely disappears.
H This means when a hole Q is in front of the lamp, a beam of

light passes through the hole. During the time taken by the light
D to travel the 10 miles to the mirror and back, the disk has

moved, and the hole ffv which was in front of the telescope

H when the light from the lamp passed through the hole

has
Q,

moved away, and a tooth is now in front of the telescope and

intercepts the light. Therefore, at the speed at which the light

disappears, the time it takes the disk to move half the pitch of a

hole is equal to the time it takes the light to travel 10 miles.

Increasing still further the velocity of the disk D, the light
appears ag dn, and increases in brilliancy, reaching a maximum

at twice the speed at which it had disappeared. Then the light

reflected from the mirror At again passes through the center of
H a hole into the telescope, but not through the same hole l

through which it would have passed with the disk stationary, but

through

the

next

hole

f/ 2,

that

IB,

the

disk

has

moved

a

distance

equal to the pitch of one hole while the light traveled 10 miles.
D Assume; for instance, that the disk has 200 holes and makes

4

RADIATION, LIGHT, AND ILLUMINATION.

94 rev. per sec.
full brilliancy

at the moment when the light has again reached
In this case, 200 X 94 = 18,800 holes pass the

telescope per second, and the time of motion by the pitch of one

hole is

Sec., and as this is the time required by the light

18 800

;

^ to travel 10 miles, this gives the velocity of light as 10 -*- -^

or 188,000 miles per sec.
The velocity of light in air, or rather in empty space, thus is
188,000 miles or 3 X 1010 cm. per sec.
For electrical radiation, the velocity has been measured by
Herz, and found to be the same as the velocity of light, and there is very good evidence that all radiations travel with the same velocity through space (except perhaps the rays of radioactive

substances).

3. Regarding the nature of radiation, two theories have been

proposed. Newton suggested that light rays consisted of extremely minute material particles thrown off by the lightgiving bodies with enormous velocities, that is, a kind of bom-

bardment. This theory has been revived in recent years to ex-

plain the radiations of radium, etc. Euler and others explained
the light as a wave motion. Which of these explanations Ls correct can be experimentally decided in the following manner:

Assuming light to be a bombardment of minute particles, if wo

combine two rays of light in the same path they must add to

each other, that is, two equal beams of light together give a beam

of twice the amplitude. If, however, wo assume light is a wave

motion, then two equal beams of light add to one of twice the

amplitude

only

in

case

the

waves

are

in

phase,

as

A l

and

/^

in

A Fig. 3 add to (7r

If, however, the two beams

and 7i are not

2

2

C in phase, their resultant

is
2

less

than their

sum,

and

if

the

two

beams

A 3

and

B3 in

Fig.

3 happen to

be

iu

opposition

(180 degrees apart), that is, one-half wave length out of phase

with each other, their resultant Ls zero, that is, they blot each

other out.
A Assuming now we take a plain glass plate (Fig. 4) and a
slightly curved plate 5, touching each other at C, and illuminate
them by a beam of uniform light as the yellow light given by coloring the flame of a bunsen burner with somo sodium Halt
a part of the light 6, is then reflected from the lower surface of

NATURE AND DIFFERENT FORMS OF RADIATION. 6

the curved glass plate B, a part c, passes out of it, and is reflected

A from the upper surface of the plain glass plate A.

beam of

\ Ba

7

\

FIG. 3.

reflected light a, thus is a combination of a beam 6 and a beam c.

The two beams of light which combine to a single one, a, differ

from each other in phase by twice the distance between the two

glass plates.

At

those

points

d i9

d 2,

etc.

at

which

the

distance

4,
between the two glass plates is } wave length, or , f, etc., the two component beams of a would differ by i, f , |, etc. wave lengths, and thus would blot each other out, producing darkness,

6

RADIATION, LIGHT, AND ILLUMINATION.

while at those points where the distance between the glass plates is |, 1, Ij, etc. wave lengths, and the two component beams a thus differ in phase by a full wave or a multiple thereof, they would add. If, therefore, light is a wave motion, such a structure
would show the contact point C of the plates surrounded by
alternate dark rings, d, and bright rings, y. This is actually the case, and therefore this phenomenon, called "interference" proves light to be a wave motion, and has lead to the universal acceptance of the Eulerian theory.
Measuring the curvature of the plate 5, and the diameter
B A of the dark rings d, the distance between the plates and at
the dark rings d, can be calculated and as this distance is onequarter wave length, or an odd multiple thereof, the wave length can be determined therefrom.
The wave length of light can be measured with extremely high accuracy and has been proposed as the absolute standard of length, instead of the meter, which was intended to be 10~7 of
the quadrant of the earth. 4. It is found, however, that the different colors of light have
different wave lengths; red light has the greatest wave length, and then in the following order: red, orange, yellow, green, blue, indigo, violet, the wave length decreases, violet light having the shortest wave length.
If in experiment (Fig. 4) instead of uniform light (monochromatic light), ordinary white light is used, which is a mixture of all colors, the dark and bright rings of the different colons appear at different distances from each other, those of the violet nearest and those of the red the furthest apart, and HO superimpose upon each other, and instead of alternately black and light ringH, colored rings appear, so-called interference rings. Wherever a
thin film of air or anything else of unequal thickness in interposod between two other materials, such interference colors tlmn appear. They show, for instance, between sheets of mica, etc. The colors of soap bubbles arc thus produced.
The production of such colors by the interference of ray of light differing from each other by a fractional wave length i
called iridescence.
Iridescent colors, for instance, are those of mother-of-pearl,
Df opal, of many butterflies, etc. Light, therefore, Is a wave motion*

NATURE AND DIFFERENT FORMS OF RADIATION. 7

The frequency of radiation follows from the velocity of light,

and the wave length.

The average wave length of visible radiation, or light, is about
lw = 60 microcentimeters,* that is, 60 X 10~6 cm. (or about = innyffe in.) and since the speed is S 3 X 10 10 cm. the frequency
o

= = X is / r- 500

10 12 ,

or 500

millions

of millions

of

cycles per

LW

second, that is, inconceivably high compared with the frequencies
with which we are familiar in alternating currents. If, as proven, light is a wave motion, there must be some thing
which is moving, a medium, and from the nature of the wave

motion, its extremely high velocity, follow the properties of this
medium: it has an extremely high elasticity and extremely low density, and it must penetrate all substances since no vacuum can

be produced for this medium, because light passes through any vacuum. Hence it cannot be any known gas, but must be essentially different, and has been called the "ether."
Whether the ether is a form of matter or not depends upon

the definition of matter. If matter is defined as the (hypotheti-
cal) carrier of energy (and all the information we have of matter

is that it is the seat of energy), then the ether is matter, as it is a

carrier of energy: the energy of radiation, during the time be-
tween the moment when the wave leaves the radiator and the

moment when it strikes a body and is absorbed, resides in the

ether.

5.

If
,

light

is

a

wave

motion

or

vibration,

it

may

be

a

longitudi-

nal vibration, or a transversal vibration. Either the particles of

the medium which transmit the vibrations may move in the

direction in which the wave travels, as is the case with sound

B waves in air. If in Fig. 5 sound waves travel from the bell in

BA m the direction

the air molecules
t

vibrate in the same direction,

A to B. Or the vibration may be transversal ; that is, if the beam

* As measures of the wave length of light, a number of metric units have survived and are liable to load to confusion:
The micron, denoted by a, equal to one thousandth of a millimeter. The ftp, equal to one millionth of a millimeter. The Angstrom unit, equal to one ten-millionth of a millimeter. As seen, the basis of those units is the millimeter, which was temporarily used as a standard unit of length before the establishment of the present absolute system of units, the (C,G,S), which is based on centimeter length, gram mass, and second time measure.
A radiation of the wave length of 60 microcentimeters thus can be expressed
also as: 6000 Angstrom units, or 0.6 /x, or 600 pp.

8

RADIATION, LIGHT, AND ILLUMINATION.

of light moves in Fig. 6 perpendicularly to the plane of the paper,
the vibrating particles move in any one of the directions oa, ob,
etc. in the plane of the paper, and thus perpendicular to the ray

FIG. 5.

of light. In the former case (a longitudinal vibration, as sound) there obviously can be no difference between the directions at

right angles to the motion of the wave. In a transversal vibra-
tion, however, the particles may move either irregularly in any

of the infinite number of directions at right angles to thc^ray

(Fig. 6) and thus no difference exists in the different directions

perpendicular

to

the

beam ;

or

they

may

vibrate

in

one

direction

only* ,'

as

the

direction

boa

wave (Fig. 7) . 11 i called

In "//

the latter case,

-i

i j)

polarized"

andi

the
i, _ _
has

is
.li-cr
differ-

ent characteristics in three direc-

tions at right angles to each other: one direction is the direction of

FIG. 6,

FIG. 7.

propagation, or of wave travel; the

^ second is the direction of vibration ;

and the tjlir(j is

direction per-

pendicular to progression and to vibration. For instance, the electric field of a conductor carrying alternating current is a
polarized wave : the direction parallel to the conductor is the
direction of energy flow; the direction concentric to the conductor is the direction of the electromagnetic component, and
the direction radial to the conductor is the direction of the

electrostatic component of the electric field.
Therefore, if light rays can be polarized, that Is, made to ex-
hibit different properties in two directions at right angles to each other and to the direction of wave travel, thin would prove tfee

light wave to be a transversal vibration* This is actually the case. For instance, ,if a beam of light is reflected a number of times under a fairly sharp angle, as shown in Fig. 8, this beam becomes
m polarized; that is, for instance, the reflection from the mirror o; m m set like the mirrors v 2 * , . which produced the polarization,

NATURE AND DIFFERENT FORMS OF RADIATION. 9

is greater, and the absorption less than from a mirror set at right

angles thereto, as w/.

Some crystals, as Iceland spar (calcium carbonate) , show

"double refraction/' that is, dissolve a beam of light, a, enter-

ing them, into two separate beams, b and c (Fig. 9) which are

polarized at right angles to each other.

K In a second crystal, 2J beam & would then enter as a single

K K beam, under the same angle as in the first crystal v if 2 were

K^ K in the same position as

while if z were turned at right angles

K to v beam 6 would enter I 2 under the same angle as beam c in

K crystal r

6. As seen, light and radiation in general are transversal wave

motions of very high speed, S 3 X 10 10 cm. per sec. in a hypo-

thetical medium, ether, which must be assumed to fill all space

and penetrate all substances.

"Radiation is visible, as light, in a narrow range of frequencies

only: between 400 X 1012 and 770 X 1012 cycles per sec. cor-

responding to wave lengths

from

76

X

6
10~"

cm.

to

39

X

10~ c cm.*

All other radiations are invisible and thus have to be observed by

other means.

I have here a pair of rods of cast silicon (10 in. long, 0.22 in. in diameter, having a resistance of about 10 ohms each), connected

X X * The visibility of radiation IB greatest between the wave lengths 50 X 10"* to 60 lO""8 and good between the wave lengths 41 X 10"8 to 76 10""%
but extends more or loss indistinctly ovor the range of wave lengths from
X X X 33 10~ to 77 10"* and faintly even as far as 30 X 10~8 to 100 NT*.

10

RADIATION, LIGHT, AND ILLUMINATION.

in series with each other and with a rheostat of about 40 ohms
resistance in a 120-volt circuit. When I establish a current
through the rods, electric energy is converted into heat by the resistance of the rods. This heat energy is converted into and
sent out as radiation, with the exception of the part carried off
by heat conduction and convection. Reducing the resistance, I increase the heat, and thereby the radiation from the silicon rods.
Still nothing is visible even in the dark; these radiations are of
too low frequency, or great wave length, to be visible. By holding my hand near the rods, I can feel the energy as heat, and show
it to you by bringing the rods near to this Crookes' radiometer,

FIG. 9.

which is an instrument showing the energy of radiation. It con-

sists (Fig. 10) of four aluminum vanes, mounted in a moderately high vacuum so that they can move very easily. One side of each vane is polished, the other blackened. The waves of radiation
are reflected on the polished Hide of the vane; on the blackened

side they are absorbed, produce heat, thus raise the temperature of

the

air

near

the vane ;

the

air

expands and

pushes

the vanes

ahead,

that is, rotates the wheel As you see, when I bring the heated rods near the radiometer, the wheel spins around at a rapid rate

by the radiation from the rods, which to the eye are invisible*

NATURE AND DIFFERENT FORMS OF RADIATION. II

Increasing still further the energy input into the silicon rods, and thereby their temperature, the intensity of radiation increases, but at the same time radiations of higher and higher frequencies appear, and ultimately the rods become visible in the dark, giving a dark red light; that is, of all the radiations sent out by the rods, a small part is of sufficiently high frequency to be visible. Still further increasing the temperature, the total radiation increases, but the waves of high frequency increase more rapidly than those of
lower frequency; that is, the average

frequency of radiation increases or
the average wave length decreases and higher and higher frequencies

appear, orange rays, yellow, green, blue, violet, and the color of the

light thus gradually changes to
bright red, orange, yellow. Now I
change over from the silicon rods
which are near the maximum tem-

perature they can stand to a tungsten lamp (a 40-watt 110-volt lamp, connected in series with a rheostat

of 2000 ohms resistance in a 240-

volt circuit)* For comparison I also turn on an ordinary 16 c. p. carbon

filament incandescent lamp, running at normal voltage and giving its usual yellow light. Gradually turn-

ing out the resistance, the light of

the tungsten lamp changes from orange to yellow, yellowish white

FIG. 10.

and ultimately, with all the resistance cut out and the fila-

ment running at more than double voltage, is practically white;

that is, gives a radiation containing all the frequencies of visible light in nearly the same proportion as exist in sunlight. If
wo should go still further and very greatly increase the temperature, because of the more rapid increase of the higher fre-
quencies (violet, blue, green) than the lower frequencies of light (red, orange and yellow) with increase in temperature, the light

12

RADIATION, LIGHT, AND ILLUMINATION.

should become bluish. However, we are close to the limit of
temperature which even tungsten can stand, and to show you light of high frequency or short wave length I use a different apparatus in which a more direct conversion of electric energy into radiation takes place, the mercury arc lamp. Here the
light is bluish green, containing only the highest frequencies of visible radiation, violet, blue and green, but practically none
of the lower frequencies of visible radiation, red or orange.

A\

SJJJLQJU

....240-VOLT8-00 CYCLES

FIG. 11.

In the tungsten lamp at high brilliancy and more still in the

mercury arc, radiations of higher frequencies appear, that in,

shorter wave lengths than visible light, and these radiations arc

again invisible. As they arc of frequencies boyomi the violet
rays of light, they arc called " ultra-violet rays/' while tho radia-

tions which we produced from tho heated silicon rods at moderate

temperatures were invisible because of too low frequency and

are

thus

called

" ultra-red

rays/'

or "infra-red

rays/'

as

they

are

outside of and below the red end of the range of visible radiation.

To produce powerful ultra-violet rays, I use a condenser clis**

charge between iron terminals, a so-called ultra-violet arc lamp.

Three iron spheres, / in Fig. 11, of about f in. diameter, are

mounted on an insulator B. The middle sphere is fixed, the

NATURE AND DIFFERENT FORMS OF RADIATION. 13

outer

ones

adjustable

and

set

for

about

3
TF

in.

gap.

This lamp is

connected across a high voltage 0.2-mf. mica condenser C, which

is connected to the high voltage terminal of a small'step-up trans-
former T giving about 15,000 volts (200 watts, 110 + 13,200
volts). The low tension side of the transformer is connected to
R the 240-volt 60-cycle circuit through a rheostat to limit the

current. The transformer charges the condenser, and when the

voltage of the condenser has risen sufficiently high it discharges

through the spark gaps I by an oscillation of high frequency

(about 500,000 cycles), then charges again from the transformer,

discharges through the gap, etc. As several such condenser dis-

charges occur during each half wave of alternating supply voltage

the light given by the discharge appears continuous.
You see, however, that this iron arc gives apparently very little

light; most of the radiation is ultra-violet, that is, invisible to the
eye. To make it visible, we use what may be called a frequency

converter of radiation. I have here a lump of willemite (native

zinc silicate), a dull greenish gray looking stone. I put it under

the iron arc and it flashes up in a bright green glare by convert-

ing the higher frequency of ultra-violet rays into the lower

frequency of green light. This green light is not given by the

iron arc, as a piece of white paper held under the arc shows only

the faint illumination given by the small amount of visible radia-

tion. I now move a thin sheet of glass, or of mica, between the

iron arc and the lump of willemite, and you see the green light

disappear as far as the glass casts a shadow. Thus glass or mica,

while transparent to visible light, is opaque for the ultra-violet

A light of the iron arc.

thick piece of crystallized gypsum (sel-

onitc) put in the path of the ultra-violet light does not stop it,
hence is transparent, as the lump of willemite continues to show

the green light, or a piece of cast glaSxS its blue light.

I have here some pieces of willemite in a glass test tube. They

appear dull and colorless in the ultra-violet light, as the glass is

opaque for this light. I shift them over into a test tube of fused

quartz, and you sec them shine in the green glare. Quartz is trans-
parent to ultra-violet light. When investigating ultra-violet

light, quartz lenwes and prisms must, therefore, be used.

Still higher frequencies of ultra-violet light than those given

by a condenser discharge between iron terminals are produced by a low temperature mercury arc. Obviously this arc must not be

14

RADIATION, LIGHT, AND ILLUMINATION.

operated in a glass tube but in a quartz tube, as glass is opaque

for these rays.

These ultra-violet radiations carry us up to frequencies of about

KT 3000 X 10 12 cycles per sec., or to wave lengths of about 10 X

8

cm. Then, however, follows a wide gap, between the highest

frequencies of ultra-violet radiation and the frequencies of X-rays.
In this gap, radiations of very interesting properties may some-

times be found.
At the extreme end of the scale we find the X-rays and the

radiations of radio-active substances if indeed these radiations

are wave motions, which has been questioned. Since at these extremely high frequencies reflection and refraction cease, but irregular dispersion occurs, the usual methods of measuring wave lengths and frequencies fail. The X-rays apparently cover quite a range of frequency and by using the atoms of a crystal as diffraction grating, their average wave length has been measured as
0.1 X 10~6 cm., giving a frequency of 0.3 X 1018 cycles per sec.
In comparing vibrations of greatly differing frequencies, the
most convenient measure is the octave, that is, the frequency scale

of acoustics. One octave represents a doubling of the frequency ;

n octaves higher then means a frequency 2n times as high, n

octaves

lower,

a

frequency

n (|)

as

high.

By this .scale all the inter-

vals are of the same character; one octave means the amo

relative increase, which ever may be the absolute frequency or

wave length. As the perceptions of our senses vary in proportion to the pcr-
centual change of the physical quantity causing the perception (Fechner's law), in the acoustic or logarithmic scale the steps are
thus proportional to the change of sensual perception caused by

them.

The visible radiation covers somewhat leas than one octave;

ultra-violet radiations have boon observed beyond this for about

two more octaves. Nine octaves higher is the estimate* I frequency

of X-rays.
On the other side of the visible range, towards lower frequencies
or longer waves, ultra-red rays, observations have been extended over more than eight octaves up to wave lengths as great as 0*03
cm. length, or frequencies of only 10 12 cycles per sec. The ultrared rays given by the heated silicon rods of our experiment do not
extend to such low frequencies, but such very low frequencies

NATURE AND DIFFERENT FORMS OF RADIATION. 15
have been observed in the radiations of bodies of very low temperature, as liquid air, or in the moon's rays.
7. Very much longer waves, however, are the electric waves. They are used in wireless telegraphy, etc. I here connect (Fig. 12)

jmq

Iju&J
X

FIG. 12.

the condenser C of the apparatus which I used for operating the

G ultra-violet arc, to a spark gap v of which the one side is con-

B nected to ground v the other side to a vertical aluminum rod A v

about 8 feet long. The charge and discharge of the aluminum

A rod l by the oscillating condenser current, send out an electric,

wave of about 50 feet length. This wave passes through you, and

when

striking

the

aluminum

rod

A 2

back

of

you,

induces

therein

B an electric charge.

A 2

is

separated

from

ground

2 by a narrow

spark gap C?2 between graphite terminals, and the arrival of the

electric

wave

at

A a

causes

a

small

spark

to

jump

across

the

gap

6 which closes the circuit of the tungsten lamp L, thereby 2,

lighting it as Jong as the wave train continues.

16

RADIATION, LIGHT, AND ILLUMINATION.

The electric waves used in wireless telegraphy range in wave

lengths from 100 feet or less to 10,000 feet or more, corresponding

to 10 7 to 10 5 cycles per sec. or less.
Still very much longer waves are the fields of alternating cur-
rent circuits: the magnetic and electrostatic field of an alterna-

ting current progresses as a wave of radiation from the conductor,

But as the wave length is very great, due to the low frequency,

^ _ X 3

10 1Q

^ a

60-cycle

alternating

current

gives

a

wave

i
length

ott

X 500

10 6 cm. or 3100 miles

the distance to which the field of

the circuit extends is an insignificant fraction only of the wave

length, and the wave propagation of the field thus is usually not

considered.

Electric waves of higher frequencies than used in wireless

telegraphy are the Herzian waves, produced by electric oscilla-

tors, "that is, a moderately long straight conductor cut in the

middle

by

a

gap

and

terminated

by

spherical

as condensers^

shown in Fig. 13. On these waves the velocity of propagation

o

< BNERGY-SU PPUY" - *
-o o
FIG. 13,

o

has been measured by Herz by producing standing waves by

combination of main wave and reflected wave.

Still much higher frequoncieH arc the oscillations between the

cylinders of multi-gap lightning arresters, and the limit of fre-

quency of electric waves would probably bo given by tho oscilla-

ting discharge of two small separated by a narrow gap.

spheres against each other when
It probably is at about 5 X 10 l

cycles, or 0.0 cm. wave length. The blank space between the shortest electric wave and the
longest ultnwecl light wave thun has become fairly narrow

from 0.0 to 0.03 cm., or only about four octavos

8. In tho following tables, tho different known forms of radia-

tion are arranged by their frequency and wave length, and are

given also in octaves, choosing as aero point the middle c of the

piano, or a frequency of 128 cycles per sec.

NATURE AND DIFFERENT FORMS OF RADIATION. 17

SPECTRUM OP RADIATION.

Zero point chosen at
Speed of radiation S

c

= =

128
3X

cycles per lo10 cm.

second.

These radiations are plotted graphically in Fig. 14, with the octave as abscissa).
As seen, the total range of frequencies of radiation is enormous, covering nearly GO octaves, while the range of sound waves is only about nine octaves, from 15 to 8000 cycles.
There are two blank spaces in the range of radiation, one between electric and light waves, and a second and longer one between light and X-rays.
It is interesting to note that the range of electric waves is far greater than that of light waves.
Only a very narrow range of radiation, less than one octave out of a total of 60, is visible. It is shown shaded in Fig. 14. This

18

RADIATION, LIGHT, AND ILLUMINATION.

exhibits the great difficulty of the problem of efficient light production: it means producing as large a part of the total radiation as possible within this very narrow range of visibility.
Regarding the range of frequencies covered by it, the eye thus is much less sensitive than the ear, which hears over ten octaves
as sound waves. While the visible radiations are the most important ones, as
light, the total range of radiation is of interest to the electrical
engineer.
The ultra-red rays are those radiations which we try to avoid is far as possible when producing light, as they consume power

SOUND WAVES

FIG, 14.

and so lower the efficiency; the ultra-violet rays are of importance in medicine as germ killers. They are more or loss destructive to life, appear together with the visible radiation, and where they are of appreciable amount, as in the arc, protection against them becomes desirable. The X-rays have become of importance in medicine, etc., as they penetrate otherwise opaque bodies and thus
allow seeing things inside of other bodies.
The total range of electric waves, between tho frequencies of alternating currents and the limits of electric waves, has been of importance to the electrical engineer a# harmful and destructive phenomena in electric circuits, which are to be guarded against, and only in recent years, with tho development of wireless telegraphy, some such electric waves have found a useful commercial application. The main object of their study which IB
the study of transient electric phenomena, is still, however, to guard against their appearance in electric circuits and discharge them harmlessly when they appear.
Considering the great difference which already exists between alternating currents of low frequency; 25 or 15 cycles, and of high

NATURE AND DIFFERENT FORMS OF RADIATION. 19
frequency, 133 cycles, and realizing that the total range of waves,
which may appear in electric circuits, is many hundred times
greater than the difference between high and low frequency alternating currents, it can be realized that the differences in the character of electric waves are enormous between the low frequency surges of near machine frequency and the high frequency oscillations of a multi-gap lightning arrester, near the upper limits of electric wave frequencies, and the problem of protecting circuits against them thus is vastly more difficult than appears at first sight and the conclusions drawn from experimental investigations
of electric waves may be very misleading when applied to waves many octaves different from those used in the experiment. This explains the apparently contradictory evidence of many experi-
mental investigations on the protection of electric circuits.

LECTUEE II.
RELATION OF BODIES TO RADIATION.
9. For convenience, the total range of known radiations can be divided into two classes, the electric waves and the light waves, which are separated from each other by the blank space in the middle of the spectrum of radiation (Fig. 14). Under light waves we here include also the invisible ultra-red radiation and
the ultra-violet radiation and the non-refrangible radiations, as X-rays, etc., separated from the latter by the second blank space of the radiation spectrum.
In the following, mainly the light waves, that is, the second or
high frequency range of radiation, will be discussed. The elec-
tric waves are usually of importance only in their relation to the radiator or oscillator which produces them, or to the receiver on which they impinge, and thus are treated in connection with the radiator or receiver, that is, the electric conductor, in the theory of transient electric phenomena and oscillations.*
The radiation may bo of a single frequency, that is, a single
wave; or a mixture of different frequencies, that is, a mixture
of different and frequently of an infinite number of waves.
Electric radiation usually is of a single frequency, that is, of the
frequency or wave length determined by the constants of the electric circuit which produces the radiation, mainly the induct-
ance L and the capacity C. They may, however, have different
wave shapes, that is, comprise, in addition to the fundamental wave, higher harmonics or multiples thereof, just as the sound waves which represent the same tone with different musical instruments are of the same frequency but of different wave
shapes, that is, contain different higher harmonics.
Light radiations usually are a mixture of a number of waves of different frequencies, and very commonly a mixture of an infinite number of frequencies, as is, for instance, the case with the
* "Theory and Calculation of Transient Electric Phenomena and Oscilla-
tions," 20

RELATION OF BODIES TO RADIATION'.

21

radiation of an incandescent body as a lamp filament, which contains all the frequencies from long ultra-red waves over visible light waves to ultra-violet waves.
In the action of vibrations on our senses there is a characteristic
difference between the perception of sound waves by the ear and that of light waves by the eye: the ear is analytic, that is, can separate the individual waves in a mixture of different 'sound waves, as an accord on the piano, and distinguish the individual components of the mixed sound which reaches the ear. Thus we can hear and distinguish an individual voice amongst a mass of other noises. The eye, however, perceives only the resultant of all the visible radiations which reach it, but cannot separate their components, and very different mixtures of radiations thus make the same impression upon the eye: thus, for instance, numerous mixtures of blue and yellow light appear alike to the eye and the same as green light, that is, appear green, while physically, it is obvious that mixtures of blue and yellow light are essentially different from green light.
It is interesting to imagine how nature would look to us if the
eye were analytic, that is, could separate the different component radiations, and if it could perceive waves over as great a range of frequency as the car, about ten octaves instead of less than one
octave as is now the case. The information given to us by the sense of sight would be infinitely increased, and we would see many differences and changes which now escape us.
10. However, while the eye cannot distinguish the different component radiations but sees only their resultant, the specific effects of the component radiations, as the physiologically harmful action of an ultra-violet component of light, still remain, even if the oye docs not see the components, and in the study of radia-
tion for the purpose- of its engineering use for illumination it is
therefore necessary to analyze the mixed radiation given by a source as a lamp, by resolving it into its component waves.
, This is done by using some feature of the radiation which varies with the frequency. Such is the case with the velocity of
propagation.
The velocity of light in empty space is 3 X 1010 cm. per sec.
It is practically the same in air and other gases. In denser bodicB, however, as water, glass, etc., the velocity of light is less
arid, as will be seen, is different for different frequencies.

22

RADIATION, LIGHT, AND ILLUMINATION.

B Assume then, in Fig. 15, a beam of light striking under an angle the boundary between two media, as air A and water W,
the vibration of the ether particles in the beam of light is at right

angles to the direction of propagation BC, and successively the

D waves

thus

reach

a l

b
lj

a z

b
2

.

.

.

As soon, however, as the back

edge of the beam reaches the boundary at its speed changes

FIG. 15.

W by entering the medium

decreases in the present instances

Let then 8 i

=

speed

of

propagation

in

medium

A,

S 2

npeed of

propagation in medium W. Then, while the center of tho txuim

moves the distance EC, the back edge, in the denser medium,
a

moves only the distance D/ = EC, and the wave front of the

i
back half of the beam thus changes to 01 while that of tho front half of the beam, which is still in the medium A, romahw GC.
Then, while the front edge of the beam movon from G to //, tho
center and the whole back half of the beam moves in tho denser

O[

medium W, only the distance CK ** ^GH, and tho wave front

&i

W of the beam, in the medium

now is EL.
}

That is, due to the

W A difference in velocity in the two media

and

the wave front
,

of the beam, and thereby its direction of propagation, IB changed

RELATION OF BODIES TO RADIATION.

23

when traversing the boundary between the two media, and the

beam BC continues its motion in the direction CM.

Let then = 1 angle of incidence, that is, the angle between

the incident beam BC and the perpendicular CN on the boundary,

= and

a 2

angle of refraction, that is, the angle between the out-

CM going or refracted beam

and the perpendicular CP on the

boundary. It is then:

FDH = a and LED = a

1

2;

hence,

FH - DH sin

and DL - DH sin
t

r

(1)

FH The front edge of the beam moves the distance

in medium

A, while the back edge moves the distance DL in medium W;

'
that is,

+ SI

s 3;

(2)

hence, substituting (1) into (2), gives:

That is, the ratio of the sines of the angle of incidence and the
angle of refraction equals the ratio of the speed of propagation iu the two media, hence the ratio of the sines of these two angles IH constant. This is the law of refraction, and this ratio of sines
A is called the refractive index between the two media and W. As
the refractive index of one medium W, then, is understood its re-
fractive index against empty space or against air :

*

sm c^

$ x

(4)

= X where S is the velocity of light in empty space

3

10 10 and ,

S l

the

velocity

in

the

medium,

of

which

<J t

is

called

the

refractive

index.

M From equation (4) it follows, that, if

is the refractive index

between medium 1 and medium 2, _ 3 3, the refractive index
M between medium 2 and medium 3, ^_3 = _ <S2 3 -* $ - refractive

index of medium 1 and medium -3; that is, the refractive inde:s

between any two media is derived as the ratio of their refractive

indices

against

a

third

medium,

as ;

for

instance,

against

air.

24

RADIATION, LIGHT, AND ILLUMINATION.

11. Incidentally, it is interesting to consider the corresponding relations in electric waves.
In an electric circuit, the speed of propagation of an electric wave is, when neglecting the energy losses in and by the con-
ductor:

S= -,L=>
VLC

(5)

where L is the inductance, C the capacity of the conductor per
unit length (the length measured in the same measure as the
speed S).
The inductance L is proportional to the permeability /*, and C the capacity proportional to the dielectric constant, or specific
capacity K of the medium surrounding the conductor, that is, the medium through which the electric wave propagates; that is,

A
A where is a proportionality constant.
The ratio of the speed of propagation of an electric wave in two media 1 and 2 thus is :

for empty space, /i
hence,

1 and K.
S

1 ; _

(8)

where

S t

is

the

speed

of

propagation

in

tho

mcdiuni

of

constants

/^ and KV

Comparing equation (S) with (4) it follows:

'/V<t --'V;

(9)

that is, the square of tho refractive index 8 equate the product of

permeability p and dielectric constant /c,

Since for most media the permeability /i *

1 7

for

all

except

the magnetic materials

* - <V-

(10)

RELATION OF BODIES TO RADIATION.

25

This relation between the constant of the electric circuit K and

the constant of optics $ was one of the first evidences of the
identity of the medium in which the electric field exists with the medium which carries the light waves. It is, however, only
approximately correct, as the refractive index 8 varies with the

frequency and is derived for the extremely high frequencies of

A light radiation, while K refers to stationary conditions.

better

agreement is thus reached when using as d the refractive index

extrapolated for infinite wave lengths.

12. It is found that the different component frequencies of a
beam of radiation are deflected differently when passing from one medium into another, and the higher frequencies are deflected

FIG. 16.

more than the lower frequencies, thus showing that the velocity

of propagation decreases with an increase of frequency, that is, a decrease of wave length.

This gives a means of resolving a mixed radiation into its com-

ponent waves, that is, into a spectrum, by refraction.
A P B narrow beam of light (Fig. 16) is passed through a prism

of transparent material, and the component frequencies then

appear on the screen A (or are seen by the eye) side by side, the

R V reel

below, the violet

above, in Fig. 16, and the green G

in the middle.

It is obvious that the material of the prism must be transparent

to the radiation; thus, when studying ultra-violet radiation, to which glass is opaque, glass prisms cannot be used, but some
material transparent to ultra-violet light such as a quartz or

fluorite prism must be used.

26

RADIATION, LIGHT, AND ILLUMINATION,

The beam of light also can be resolved into its components by
a diffraction grating, in which case the lower frequencies are deflected more than the higher frequencies; that is, the red more
than the violet.
These two forms, the refracting spectroscope and the diffracting spectroscope, now enable us to resolve a beam of mixed radiation into its components and thus study its spectrum.
13. I show you here a number of typical spectra: (1). The spectra of an incandescent lamp and an alcohol lamp with Welsbach mantel. These are continuous spectra, that is, show all the radiations from red over orange, yellow, green,
blue, indigo to violet, uniformly shading into each other,
(2a). The spectrum of the mercury lamp. This is a line spectrum, that is, shows only a finite number of bright lines on
black background. It contains five bright linos ; greenish yellow, bright green, indigo and two violet, one faint dark green line, aucl

Fia. 17.
a number of very faint red and orange lines, of which throo arc
indicated dotted in Fig. 17.
(25). The spectrum of an arc between titanium carbide elec-
trodes. This also is a line spectrum, but unlike the mercury spectrum, which has only six bright lines, the titanium spectrum
contains many thousands of bright linos, so that with the low
power of the spectroscope which you have, the linen blurr into each other and we see only the most prominent or brightest lines on a uniformly luminous background, which latter requires a more powerful spectroscope to resolve into linen.
(3) . The band spectrum. This shows a number of bright bands, frequently gradually fading out at their edge and separated by dark spaces. It thus differs from the continuous spectrum (1) in
being discontinuous, that is, missing certain ranges of frequency,
and differs from the line spectrum (2) in that the band spectrum has a number or range of frequencies in each band, where the line

RELATION OF BODIES TO RADIATION.

27

spectrum has only one single frequency in each line. Such

band spectra are usually characteristic of luminescent compounds

or of gases and vapors at high pressure, while elementary gases

or vapors give line spectra. Absorption and fluorescence also

give band spectra, and I thus show you a band spectrum by opera-

ting a mercury lamp in a tube of uranium glass, behind a trans-

parent screen colored by rhodamine (an aniline dye which

fluoresces red). As you see, the spectrum shows a broad red

band,

due

to

the

reddish

screen ;

and

a

greenish

yellow

band

due

to the uranium glass, while the normal mercury lines are de-

creased in intensity.

(4). If you now look with the spectroscope at the Welsbach

mantel through the mercury arc stream, you see the continuous

spectrum of the mantel and superimposed upon it the line spec-

trum of the mercury lamp. The light giving mercury vapor thus

is transparent for the light of the Welsbach mantel back of it, and

lets it pass through, with the exception of those particular fre-

quencies which it gives itself; that is, a luminous gas absorbs

those frequencies of radiation which it produces, but is trans-

parent for all other frequencies. This is easily understood: an

atom on which a vibration impinges will be set in motion by it

and thus absorb the energy of the impinging vibration if it is able

to vibrate with the frequency of the impinging vibration; that is,

to resonate with it, but will not be affected by any other frequency

to which it cannot respond, and thus is transparent to all frequen-

cies of vibration, except to those to which it can respond; that is,

which it produces when vibrating.
When looking at a continuous spectrum through a luminous

gas or vapor, two cases thus may occur: either the spectrum lines

of the gas arc brighter than the continuous spectrum, as in the

present case, and then appear as bright lines on a bright back-

ground, or the continuous spectrum is brighter than the lines of

the gas spectrum in front of it and the lines of the gas spectrum

appear less bright than the background, that is, appear as dark

lines on a bright background. Such a spectrum is called a

reversed spectrum, or absorption spectrum. It shows the lines of

the gas or vapor spectrum, by contrast, dark on the brighter back-

ground of the continuous spectrum.

The sun and many fixed stars present such a reversed spectrum :

the sun's spectrum shows the spectrum lines of all the elements

28

RADIATION, LIGHT, AND ILLUMINATION.

which are in the sun's atmosphere as dark lines on the continuous
spectrum given by the inner core of the sun. Whether the line spectrum of a gas or vapor is reversed by the
continuous spectrum of a solH or liquid back of it or not depends upon the relative intensity, and thus, to some extent, on the rela-
tive temperature. Some fixed stars show bright lines on a less luminous background, due possibly to a higher temperature and greater thickness of their atmosphere, and sometimes bright lines and dark lines occur simultaneously, or dark lines may change to bright lines at such places at which, by some activity, as a tem-
perature rise, their brilliancy is greatly increased.

18.
Combinations of the different types of spectra: continuous spectrum, line spectrum, band spectrum, reversed spectrum,
frequently occur, as we have seen bands and linos together in the modified mercury spectrum, and in this case, by turning on an incandescent lamp, we can still add a continuous spectrum due to the light of the incandescent lamp reflected from the walls of the room. So also in the continuous spectrum of incandescent
bodies, bright bands or dark bands occasionally appear, that in,
m regions in the spectrum of greater or lessor intensity, will be
discussed in the paragraphs on colored radiation and selective
radiation.

RELATION OF BODIES TO RADIATION.

29

14. When a beam of radiation impinges upon a body it is
resolved into three parts : one part is reflected, that is, does not
enter the body at all, but is thrown back. The second part is absorbed in the body, that is, converted into another form of
energy (which other form of energy usually is heat, but may be
chemical energy, some other frequency of radiation, etc.) and the
third part is transmitted, that is, passes through the body, and
out of it, if the body is not too thick. No body reflects, or
absorbs, or transmits all the radiations, but even the most perfectly reflecting body absorbs and transmits some radiation, the most transparent body reflects and absorbs some radiation, etc.
Reflection may be either regular reflection, or irregular reflection. In the former case (Fig. 18) the beam of light is reflected
under the same angle under which it impinges upon the body, and the body thus acts as a mirror, that is, gives a virtual image

FIG. 19.

back of it as shown in clotted lino in Fig. 18. In the latter case
(Fig. 19) the light is reflected irregularly in all directions.
A body which reflects all the frequencies of radiation uniformly,
that is, in which the percentage of the impinging radiation, which is reflected, is the same for all frequencies of radiation, is called a colorless b0dj/? and a body which reflects a higher percentage of the radiation of some frequency than of other frequencies, is called a colored body, and its color is the color of radiation, that is, the frequency or frequencies which it reflects more than other

frequencies.
A colorless body which reflects all the radiation impinging upon

it is called a while body. Most nearly white bodies are silver,

A magnesia, chalk, etc.

body which reflects none of the radiation

impinging upon it, but absorbs all, is called a black body. The

30

RADIATION, LIGHT, AND ILLUMINATION.

A most nearly black bodies are lampblack, charcoal, etc.

body

which reflects a constant part of the impinging radiation, that is,

the same part or percentage for all frequencies, is called a grey

A body, and the ratio of the reflected light to the total impinging

light is called its whiteness or albedo.

perfectly white body

thus has albedo 1, a perfectly black body albedo 0, and a body

which reflects one-quarter and absorbs the other three-quarters

of the radiation of any wave length impinging upon it, would be

said to have albedo 0.25. Black, white and grey thus -are not considered as colors in

physics.
As examples of colorless bodies I show you here:

Regular reflection: polished silver, white; polished iron, grey.

Irregular reflection: powdered magnesia, white; lampblack,

black; powdered zinc, barium sulphide, grey. As example of colored bodies I show you:
Regular reflection: polished copper, red;

polished

gold or

brass, yellow.
Irregular reflection: mercury sulphide (cinnabar), red; potas-
sium bichromate, orange; magnesium chromate, yellow; copper acetate-arsenite (paris green), green; copper oxide hydrate precipitated by ammonia, blue; ultra-marine, indigo; magnesium permanganate mixed with magnesia, violet.
15. Of the radiation which enters a body, that part which is
absorbed is usually converted into heat. Thus a black body, when exposed to radiation, becomes hotter than a white body, which reflects, or a transparent body, which transmits, most of the radiation. Thus the globe of a colored incandescent lamp, which absorbs more of the radiation than a transparent globe,
becomes hotter than a clear glass globe. When scattering dirt
on the snow it can be made to melt down far more rapidly in the
spring, under the rays of the sun, than when remaining clean, etc. Some bodies convert the absorbed radiation into chemical

energy, into other frequencies of radiation, etc.
Bodies which convert the absorbed radiation, or rather a part
thereof, into radiation of different, as far as known always lower, frequencies, are called fluorescent bodies. Thus the solution of rhodamine in alcohol, which 1 show you here, fluoresces red. It transmits red light, but absorbs green, blue and violet
light, and converts a part thereof into red light. This is best

RELATION OF BODIES TO RADIATION.

31

illustrated by exhibiting it in a source of light which contains no red rays, as the mercury lamp. You see in the rays of the mer-

cury lamp the rhodamine solution looks bright red, the red light

seems

to

come

from

the

inside

of

it ;

and

especially

through

a

red

glass the solution looks like a red hot incandescent body. Here

then, as no red light reaches the solution, the red light given by it

must be produced by frequency conversion from other radiation.

The spectroscope shows especially the bright green mercury line

weakened.

The phenomena of conversion of absorbed light into other forms of energy will be more fully discussed in the following

paragraphs.
16. By the transmitted light, that is, the radiation which
passes through them, bodies are again divided into colorless bodies; that is, such bodies which transmit the same percentage of radiation for every wave length or frequency, and colored bodies; that is, bodies which transmit a larger percentage of radiation of some frequencies than of others, and as the transparent color of a body, then, is understood the color, that is, the frequency, of that radiation of which the greatest percentage is transmitted. Thus a red glass is one which transmits a higher percentage of red radiation than of any other radiation.
A body, then, is called transparent, if it transmits all the radia-
tion, and opaqv&, if it transmits no radiation, but absorbs or
reflects all. If only a part of the radiation is transmitted, but
in such manner that it is the same part for all frequencies, the body is called grey; or imperfectly transparent, if the part which is not transmitted is absorbed in the body; and translucent, if the part which is not transmitted is irregularly reflected inside
of the body.
The most perfectly transparent bodies, for visible light, are glass, water, quartz, etc.; the most opaque are the metals, and perfectly, or almost perfectly opaque are the magnetic metals, perhaps due to the very low speed of propagation in these metals, which would result from the high value of the permeability /* by

equation (8) paragraph 11. As example of colorless bodies I show you here a glass tube
filled with water, transparent; a tube filled with nigrosine solution in alcohol, opaque and black; a very diluted solution of nigrosine with traces of other aniline dye for color correction, in

32

RADIATION, LIGHT, AND ILLUMINATION.

alcohol, as grey, and a tube filled with an emulsion of water with a solution of chloroform in white paraffin oil, which latter solu-

tion has the same specific gravity as water, translucent.
Samples of transparent colored bodies are: carmine solution, red; potassium bichromate solution, orange; potassium chromate solution, yellow; nickel sulphate solution, green; copper nitrate solution, blue; diluted potassium permanganate solution, or

diluted solution of iodine in chloroform, violet.

As

seen, the

terms

" colorless"

and

"colored 7 ' have

two

dif-

ferent meanings when applied to the reflected radiation and

when applied to the transmitted radiation, and the color of a

body in reflected light maybe different, and frequently is differ-

ent, from its color in transmitted light, and some bodies may be

colorless in reflected light, but colored in transmitted light, and

inversely. In materials of low absorption, the transmitted and

the reflected colors must be approximately complementary; thus

the transmitted color of the atmosphere is orange, the reflected

color blue.
17, Colors are, therefore, distinguished into opaque colors and transparent colors. The opaque colors are those shown by the light reflected from the body, the transparent colors those shown by the light transmitted through the body. In reflected light, the transparent colors, therefore, show only when covering a white, that is reflecting, surface, and then, because the light
reflected from the white background of the transparent coloring body traverses this body twice, before and after reflection, and,
therefore, depend in their brilliancy on the background. The difference between opaque and transparent colors, the former
reflecting from the surface, the latter reflecting from back of
the colored substance, is seen by comparing the appearance of the two classes of colors shown in 14 and in 16.

In its general use, the terms colorless, white, black, transparent, opaque, refer only to the visible radiation, that iw, to the frequen-
cies within that octave which the eye perceives as light. More
broadly, however, these terms may in physics bo applied to the total range of radiation, and then many substances which are
colorless for visible light, would be considered as strongly colored, that is, show for different frequencies great differences in the per-
centage of radiation which they reflect or transmit. Thus we
have seen that glass, which is transparent for visible light, is

RELATION OF BODIES TO RADIATION.

33

entirely opaque for some ultra-violet light and also opaque for ultra-red light of low frequency; so in this broader sense would
have to be called colored] the color of clear glass, however, is that
of the visible spectrum; or, for instance, iodine solution, which is
opaque for visible light, is transparent for ultra-red light, that is,
its color is ultra-red, etc.
In this broader sense, referring to the total range and not merely to the visible range, glass, water, mica, etc., are not colorless transparent but colored, and quartz is probably the most transparent and colorless body.
18. The color of the body, thus, is represented b^ that frequency or those frequencies of radiation of which a higher per-
centage are reflected or transmitted than of the other frequencies of radiation. This color, therefore, is a characteristic property
of the body and independent of the character of the light and of its physiological effect on the eye, and can thus be called the actual or objective color of the body. If we consider diffused daylight as white, then the body appears to the eye in its objective or actual color when compared with a white body, that is, a body uniformly reflecting all radiation in the diffused daylight. Under other conditions, as, for instance, in artificial illumination, bodies do not always appear to the eye in their
objective colors, but may show a very different color depending
on the character of the source of light. For instance, I have
here a plate of colored glass : looking through it at the mercury lamp you sec the glass has an olive green color; but when I turn on an incandescent lamp you see that it is ordinary red glass.
Its objective color is red, its subjective color in the mercury
light is green. Looking through this glass in daylight it appears red as it transmits more red light than other colors of light, and the transmitted light thus contains a higher percentage of red rays than diffused daylight. The rays of the mercury lamp, however, contain very little red light and very
much green light, and while by this red glass a much higher percentage of the rod light from the mercury lamp is trans-
mitted than of its green light, this higher percentage of trans-
mitted red light is very much less than the lower percentage of
the transmitted green light, and, therefore, in the transmitted
light, green still preponderates more than in the diffused daylight, that is, the glass appears green. For instance, if in the

34

RADIATION, LIGHT, AND ILLUMINATION.

mercury lamp the ratio of red light to green light is only one hundredth of what it is in daylight, and the red glass transmits ten times as high a percentage of red as of green light, then in the light of the mercury lamp transmitted through this red glass the ratio of red light to green light is still only one-tenth of what
it is in daylight, and the glass thus appears green.
We have to distinguish between the actual or objective color of a
body, which is a constant of the body, and its apparent or sub-
jective color, which depends upon the light in which we view the body, and therefore may be very different for different illuminants, and bodies which have the same colors in one illuminant may have entirely different colors in another illuminant and inversely. It is, however, the subjective color of the body corresponding to the particular illuminant used which we see, and
which is, therefore, of importance in illuminating engineering, and the study of the subjective colors, therefore, is of foremost importance, and the success or failure of an illumination depends on the production of the desired subjective colors.
19. Broadly, an illuminant discriminates for the color in which it is deficient and the color in which it is rich. The color
in which the illuminant is deficient as red in the mercury lamp, blue and violet in the incandescent lamp appears black; the
color in which the illuminant is abnormally rich as yellow in
the incandescent lamp, green in the mercury lamp appears as white; that is, both colors disappear, more or less; as colors, become colorless. Thus in the yellow incandescent lamp, opaque yellow appears the same as white, opaque blue and violet appear more or less as black; transparent yellow appears colorless, transparent blue and violet appear colorless and from light transparent
grey to opaque black. In the green mercury lamp, opaque green and white appear the same, opaque red appears as black; transparent green appears colorless, and transparent red appears colorless, from "clear transparent to grey, to opaque black, de-
pending upon its intensity. It is interesting to see the difference between opaque and
transparent colors in this respect: 'as opaque colors the deficient
color turns black, the excess color white; but as transparent
colors both become colorless and more or less transparent. Thus,
in the mercury lamp, red and green as transparent colors both
vanish, or rather, very greatly decrease in their prominence.

RELATION OF BODIES TO RADIATION.

35

As the eye perceives only the resultant of radiation, very dif-
ferent combinations of radiation may give the same impression to the eye, but when blotting out certain radiations, as red and
green, in the mercury lamp, these different combinations of radia-
tion may not give the same resultant any more, that is, become
of different colors, and inversely, different colors, which differ only by such component radiations as are blotted out by an illuminant, become equal in this iUuminant. For instance, a mixture of red and blue, as a diluted potassium permanganate solution, appears violet in daylight. In the mercury light it appears blue, as the red is blotted out, and in the light of the incandescent lamp it appears red, as the blue is blotted out.
I show you here, in the light of an incandescent lamp, two pieces of black velvet. I turn off the incandescent lamp and turn on the mercury lamp, and you see the one piece is blue, and
the other black. Now I show you two pieces of brownish black
cloth in the mercury light. Changing to the incandescent lamp you see that the one is a bright crimson, and the other still practically black. In both cases the color deficient in the illuminant
appeared as black. This tube of copper chloride crystals appears bright green in
the incandescent lamp. In the mercury light it is a dirty white. The excess color, green, is blotted out.
These crystals of didymium nitrate, which are a light pink in daylight, are dark pink in the incandescent light. In the mercury light they are blue: the color is a mixture of red and blue, and the one is blotted out in the mercury light and the
*
other in the incandescent light.
These two tubes, one containing a concentrated solution of
manganese chloride, the other a solution of didymium nitrate, are both a dark pink in the incandescent light. In the mercury light the first becomes a faint pink, the second becomes grass
green.
These tubes, one containing a solution of didymium nitrate, the other a diluted solution of nickel sulphate, appear both light green in the mercury light. In the incandescent lamp the former is dark pink, the latter dark green. [Didymium, which formerly Was considered as an element, has been resolved into two elements, praseodymium, which gives green salts, and neodymium, which gives pink salts. It is interesting to see that this separa-

36

RADIATION, LIGHT, AND ILLUMINATION.

tion is carried out photometrically by the light: the mercury lamp showing only the green color of the praseodymium, the incandescent lamp the pink color of neodymium].
I have here a number of tubes, which seen in the light of the incandescent lamp contain red solutions of nearly the same shade. Changing to the mercury lamp you see that they exhibit almost any color. As the red disappeared in the mercury lamp the other component colors, which did not show in the incandescent lamp as they were very much less in intensity than the red, now predominate : potassium permanganate solution turns blue, carmine blue; potassium bichromate, greenish brown; coralline,
(an aniline dye), olive green, etc., etc.
Again, a number of tubes, which in the mercury light appear of the same or nearly .the same blue color, turn to very different colors when seen in the incandescent lamp, due to the appearance of red and green, which were not seen with the mercury light.
A solution of rhodamine, however, which looks a dull red in the
light of the incandescent lamp, turns a glowing crimson in the mercury lamp, due to its red fluorescence. This diluted solution of rhodamine and methyl green (aniline dyes), which is grey in the light of the incandescent lamp, turns brownish red in the mercury lamp, the green is blotted out, while the rhodamine shows its
red fluorescence. Thus, you see, the already very difficult problem of judging the subjective colors of bodies under different iiluminants is still greatly increased by phenomena as fluorescence.
To conclude then: we have to distinguish between colorless and colored bodies, between opaque colors and transparent colors,
between color, as referred to the visible range of radiation only, or to the total range, including ultra-reel and ultra-violet, and
especially we have to realize the distinction between objective or actual color, and between subjective or apparent color, when
dealing with problems of illuminating engineering.

LECTURE III.
PHYSIOLOGICAL EFFECTS OF RADIATION.
Visibility.
20. The most important physiological effect is the visibility of the narrow range of radiation, of less than one octave, between
wave length 76 X 10~6 and 39 X 1Q-6 .
The range of intensity of illumination, over which the eye can see with practically equal comfort, is enormous: the average intensity of illumination at noon of a sunny day is nearly one million times greater than the illumination given by the full moon, and still we can see fairly well in either case; that is, the human eye can adapt itself to enormous differences in the intensity of illumination, and that so perfectly that it is difficult to realize the differences in intensity without measuring them. The photo-
^ graphic camera realizes it. An exposure taken in T second
with iV opening of the diaphragm in full sunlight usually gives a better photograph than an exposure of 10 minutes at full opening, in the light of the full moon. The ratio of time of exposure in the two cases, however, is about 1 to 1,000,000, thus showing the
difference in the intensity of illumination. Also, the disk of the
moon, when seen in daylight, has about the same intensity as the sky somewhat more than the cloudless sky, less than white reflecting clouds. As the surface of the moon's disk, of one-half degree diameter, is about JV^QW the surface of the sky, it thus follows that the daylight reflected from the sky is about 100,000 times more intense than the light of the full moon.
The organ by which we perceive the radiation, the human eye (Fig. 20), contains all the elements of a modern photographic
camera an achromatic lensc : the lensc L, of high refractive
power, enclosed between the two transparent liquids A and B
which correct the color dispersion, that is, give the achromatic property; a diaphragm: the iris /, which allows the increase or decrease of the opening P, the pupil; a shutter: the eyelids and

38

RADIATION, LIGHT, AND ILLUMINATION

the sensitive plate or retina R. The nerves of vision end at the back of the retina, and in the center of the retina is a spot F,
the "sensitive spot " or " fova," at which

the retina is very thin, and the nerve
ends specially plentiful. At this spot we
thus see sharpest and clearest, and it is this spot we use for seeing by turning

the eye so as to fix on it the image of

the

subject

we

desire

to

see while ;

the

image on the rest of the retina is used

merely for orientation.

FlG- 20-

The adaptability to the enormous range of intensity of illumination, which

as seen we meet in nature, is secured:

(1). By changing the opening and thereby the amount of light

admitted to the eye, by contracting or opening the pupil \ This

action is automatic. In low intensity of illumination the pupil
thus is wide open and contracts at higher intensities. As this

automatic action takes an appreciable, though short time, a flash

light photograph shows the pupil of the eye fully open and thereby

gives a staring impression to the faces which is avoided by keep-

ing a photographically inactive light, as a candle, burning outside
of the field of the camera when preparing for a flash light photo-

graph.
(2). By the fatigue of the optic nerves, exposed to high inten-
sity of illumination, the nerves becomes less sensitive, while at low intensity they rest and thus become more sensitive, and the
differences of sensation are hereby made very much less than
corresponds to the differences of intensity of radiation. There-
fore, when entering a brightly illuminated room from the darkness we are blinded in the first moment, until the eye gets accustomed to the light, that is, the nerves become fatigued and so reduce the sensation of light. Inversely, when stepping from a bright room into the darkness wo first see almost nothing until
the eye gets accustomed to the darkness, that is, the nerves of vision are rested and their sensitivity thus increased so as to per-
ceive the much lower intensity of illumination. (3). By the logarithmic law of sensation. The impression made
on our senses, eye, ear, etc., that is, the sensation, is not proportional to the energy which produces the sensation, that is, the

PHYSIOLOGICAL EFFECTS OF RADIATION.

39

intensity of the light, the sound, etc., but is approximately

proportional to its logarithm and the sensation, therefore,
changes very much less than the intensity of light, etc*, which

causes the sensation. Thus a change of intensity from 1 to

1000 is 1000 times as great a change of intensity as from

= 1 to 2 }

but the change

of

sensation in the first

case,

log 1000

3,

is only about 10 times as great as the change in the latter case,
log 2 = 0.301.

This logarithmic law of sensation (Fechner's Law), while usu-

ally not clearly formulated, is fully familiar to everybody, is con-

tinuously used in life, and has been used from practical experience since by-gone ages. It means that the same relative or percent-

age change in intensity of light, sound, etc., gives the same change

of sensation, or in other words, doubling the intensity gives the

same change in sensation, whether it is a change of intensity from

one candle power to two candle power, or from 10 to 20, or from

1000 to 2000 candle power.

It is obvious that the change of sensation is not proportional to

the change of intensity; a change of intensity of light by one

candle power gives a very marked change of sensation, if it is a

change from one to two candle power, but is unnoticeable, if it is a change from 100 to 101 candle power. The change of sensation

thus is not proportional to the absolute change of intensity one

candle power in either case but to the relative or percentage

change of intensity, and as this is 100 per cent in the first, 1 per

cent in the latter case, the change of sensation is marked in the

first, unnoticeable in the latter case.
This law of sensation we continuously rely upon in practice.

For instance, when designing an electrical distribution system for

lighting, we consider that the variation of voltage by 1 per cent is

permissible as it gives a change of candle power of about 5 per

cent, and 5 per cent variation is not seriously noticeable to the eye.
Now this 5 per cent change of candle power may be a change from 1 to 0.95, or by fa candle power, or it may be a change from 1000
to 90, or by 50 candle power, and both changes we assume, and

are justified herein from practical experience, to give the same

change of sensation, that is, to be near the limits of permissi-

bility.
This law of sensation (Fechner's Law) means :
- If i intensity of illumination, as physical quantity, that is,

40

RADIATION, LIGHT, AND ILLUMINATION.

in meter-candles or in watts radiation of specified wave length,
the physiological effect given thereby is:

n

L

=

A

~
log

x

o

A where is a proportionality constant (depending on the physio-
logical measure of L) and \ is the minimum perceptible value of
illumination or the " threshold value/' below which sensation

ceases. . The minimum value of change of intensity i, which is still
just perceptible to the average human eye, is about 1.6 per cent. This, then, is the sensitivity limit of the human eye for changes

of illumination.
Obviously, when approaching the threshold value \, the sensi-
tivity of the eye for intensity changes decreases.
The result of this law of sensation is that the physiological effect

is not proportional to the physical effect, as exerted, for instance,
on the photographic plate. The range of intensities permissible on the same photographic plate, therefore, is far more restricted.
A variation of illumination within the field of vision of 1 to 1000,

as between the ground and the sky, would not be seriously felt by

the eye, that is, not give a very great difference in the sensation.
On the photographic plate, the brighter portions would show 1000

times more effect than the darker portions and thus give bad

A halation while the latter are still under exposed.

photographic

plate, therefore, requires much smaller variations of intensity in the field of vision than permissible to the eye. In the same man-
ner the variations of intensity of the voice, used in speaking, are
far beyond the range of impression which the phonograph cylinder can record, and when speaking into the phonograph a more
uniform intensity of the voice is required to produce the record,
otherwise the lower portions of the speech arc not recorded, while
at the louder portions the recording point jumps and the voice

breaks in the reproduction.
2L The sensitivity of the eye to radiation obviously changes
with the frequency, as it is zero in the ultra-red, and in the ultraviolet where the radiation is not visible and thus gradually
increases from zero at the red end of the spectrum to a maximum
somewhere near the middle of the spectrum and then decreases

again to zero at the violet end of the spectrum; that is, the physi-

PHYSIOLOGICAL EFFECTS OF RADIATION.

41

ological effect produced by the same radiation power as one
watt of radiating power is a maximum near the middle of the
visible spectrum and decreases to zero at the two ends, about as illustrated by the curves in Fig. 21. Inversely, the mechanical equivalent of light, or the power required to produce the same physiological effect as one candle power of light is a
minimum near the middle of the spectrum and increases from
there to infinity at the end of the visible range, being infinite

FIG. 21.
in the ultra-reel and ultra-violet, where no power of radiation can produce visibility. It thus varies about as indicated in Fig. 22.
The mechanical power equivalent of light, thus, is not constant, as the mechanical energy equivalent of heat which is 426 kgm. or 4.25 kilo-joule per calorie but is a function of the frequency, that is, of the color of radiation, with a maximum, probably not very far from 0.02 watt per candle power in the middle of the
spectrum.
When comparing, however, the physiological effects of different
frequencies of radiation, that is, different colors of light, the diffi-
culty arises that different colored lights cannot be compared photometrically, as all photometers are based on making the illumination produced by the two different sources of light equal, and when these sources of light are of different color they can never become equal. As long as the colors are not very different two different shades of yellow or yellowish white and white the eye can still approximately estimate the eaualitv of intensity and

42

RADIATION, LIGHT, AND ILLUMINATION.

thus compare them, though not as accurately as when the two sources of light are of the same color. With very great color
differences, as green light and orange light, this is no longer feasible. However, an accurate comparison can still be made on the basis of equal ease in distinguishing objects. As the pur-

YELLOW

GREEN

FIG. 22.

pose for which light is used is to distinguish objects, the correct comparison of lights obviously is on the basis of equal distinctness of objects illuminated by them; that is, two lights, regardless whether of the same or of different colors, give the same candle power, that is, the same physiological effect, if they enable us to distinguish objects with the same ease at the same distance. Experience has shown that the sharpest distinction, that is, the
greatest accuracy in comparing different lights in this manner, is reached by determining the distance from the source of light at

PHYSIOLOGICAL EFFECTS OF RADIATION.

43

which print of moderate size just ceases to be readable. For this purpose the print must be a mixture of letters which do not form intelligible words and the point which can be determined most

accurately is where large letters, as capitals, are still readable,

while small letters are already unreadable (see p. 174). Obviously,
in comparing different colors of light the object must be colorless, that is, the print be black on white. This method of comparison of the physiological effect, by what has been called the "luminometer," is theoretically the most correct, as it is independent of

the color of light. It is, however, not as accurate as the compari-
son by photometer, and thus the average of a number of observations must be used. The only error which this method leaves is

that due to the difference in the sensitivity of different eyes, that

is, due to the differences between the sensitivity curves (Fig. 21),

and this in most cases seems to be very small.

22. It is found, however, that the sensitivity curve for different

colors of radiation is a function of the intensity of radiation; that

is, the maximum sensitivity point of the eye is not at a definite
frequency or wave length, but varies with the intensity of illumination and shifts more towards the red end of the spectrum for

high, towards the violet end of the spectrum for low intensity of illumination, and for illumination of very high intensity the maxi-
mum physiological effect takes place in the yellow light, while for
very low intensity of illumination it occurs in the bluish green

light; that is, at high intensity yellow light requires less power for the same physiological effect than any other color of light, while for low intensity, bluish green light requires less power for

the same physiological effect than any other color of light. Thus,

if an orange yellow light, as a flame carbon arc, and a bluish green

light, as a mercury lamp, appear of the same intensity from the

distance of 100 feet, by going nearer to the lamps the orange

yellow appears to increase more rapidly in intensity than the

bluish green, and from a veiy short distance the former appears

glaring bright, while the latter is disappointing by not showing anywhere near the same apparent intensity. Inversely, when

going further and further away from the two lamps the orange

yellow light seems to fade out more rapidly than the bluish green,

and has practically disappeared while the bluish green is still

A markedly visible.

mercury lamp, therefore, can be seen from

distances from which a much brighter yellow flame arc is practi-

44

RADIATION, LIGHT, AND ILLUMINATION.

cally unnoticeable, but inversely, from a very short distance the yellow light appears dazzling, while a mercury lamp of higher
candle power appears less bright. Fig. 23 illustrates the change of sensitivity with intensity, by
approximate curves of the variation of the relative sensitivity of
the average human eye with the intensity i of illumination in

FIG. 23.
meter candles (or rather log i] as abscissas, for red light, wave length 65.0; orange yellow light, wave length 59; bluish green light, wave length 50.5; and violet light, wave length 45.0.
As seen for red light as well as violet light the two ends of the visible spectrum the sensitivity is low, while for orange yellow as well as bluish green light near the middle of the
visible range the sensitivity is high. For bluish green light, however, the sensitivity is high at low
and moderate intensities but falls off for high intensities, while for orange yellow light the sensitivity is high at high intensities
and falls off at medium and low intensities and ultimately vanishes, that is, becomes invisible at intensities many times higher than
those at which green light is still well visible. Red light vanishes from visibility still earlier than orange yel-
low light, while violet light remains visible even at very low
intensities.
The vanishing points of the different colors of light, that is,

PHYSIOLOGICAL EFFECTS OF RADIATION.

45

the minimum intensities which can just be perceived are, approxi-
mately, at:

Color

red

Wave length lw =

67

Meter-candles in-

tensity.. .?,= 0.06

Relative radiation

power

po = 10,000

orange 60 5
0.0056
1000

yellow 57.5
0.0029
100

green 50.5
0.00017
1

blue 47
0.00012
2

violet
43 X KT6
0.00012
20

That is, the minimum visible amount of green light represents the least amount of power; the minimum visible amount of blue light requires twice as much power as green light; violet
light 20 times as much, but yellow light 100 times and red light
even 10,000 times as much power as green light at the threshold
of visibility.
While the intensity of radiation varies inversely proportional to the square of the distance, it follows herefrom that the physiological effect of radiation does not vary exactly with the square of the distance, but varies somewhat faster, that is, with a higher power of the distance for orange yellow or the long-wave end of the spectrum, and somewhat slower, that is, with a lesser power of the distance than the square, for bluish green or the short-wave
end of the spectrum.
This phenomenon is appreciable even when comparing the enclosed alternating carbon arc with the open direct current carbon arc : by photometer, where a fairly high intensity of illumination is used, the relative intensity of the two arcs is found somewhat different than by luminometer, that is, by reading distances nearer the lower limit of visibility. For low intensities, the alternating arc compares more favorably than for high
intensities.
It follows, therefore, that in the photometric comparison of illuminants, where appreciable color differences exist, the inten-
sity of illumination at which the comparison is made must be given, as it influences the result, or the candle power and the
distance of observation stated.
23. Not only the sensitivity maximum is different for low and
for high intensity of illumination, but the shape of the sensitivity curve also is altered, and for low intensity is more peaked,
that is, the sensitivity decreases more rapidly from a maximum
towards the ends of the spectrum than it does for high intensity

46

RADIATION, LIGHT, AND ILLUMINATION.

of illumination as indicated by the curves in Fig. 24 which
shows approximate sensitivity curves of the average human eye:
(a) for every low illumination near the treshold value of visi-
bility or 0.001 meter-candles; (b) for medium illumination, 4.6
meter-candles; (c) for very high illumination, 600 meter-candles.

1.70 50.0

FIG. 24.

(1 meter-candle is the illumination produced by 1 candle power
N of light intensity at 1 meter distance; meter-candles, thus, the N illumination produced by a light source of candle power at 1

VN meter distance or of 1 candle power at =r meter distance, etc.).

= X As-seen, curve (a), ends at wave length lw

61

10~ 6 ;

that

is,

for longer waves or orange and red light, 0.001 meter-candles is below the threshold value of visibility; hence is no longer visible.
The maximum visibility, that is the sensitivity maximum of

the human eye, lies at wave length.

Z = 51.1, bluish green for very low intensity, curve (a). Z = 53.7, yellowish green for medium intensity, curve (6) .

Z

56.5, yellow for high intensity, curve (c).

The sensitivity maximum varies with the intensity about as
shown in Fig. 25; that is, it is constant in the bluish green for low intensities, changes at medium intensities in the range between 0,5 and 50 meter-candles and again remains constant in
the yellow for still higher intensities.

PHYSIOLOGICAL EFFECTS OF RADIATION.
The sensitivity curves, as given in Fig. 24, have the general
character of probability curves :

H where lwo is the wave length at maximum sensitivity and Q is
the sensitivity at this wave length, that is, the maximum sensi-
tivity and ks is a constant which is approximately 120 for low,

LOG \ - -

42

-i

0.01

01

Ji-

3 57

FIG. 25.
62 for high intensities and changes in approximately the same range of intensities in which 1WQ changes; &s is also plotted in
Fig. 25.
This effect of the intensity of illumination on the sensitivity of the eye is very important in illuminating engineering as it determines the color shades which are most effective for the particular
purpose. For instance, in sending the light to great distances, for signalling, etc., the bluish green of the mercury lamp is best suited, carries farthest, and the yellow flame arc the poorest; the white carbon arc superior to the yellow flame arc, even where the latter is of greater intensity. Inversely, where a big glare of
light is desired, as for decorative purposes, for advertising, etc., the yellow flame carbon arc is best suited, the bluish green mer-
cury lamp disappointing.
Apparent exceptions may exist: for instance, the long waves of
the orange yellow penetrate fog better than the short waves of bluish green, and for lighthouses, where the important problem is
to reach the greatest possible distance in fog, yellow light, thus,
may be superior. la general, however, the bluish green is superioi

48

RADIATION, LIGHT, AND ILLUMINATION.

in visibility to the orange yellow for long distances, and inversely,
the orange yellow is superior for short distances.
At the limits of visibility the eye is very many times more
sensitive to green light and, in general, high-frequency light, than
to orange yellow and, in general, low-frequency light.
A necessary result of the higher sensitivity of the eye for green
light is the preponderance of green in gas and vapor spectra. As no special reason exists why spectrum lines should appear more frequently at one wave length than at any other and as the radiation is most visible in the green, this explains, somewhat, the
tendency of most highly efficient illuminants towards a greenish or yellow color (as, for instance, the Welsbach mantel, the Nernst
lamp, etc.).

Pathological and Other Effects on the Eye.

24. Radiation is a form of energy, and thus, when intercepted

and absorbed, disappears as radiation by conversion into another form of energy, usually heat. Thus the light which enters the

eye is converted into heat, and if its power is considerable it may
be harmful or even destructive, causing inflammation or burns.

This harmful effect of excessive radiation is not incident to any

particular frequency, but inherent in radiation as a form of energy. It is, therefore, greatest for the same physiological effect, that is, the same amount of visibility, for those frequencies of

light which have the lowest visibility or highest power equiva-

lent, that is, for the red and the violet and least for the green and

the yellow, which for the same amount of visibility represent

least power. Hence, green and greenish yellow light arc the most

harmless, the least irritating to the eye, as they represent the

We least power.

feel this effect and express it by speaking of

the

green

light

as

"cold

7
light'

and

of

the

red

and

orange

light

as

"hot" or "warm." The harmful effect of working very much

under artificial illumination is largely due to this energy effect,

incident to the large amount of orange, reel, and ultra-rod

in the radiation of the incandescent bodies used for illuminants and thus does not exist with " cold light/' as the light of the

mercury lamp. Blue and violet light, however, are just as energetic, or "hot/'
as orange and red light, and the reason that they arc usually not recognized as such is that we have no means to produce efficiently

PHYSIOLOGICAL EFFECTS OF RADIATION.

49

powerful blue and violet light, and if we could produce it would
not be able to use it for illumination, clue to the specific effects of
this light which will be described in the following.
A In Fig. 26, let the curve represent roughly the mechanical
power equivalent of light for average intensity, that is, the power required to produce the same physiological effect or the same candle power. The distribution of power in an incandescent

FIG. 26.
lamp carbon filament would be somewhat like C. Hence, the
physiological effect falls off somewhat towards the green, as C
drops more than A, and almost vanishes in the blue and violet, as
C rapidly decreases, while A, the power required to give the same
physiological effect, rapidly increases. From the yellow towards
the red the physiological effect again decreases somewhat, but

50

RADIATION, LIGHT, AND ILLUMINATION.

C the radiation still increases towards the ultra-red. Dividing

A by then gives the distribution of the physiological effect, curve

C", that is, of visibility, in the incandescent lamp spectrum, show-

ing that the color of the light is yellow.

Hg

gives

distributhe^

tion of power in the mercury spectrum. It is shown in dotted

lines, as the distribution is not continuous, but the power massed
at definite points, the spectrum lines of mercury. Hgf then gives

the visibility curve by dividing Hg by A. As seen, the ratio of

Hg the area of f to Hg, that is, the ratio of the physiological effect

to the power, is much less than the ratio of the area of C' to C;

that is, the former produces for the same amount of visibility far

less heat and thus is safer,

25. Excessive intensity, such as produced at a short-circuiting

arc, is harmful to the eye. The human organism has by evolution, by natural selection, developed a protective mechanism
against the entrance of radiation of excessive power into the eye:
at high intensity of illumination the pupil of the eye contracts and thus reduces the amount of light admitted, and a sudden

exposure to excessive radiation causes the eyelids to close. This

protective mechanism is automatic; it is, however, responsive
mainly to long waves of radiation, to the red and the yellow light, but not to the short waves of green, blue and violet light. The

reason for this is apparently that all sources of excessive radia-

tion which are found in nature, the sun and the fire, are rich in

red and yellow rays, but frequently poor in rays of short wave length, and, therefore, a response to short wave lengths alone would

not be sufficient for protection as they might be absent in many

intense radiations, while a response to long waves would be

sufficient since these are always plentiful in the intense radiations

found in nature.

It is only of late years that illuminants, as the mercury lamp, which are deficient in the long waves, have been produced, and for

these the protective action of the eye, by contracting the pupil,
fails. This absence or reduction of the contraction of the pupil
of the eye in the light of the mercury lamp is noticed when passing from a room well illuminated by incandescent lamps, to one equally
well illuminated by mercury lamps and inversely. When changing

from the incandescent light to the mercury light, the illumination given by the latter at first appears dull and inferior as the pupil

is still contracted, but gradually gains in intensity as the pupil

PHYSIOLOGICAL EFFECTS OF RADIATION.

51

opens; and inversely, coming from the mercury light to the incan-

descent light, the latter first appears as a big glare of light, the
pupil still being open, but gradually dulls down by the contraction

of the pupil.

This absence of the automatic protective action of the eye

against light deficient in long waves is very important, as it means

that exposure to excessive intensity of illumination by mercury

light may be harmful, due to the power of the light, against which

the eye fails to protect, while the same or even greater power of

radiation in yellow light would be harmless, as the eye will pro-

tect itself against it. The mercury lamp, therefore, is the safest

illuminant, when of that moderate intensity required for good

illumination, but becomes harmful when of excessive intensity, as

when closely looking at the lamp for considerable time, when

operating at excessive current. The possibility of a harmful

effect is noticed by the light appearing as glaring. This phe-

nomenon explains the contradictory statements occasionallymade

regarding the physiological effect of such illuminants.

Up 26.

to and including the green light, no specific effects,

that is, effects besides those due to the power of radiation, seem

yet to exist. They begin, however, at the wave length of blue

light.
I show you here a fairly intense blue violet light, that is, light containing only blue; and violet radiation. It is derived from a vertical mercury lamp, which is surrounded by two concentric glass cylinders welded together at the bottom. The space between the cylinders is filled with a fairly concentrated solution of potassium permanganate (strong copper nitrate solution or a
cupric-ammon salt solution, though not quite so good, may also
be used) which is opaque to all but the blue and violet radiations. As you see, the light has a very weird and uncanny effect, is extremely irritating: you can see by it as the intensity of illumination is fairly high, but you cannot distinguish everything, and especially the lamp is indefinite and hazy : you see it, but when you look at it it disappears, and thus your eye is constantly trying to look at it and still never succeeds, which produces an irritating restlessness. It can well be believed that long exposure to such illumination would result in insanity. The cause of this weird effect
which is difficult to describe probably is that the sensitive
spot on the retina, that is, the point on which we focus the image

52

RADIATION, LIGHT, AND ILLUMINATION.

F of the object which we desire to see, or the fova in Fig. 19, is
blue blind, that is, does not see the blue or violet light. Thus we

see the lamp and other objects indistinctly on the outer range of the retina, but what we try to see distinctly disappears when

focused on the blue blind spot F. This spot, therefore, is often

called

the

"yellow

7
spot/

as

we

see

yellow

on

it

due
to^the

absence of the vision of blue at this particular place of the retina.

To produce this effect requires the mercury lamp; most other

illuminants do not have sufficient blue and violet rays to give

considerable illumination of this color and even if they do, no

screen which passes blue and violet is sufficiently opaque to the

long waves not to pass enough of them to spoil the effect, if the illuminant is rich in such long waves. The mercury lamp, how-

ever, is deficient in these, and thus it is necessary only to blind off the green and yellow rays in order to get the blue and violet light,
I show you here a mercury lamp enclosed by a screen consisting of a solution of naphtol green (an aniline dye) which transmits only the green light. As you see, in the green the above-described effect does not exist, but the vision is clear, distinct and restful.
27. Beyond the violet the radiation is no longer visible to the eye as light. There is, however, a faint perception of ultra-violet light in the eye, not as distinct light, but rather as an indistinct, uncomfortable feeling, some form of dull pain, possibly
resulting from fluorescence effects caused by the ultra-violet
radiation inside of the eye. With some practice the presence of
ultra-violet radiation thus can be noticed by the eye and such

light avoided. In the ultra-violet, and possibly to a very slight extent in the violet and even in the blue, a specific harmful effect

appears, which possibly is of chemical nature, a destruction by
chemical dissociation. This effect increases in severity the fur-

ther we reach into the ultra-violet, and seems to become a

maximum in the range from one to two octaves beyond the violet.

These very short ultra-violet rays are extremely destructive to
the eye: exposure even to a moderate intensity of them for very

few minutes produces a severe and painful inflammation, the

after effects of which last for years, and long exposures would

probably result in blindness. The chronic effects of this inflam-

mation are similar to the effect observed in blue light: inability

or difficulty in fixing objects on the sensitive spot F, so that with-

out impairment of the vision on the rest of the retina clear dis-

PHYSIOLOGICAL EFFECTS OF RADIATION.

53

tinction is impaired and reading becomes difficult or impossible,
especially in artificial illumination. It appears as if the sensitive
spot Fj or the focusing mechanism of the eye, were over-irritated and when used, for instance in reading, becomes very rapidly fatigued and the vision begins to blur. If further irritation by ultra-violet light or by attempting to read, etc., is avoided,
gradually the rapidity of fatigue decreases, the vision remains distinct for a longer and longer time before it begins to blur and ultimately becomes normal again.
The inflammation of the eye produced by ultra-violet light appears to be different from that caused by exposure to highpower radiation of no specific effect, as the light of a short circuit of a high-power electric system, or an explosion, etc.
The main differences are:
1. The effect of high-power radiation (power burn) appears
immediately after exposure, while that of ultra-violet radiation (ultra-violet burn) appears from 6 to 18 hours after exposure.
2. The external symptoms of inflammation: redness of the eyes and the face, swelling, copious tears, etc., are pronounced in the power burn, but very moderate or even entirely absent
in the ultra-violet burn.
3. Complete recovery from a power burn even in severe cases usually occurs within a few days, leaving no after effects, while recovery from an' ultra-violet burn is extremely slow, taking months or years, and some after effects, as abnormal sensitivity
to radiation of short wave lengths, may be practically permanent.
The general phenomena of a severe power burn are: Temporary blindness immediately after exposure, severe pains in the eyes and the face, redness of eyes and face, swelling, copious tears, etc. These effects increase for a few hours and then
decrease, yielding readily to proper treatment: application of
ice, cold boric acid solution, etc., and complete recovery occurs within a few days.
In chronic cases, as excessive work under artificial illumination, the symptoms appear gradually, but recovery, if no structural changes in the eyes have occurred, is rapid and complete by proper treatment and discontinuance of work under artificial
illumination.
Most artificial light is given by temperature radiation (incandescent lamp, gas and kerosene flame), and therefore its radiation

54

RADIATION, LIGHT, AND ILLUMINATION.

consists of a very small percentage only of visible light (usually

less than 1 per cent), while most of its energy is in the ultra-red

and invisible, and for the same amount of visible radiation or

light the total with daylight.

radiated power thus is many times greater than

Regarding

chronic

"
power

burn,"

artificial

light,

therefore, is much more harmful than daylight, that is, much

more energy enters the eye under incandescent illumination than

under much more powerful daylight illumination.

In a severe ultra-violet burn no immediate symptoms are

noticeable, except that the light may appear uncomfortable
while looking at it. The onset of the symptoms is from 6 to 18
hours later, that is, usually during the night following the ex-
posure, by severe deep-seated pains in the eyes; the external appearance of inflammation is moderate or absent, the vision is not impaired, but distinction made difficult by the inability to focus the eye on any object. The pains in the eyes and headache yield very slowly; for weeks and even months any attempt
of the patient to use the eyes for reading, or otherwise sharply distinguishing objects, leads to blurring of the vision; the letters of the print seem to run around and the eye cannot hold on
to them, and severe headache and deep-seated pains in the eyes follow such attempt. Gradually these effects become less;
after some months reading for a moderate length of time during

daylight is possible, but when continued too 'long, or in poor
light, as in artificial illumination, leads to blurring of the vision and head or eye ache. Practically complete recovery occurs
only after some years, and even then some care is necessary, as any very severe and extended strain on the eyes temporarily brings back the symptoms. Especially is this the case when looking at a light of short wave length, as the mercury arc; that

is, there remains an abnormal sensitivity of the eye to light of short wave lengths, even such light which to the normal eye is

perfectly harmless, as the mercury lamp.
In chronic cases of ultra-violet burn, which may occur when

working on unprotected arcs, and especially spark discharges

(as in wireless telegraphy), the first symptoms are: occasional
headaches, located back of the eyes, that is, pains which may

be characterized either as headache or as deep-seated eye ache.

These recur with increasing frequency and severity. At the

same time the blurring of the vision begins to be noticeable

PHYSIOLOGICAL EFFECTS OF RADIATION.

55

and the patient finds it more and more difficult to keep the eye focused for any length of time on objects, as the print when reading. These symptoms increase in severity until the patient is obliged to give up the occupation which exposed him to ultraviolet light, and then gradual recovery occurs, as described above, if the damage has not progressed too far.
In mild cases recovery from power burns may occur in a few
hours and complete recovery from mild ultra-violet burns in a few weeks.
Both types of burn may occasionally occur simultaneously and their symptoms then successively.
For instance, in a case of an exposure while working for about half an hour with a flame-carbon arc without enclosing glass globe (such an arc contains large amounts of high-power radiation, of yellow and orange color, but also a considerable amount
of ultra-violet rays), the symptoms of the power burn increased in severity for a few hours, and then rapidly vanished by the application of cold water, and recovery was practically complete six hours after exposure; then some hours later, in the middle of the night, the patient was awakened by severe pains in the eyes, the symptoms of the ultra-violet burn, and had to seek medical attendance. Under proper treatment recovery occurred in a few days, but the blurring of the vision was appreciable for some days longer, and the sensitivity to high-frequency light for some weeks.
28. Arcs produce considerable amount of ultra-violet light,* and in former experiments we have used a high frequency iron arc for producing ultra-violet light and also have seen that even a very thin sheet of glass is opaque for these radiations. For very
long ultra-violet rays, that is, the range close to the visible violet, glass is not quite opaque, but becomes perfectly opaque for about one-quarter to one-half octave beyond the violet, and in this first quarter of an octave the harmful effect of the ultra-violet radia-
tion is still very small and becomes serious only when approach-
ing a distance of about one octave from the visible end of the violet. Clear transparent glass thus offers a practically complete protection against the harmful effects of ultra-violet light, except when the latter is of excessive intensity, and thus arcs enclosed
* An arc between silicon terminals emits especially powerful ultra-violet
radiation accompanied by little visible light.

56

RADIATION, LIGHT, AND ILLUMINATION.

by glass globes are harmless. It is, however, not safe to look into and work in the light of open metal arcs for too long a
time.
The carbon arc gives the least ultra-violet rays, so little that even without enclosure by glass it is fairly safe; metal arcs give more and the mercury arc gives the greatest amount and reaches to the farthest distance beyond the visible, and these very destructive very short ultra-violet rays have so far only been observed in the radiation of a low temperature mercury arc in a quartz tube:
quartz being transparent to these rays while glass is opaque.
The high temperature mercury arc in a quartz tube, that is, arc operated near atmospheric pressure as it is used to some extent for illumination, especially abroad, seems to be much less dangerous than the low temperature or vacuum arc, but it also
requires a protecting glass globe. In general, no metal arc, spark discharge, or glow discharge
should ever be used industrially or otherwise without being enclosed by a glass globe, preferably of lead glass, if located so that
it may be looked at. Those experimenting with arcs or other
electric discharges should always protect their eyes by the inter-
position of a glass plate. Thus the sparks of wireless telegraph stations, the discharges
of ozonizers, the arcs of nitric acid generators, electric furnaces,
etc., may be dangerous without glass enclosure.
While artificial illuminants, and especially metal arcs, give
an appreciable amount of ultra-violet light, these ultra-violet rays extend only to about one-quarter octave beyond the visible violet and if, as is always the case, the illuminant is enclosed by-
glass, the harmful effect of these long ultra-violet rays is negli-
gible. The radiation of the sun also contains ultra-violet rays, and a larger percentage compared with the total radiation than any glass-enclosed artificial illuminant, and as the light of the sun,
that is, daylight, is recognized as perfectly harmless, as far as
this specific destructive action is concerned, the same applies to
the artificial illuminants, as they contain less ultra-violet rays
than the light of the sun.
This specific destructive action on the eye of short ultra-violet
radiation extends beyond the blank space in the spectrum of
radiation (Fig. 14) and still exists, though possibly to a lesser
extent; in the X-rays.

PHYSIOLOGICAL EFFECTS OF RADIATION.

57

Pathological and Therapeutic Effects of Radiation.
29. Radiation impinging on the tissue of the human body
or other living organisms exerts an influence depending on intensity, power and frequency. The effect on the eye has been discussed in the preceding paragraphs. The specific chemical effect in supplying the energy of plant life will be more fully discussed in the following under chemical effects. As is to be expected, the effect of radiation on the living protoplasm of the cells is stimulating if of moderate intensity, destructive if of excessive intensity; that is, by the energy of the radiation the motions of the parts of the protoplasm-molecule are increased, and, if the intensity of radiation is too high, the mole-
cule thus is torn asunder, that is, destroyed, the living cell
killed and inflammation and necrosis (mortification) result. If the intensity is moderate, merely an increase of the rapidity of the chemical changes in the protoplasm, which we call life,
results; that is, the radiation exerts a stimulating effect, increasing the intensity of life, causing an increased renewal of worn-out tissue and reconstruction, and thus is beneficial or curative, especially where the metabolism is sluggish.
Just as in the action on the eye, two different effects probably exist: a general effect due to the energy of the radiation which
with sunlight is a maximum beyond the visible close to the red
end of the spectrum, and with most artificial illuminants (those
based on incandescence) reaches a maximum still further in
the ultra-red and a specific effect depending on the frequency. The power effect is general and probably fairly uniform
throughout the exposed tissue, appears simultaneous with or immediately after the exposure, and thus practically no danger of harmful results from destruction of tissue exists, as excessive intensity makes itself felt immediately, before far-going destruction of tissue can occur, and, therefore, the only possible danger which could exist would be in the indirect effect of stimulation
on other organs of the body, as the heart. Thus the use of in-
candescent light as stimulant appears fairly harmless. Different is the specific action of high-frequency radiation.
This occurs only some time after exposure, from a few hours to several weeks (with X-rays). As these higher frequencies are not felt by the body as such and exert a powerful action even at such

58

RADIATION, LIGHT, AND ILLUMINATION.

low intensities that their energy is not felt as heat, and, further-
more, the susceptibility of different people may be different,
there is nothing to guard against excessive and thereby harmful exposure. Furthermore, the damage is far more severe and lasting than with the power effect, and fatal cases have occurred
years after exposure. Possibly, as may be expected from
selective action, only a few cells in the living tissue are killed by the radiation, and the disintegration products of these dead cells then gradually involve the surrounding living cells, causing
their destruction or degeneration, so that the harm is far out of
proportion with the immediate destructive effect of the radiation proper, especially with penetrating forms of radiation, as X-rays and radium rays, in which the lesions are correspondingly
deep-seated. High-frequency radiation (violet, ultra-violet, X-ray) should
therefore be used only under the direction of experts fully
familiar with their physiological action and clanger. The specific action of high-frequency radiation is still absent
in the green, begins slightly in the blue and violet, increases into the ultra-violet and persists up to the highest frequencies of
the X-rays. It is shared also by the radiation of the radio-active substances, as the alpha and beta rays of radium. While the
maximum of this effect probably also lies in the ultra-violet,
from one to two octaves beyond the visible spectrum, the
effect is profoundly modified by the transparency or opacity of the tissue for different frequencies, and the character of the stimu-
lating and pathological effects greatly depends on the depth to
which the radiation penetrates the body. The largest part of the organism is water. Water is trans-
parent for visible light, becomes more and more opaque in the ultra-red as well as in the ultra-violet, and is again fairly trans-
parent for X-rays. Blood is fairly transparent for the long visible rays of red and yellow, but nearly opaque for the shorter violet and ultra-violet rays. Hence next to the X-rays which can pass through the body, the longest visible rays of red and yellow penetrate relatively deepest into the body, though even they are practically absorbed within a short distance from the
surface. Thus while the energy maximum of the sunlight is in the ultra-red, the maximum physiological effect probably
is that of the red and yellow rays : the same which are the active

PHYSIOLOGICAL EFFECTS OF RADIATION.

59

rays in plant life. The violet and ultra-violet rays are absorbed

close to the surface of the body by the blood, which is opaque for them. They can thus be made to penetrate deeper as is

done in their therapeutic use by freeing the tissue of the body from blood by compression or other means. Even then, how-

ever, probably only the longest ultra-violet rays penetrate, the

very short ones being kept out by the opaque character of the

water in the tissue.

The penetration of the radiation of the sunlight into the human

body is very greatly reduced by acclimatization, which leads

to the formation of a protective layer or pigment, more or less

opaque to the light. Such acclimatization may be permanent
or temporary. Permanent acclimatization has been evolved

during ages by those races which developed in tropical regions,

as the negroes. They are protected by a black pigment under

the skin, and thereby can stand intensities of solar radiation

A which would be fatal to white men.

temporary acclimatiza-

tion results from intermittent exposure to sunlight for gradually

increasing periods : tanning, and enables the protected to stand

without harmful effects exposure to sunlight which would pro-

duce severe sunburn in the unprotected. This acquired protection mostly wears off in a few weeks, but some traces remain

even after years.
A slight protection by pigmentation also exists in white men,

and its differences lead to the observed great differences in sensitivity to solar radiation: blondes, who usually have very light pigmentation, are more susceptible to sunburn and sunstroke than

the more highly pigmented brunette people. In sunburn we probably have two separate effects super-

imposed upon the other: that due to the energy of the solar radiation and the specific effect of the high frequencies, which to a small extent are contained in the sunlight. The two effects

are probably somewhat different, and the high-frequency effect

tends more to cause inflammation of the tissue, while the energy

effect tends towards the production of pigmentation (tanning),

and the symptoms of sunburn thus vary with the different pro-

portions of energy radiation and of high-frequency radiation as

depending on altitude, humidity of the air, the season, etc.

30. The action of radiation on living organisms is stimulating if of moderate intensity, destructive if of high intensity. Thus

60

RADIATION, LIGHT, AND ILLUMINATION.

it is analogous with that of any other powerful agent or drug, as alcohol, caffeine, etc. The intensity of light which is destructive to life largely depends on the amount of light to which the organism is accustomed. Those organisms which live in the
dark may be killed by an amount of light which is necessary
for the life of other organisms. Amongst the saprophytic bacilli,
for instance (the germs of putrefaction), many species live in the
light, and die, or at least do not multiply, if brought into the dark, while other putrefactive bacilli live in the dark and are killed by light. The latter also is the case with the pathogenic
bacilli, that is, the disease germs, as the bacillus of tuberculosis,
cholera, etc. As these live in the dark, the interior of the body, they are rapidly killed by light. Light, and radiation in general, therefore is one of the most powerful germicides and disinfect-
ants. One of the most effective prophylactic measures, espe-
cially against the diseases of civilization, as tuberculosis, etc., thus
is to flood our homes with light, especially direct sunlight, while
our habit of keeping the light out of our houses by curtains, shades, etc., closing our residences almost light-tight, when leaving them for some time, converts them into breeding places of disease germs, and then we wonder about mortality.
Obviously excessive light intensity ultimately becomes harm-
ful even to the human organism, and it is therefore advisable to protect ourselves against the light when it becomes annoying by its intensity. It has even been claimed that the impossibility of white men to become permanently acclimatized in the tropics
and the change in the temperament of the population of our
country within a few generations from their immigration: the increased nervousness, restlessness and "strenuousness,"
are the result of the greater intensity of the sunlight, especially
its high-frequency radiation, compared with the more cloudy climate of our original European home. Whether this is the
case remains to be further investigated. It is hard to believe,
however, that such a profound effect should result from the exposure of a small part of the body, face and hands, to a more intense light, and the failure of acclimatization in the tropics could well be explained by the higher temperature and its damag-
ing effect, while the change from Europe to America is not merely a change from a more cloudy to a more sunny climate, but from a maritime climate, that is, climate having fairly uniform

PHYSIOLOGICAL EFFECTS OF RADIATION.

61

and slowly changing temperatures, to a continental climate, with its rapid changes of temperature and enormous temperature extremes, and the difference between continental and maritime
climate may be suspected as the cause in the change of the tem-
perament of the races.
As men have lived for ages in the light, the cells of the human
body are far more resisting to the light than the disease germs, which for ages have lived in the dark; and light, and more particularly the high-frequency violet and ultra-violet radiation and the X-rays, thus have found a useful therapeutic application
in killing disease germs in the human body. Thus, by expos-
ing the diseased tissue to high-frequency radiation, the disease
germs are killed, or so far damaged that the body can destroy them, while the cells of the body are still unharmed, but stimulated to greater activity in combating the disease germs. As seen, for this purpose the radiation must be of sufficient intensity and duration to kill or damage the bacilli, but not so intense as to harm the cells of the body. Surface infections, as tuberculosis of the skin (scrofulosis, lupus), thus are effectively and rapidly cured by high-frequency light. More difficult and less
certain the effect is if the infection is deeper seated, as then the radiation must penetrate a greater thickness of tissue to reach the bacilli, and is thereby largely absorbed, and the danger thus exists that, before a sufficient intensity of radiation can be
brought to the seat of the infection, the intensity at the surface
of the tissue may become harmful to the cells of the body. In
this case the more penetrating X-rays would be more applicable, as they can penetrate to any depth into the body. They are, however, so far distant in frequency from the light radiation, that the acclimatization of the body to the light radiation probably exists only to a lesser extent against the X-rays; that is, the difference in the destructive effect on the bacilli and on the cells of the body, on which the curative effect is based, probably is less with the X-rays than with the long ultra-violet waves.
Since Dr. Finsen introduced phototherapy and radiotherapy, some twenty years ago, it thus has found a very extended and
useful field, within its limitation.
This greater destructive action of -radiation on micro-organisms
than on the cells of the human body, extends not merely to the
pathogenic bacilli, but to all organisms living in the dark.

62

RADIATION, LIGHT, AND ILLUMINATION.

Thus the spermatozoa which biologically are independent living organisms seem to be killed by X-rays before any damage is done to the body, and permanent sterility then results. Amongst the cells of the body differences seem to exist
in their resistivity. It is claimed, for instance, that the sensory-
nerves are first paralyzed by violet radiation and that intense violet light can thus be used to produce local anaesthesia, sufficient for minor operations.
Occasionally the effect of light may be harmful in the relation of the human body to invading bacilli. In some eruptive in-
fections, as smallpox, ulceration of the skin (leading to marking) seems to be avoided if the patient is kept from the light, and the course of the disease mitigated. As red light, however, seems to have no effect, instead of perfect exclusion of light, which is not very feasible, the use of red light thus seems to offer an essential advantage.

LECTURE IV.
CHEMICAL AND PHYSICAL EFFECTS OF RADIATION.

Chemical Effects.

31. Where intense radiation is intercepted by a body chemical action may result by the heat energy into which the radiation is

converted. This, however, is not a direct chemical effect of radiation but an indirect effect, resulting from the energy of the

radiation, i

Direct chemical effects of radiation are frequent. It is such an effect on which photography is based : the dissociating action of radiation on silver salts, the chloride in ordinary photographic paper, the bromide and iodide in the negative plate and the quick printing papers. This chemical action is greatest in the violet and ultra-violet and decreases with increasing wave length, hence is less in the green, small in the yellow, and almost absent in the red and ultra-red, so that the short waves, blue, violet and ultra-violet, have sometimes been called "chemical rays." This, however, is a misnomer, just as the term "heat rays" sometimes applied to red and ultra-red rays. In so far as when intercepted they are converted into heat, all rays are heat rays, but neither
the ultra-red nor any other radiation is heat, but it may become
heat when it ceases to be radiation. Thus all radiations are

chemical rays, that is, produce chemical action, if they strike a

body which is responsive to them.

The chemical action of radiation is specific to its frequency and

We seems to be some kind of a resonance effect.

may picture to

ourselves that the frequency of vibration of a silver atom is that

of violet or ultra-violet light, and therefore, when struck by a

wave of this frequency, is set in vibration by resonance, just as a

tuning fork is set in vibration by a sound wave of the frequency

with which it can vibrate, and if the vibration of the silver atom,

in response to the frequency of radiation, becomes sufficiently

intense, it breaks away from the atom with which it is chemically

63

64

RADIATION, LIGHT, AND ILLUMINATION.

combined in the compound, the silver bromide, etc., and this

compound thus splits up, dissociates. The phenomenon, how-

ever, must be more complex, as a simple resonance vibration

would be especially pronounced at one definite frequency, the

frequency of complete resonance, and rapidly decrease for higher and for lower frequencies. The chemical action of radiation on

silver compounds, however, does not show such a response to any

definite

frequency,

but,

while

strongest

in

the

exultraviolet^

tends over the entire range from the frequency of green light

beyond the ultra-violet and up to the highest frequencies of X-rays. That the chemical activity of radiation is some form of resonance, is, however, made very probable by the relation which
exists between the active frequency range and the weight of the

atom or molecule which responds to the radiation. Thus, while

the fairly heavy silver atom (atomic weight 108) responds to rays near the violet end of the visible spectrum, the much lighter oxygen atom (atomic weight 16) responds only to much higher
frequencies, to those of the physiologically most destructive rays, about one to two octaves beyond the visible spectrum. These

very short radiations energetically produce ozone 3 , from oxygen

+ O = 2, probably by dissociating oxygen molecules 2 ,into free atoms,

and these free atoms then join existing molecules :

3

3,

thus forming ozone. Possibly their destructive physiological

action is due to this ability to cause resonance with the oxygen

atom and thereby destroy molecular structures.-
32. Response to the long waves of red and ultra-red light thus
may be expected from atoms or groups of atoms which are very much heavier than the silver atom, and this indeed seems to be

the case in the action of radiation on the life of the plants. There

the response is not by atoms, but by the much heavier groups of
atoms, radicals of carbon compounds, which separate and recom-

bine in response to radiations and thus produce in vegetable
organisms the metabolism which we call life. The action of radiation on plant life thus seems to be a chemi-

cal action, and this would be the most important chemical action, as on it depends the life of the vegetation and thereby also the existence of animal life and, thus, our own. This action by which

the vegetation converts the energy of radiation into chemical

energy is related to the presence of chlorophyl, a green body which exhibits a red fluorescence, I show you here a solution

CHEMICAL AND PHYSICAL EFFECTS OF RADIATION. 65

thereof in alcohol. This use of the energy of radiation occurs only in those parts of the plant in which chlorophyl is present,
usually shown by its green color, that is, in the leaves and young stems* In those plants in which the leaves have lost their chloro-

phyl in taking up other functions as the function of protection

against attack by conversion into spines in the cacti the stems

and trunks have acquired the function of energy supply from

radiation, and show the green color of chlorophyl. When the

leaves die in the fall their chlorophyl disappears and they change

to yellow or red color. Those parts of the plants which contain

chlorophyl, mainly the leaves, take carbon dioxide (C02) from

the air through breathing openings (stomata), absorb the radia-

tion, and convert its energy into chemical energy, and use this

energy

in

splitting

up

or

dissociating

the

C0 2,

exhausting

the

oxygen 3 and using the carbon in producing the complex carbon

compounds of their structure: fiber (cellulose), starch, protoplasm, etc. The energy of plant life thus is derived from radiation and their work is constructive or synthetic, that is, they produce complex chemical compounds from simple ones: the carbon dioxide of the air, the nitrates and phosphates of the soil,

etc. Inversely, the animal organism is analytic, it converts the

chemical energy of complex compounds into mechanical and heat energy by splitting them into simpler compounds, burning them in the lungs or gills. For the supply of mechanical energy which maintains the life, the animal organism thus depends upon the synthetic work of the vegetation by consuming as food the complex compounds constructed by the plants from the energy

of radiation, eith'er directly (vegetarians), or indirectly, by eating

other animals, which in their turn live on the vegetation. Thus,

while

the

plants

take

in

from

the

air

carbon

dioxide

C0 2,

exhaust

the oxygen 2, and convert the C into complex compounds, the

animal takes in oxygen 2, by it burns up the complex carbon

compounds

derived

from the

plants,

and

exhausts

C0 2

as

product

of combustion, but in its ultimate result, all life on the earth de-

pends for its energy on radiation, which is made available in the

plants by conversion to chemical energy and used as such by the

animals.

The radiations which supply the energy of plant life, probably

are the long waves of yellow, red and ultra-red light, while the short waves of blue, violet and ultra-violet cannot be used by the

66

RADIATION, LIGHT, AND ILLUMINATION.

plant, but are harmful, understood: to the long

kill the vegetation. This can easily be waves of red and yellow light the atoms

do not respond, but only the much heavier groups of atoms or car-

bon radicals, and these thus separate and recombine and thereby

constitute what we call life. To very short waves, that is, high

frequencies, these heavy groups of atoms cannot respond, but

single atoms would respond thereto and thus by their separation

break up and destroy the atomic groups. That is, the resonant

dissociation produced by low frequency of radiation extends only to the groups of atoms and thereby results in their separation and

recombination to heavier molecules: life, while the resonant dis-

sociation produced by high frequencies extends to the atom and

thereby splits up and destroys the molecules of the living organ-

ism, that is, death. Therefore the short waves of radiation,

green, blue, etc., which are more or less harmful to plants, are

not used but are reflected by the ehlorophyl; hence the green

color. To some extent violet radiation is absorbed by chloro-

phyl, but it is questionable whether the energy of violet light

directly

contributes

to the

chemical

action,

and

it

rather
is^

probable that the violet radiation is converted into red light by

fluorescence

chlorophyl fluoresces red

and used as red

light. Excessive violet radiation seems to be harmful.

Physical Effects.
33. Some of the most interesting physical effects of radiation are those by which it is converted into another form of radiation:
fluorescence and phosphorescence.
Many substances have the property of converting some of the radiation which is absorbed by them into radiation of a different
wave length, that is, act as frequency converter of radiation, fluorescence. Many bodies when exposed to radiation store some of the energy of radiation in such a manner as to give it out again afterwards and thus, after exposure to light, glow in the darkness with gradually decreasing intensity, phosphorescence. These phenomena probably belong to the least understood effects of radiation. They are very common, but phosphorescence usually lasts such a short time that it can be observed only by special apparatus, although a few bodies continue to phosphoresce for hours and even days. Fluorescence also is usually

CHEMICAL AND PHYSICAL EFFECTS OF RADIATION. 67
so weak as to escape notice, although in a few bodies it is very
strong.
The change of frequency in fluorescence always seems to be a lowering of the frequency, that is, an increase of wave length, and in phosphorescence also the light given out seems always to be of lower frequency than the light absorbed and indeed, fluorescence and phosphorescence seem to be essentially the same phenomenon, radiation is absorbed and its energy given out again as radiation of lower frequency and that part of the returned radiation which appears during the absorption we call fluorescence, that part which appears later, phosphorescence. There is,
however, frequently a change of the color of the light between fluorescence and phosphorescence and also between phosphorescence immediately after exposure to light and some time afterwards. For instance, some calcite (calcium carbonate or limestone) fluoresces crimson, but phosphoresces dark red. The phosphorescence of calcium sulphide changes from blue in the beginning to nearly white some time after, etc.
Due to the change of frequency to longer waves the longest visible rays, red, orange and yellow, produce no fluorescence or
very little thereof, as their fluorescent and phosphorescent radiation would usually be beyond the red, in the invisible ultra-red. Blue, violet and ultra-violet light produce the most intense effects, as a lowering in frequency of these radiations brings them
well within the visible range.
Ultra-violet light is best suited for studying fluorescence as it is not visible, and thus only the fluorescent light is visible; white
light, for instance, does not show the same marked effect, since the direct white light is superimposed upon the light of fluorescence. Most brilliant effects, however, are produced by using a source of light which is deficient in the frequencies given by fluorescence and then looking at the fluorescent body through a glass having the same color as that given by fluorescence. Thus the least traces of red fluorescence can be discovered by looking at the body through a red glass, in the illumination given by the mercury lamp. As the mercury lamp contains practically no red rays, seen through a red glass everything appears nearly black or invisible except red fluorescent bodies, which appear self-luminous, glowing in a light of their own, and appear like red hot
bodies.

68

RADIATION, LIGHT, AND ILLUMINATION.

In the illumination given by the mercury lamp I here drop a

B few drops of a solution of rhodamine 6 G, rhodamine

and

uranine (aniline dyes) into a large beaker of water. As you see,

when sinking down and gradually spreading, they appear
especially against a dark background as brilliant luminous clouds of orange, red and green, and seen through a red glass
they appear like clouds of fire. I change to the illumination' given by the incandescent lamp and all the brilliancy disappears,
fluorescence ceases and we have a dull red colored solution. I

show you here the sample card of a silk store of different colored silks. Looking at it through a red glass, in the mercury light all
disappear except a few, which you can pick out by their luminosity : they are different colors, pinks, reds, heliotrope, etc., but all containing the same red fluorescent aniline dye, rhodamine.
A glass plate coated with a thick layer of transparent varnish,
colored by rhodamine, appears like a sheet of red hot iron in the mercury light, especially through a red glass, while in the light of

the incandescent lamp it loses all its brilliancy.
G This solution of rhodamine 6 in alcohol, fluoresces a glaring

orange in the mercury light, in the light of a carbon arc lamp (or in daylight) it fluoresces green and less brilliant. Thus you see that the color of the fluorescent light is not always the same, but
depends to some extent on the frequency of radiation which

causes the fluorescence.

Here I have a sheet of paper covered with calcium sulphide

and a lump of willemite (zinc silicate) and some pieces of calcite. As you see, none of them show any appreciable fluorescence in the mercury light. But if I turn off the mercury light, the

calcium sulphide phosphoresces brightly in a blue glow, the others
do not. Now I show you all three under the ultra-violet rays of

the condenser discharge between iron terminals, or ultra-violet

lamp (Fig. 11) and you see all three fluoresce brilliantly, in blue,

green and red. Turning off the light all three continue to glow with about the same color, that is, phosphoresce, but the red

fluorescence of the calcite very rapidly decreases, the green glow

of the willemite a little slower, but the blue glow of the calcium
my sulphide screen persists, decreasing very little. I now hold
hand back of it and close to it and you see the picture of the hand

appear on the screen by an increase of the luminosity where by contact with the hand the temperature of the screen was slightly

CHEMICAL AND PHYSICAL EFFECTS OF RADIATION. 69

raised, thus showing the effect of the temperature rise in increas-

ing phosphorescence.

These substances which I show you, calcium sulphide, cal-

cium carbonate (calcite), zinc silicate (willemite), are not fluo-

rescent or phosphorescent themselves, but their luminescence is

due to a small percentage of some impurities contained in them.

Chemically pure substances and concentrated solutions of the

aniline dyes, or these dyes in their solid form, do not show the

luminescence, but only when in very dDuted solutions; that is,

luminescence as fluorescence and phosphorescence seems to be

the property of very diluted solutions of some substances in

others. Thus a sheet of paper or cardboard colored red by

rhodamine does not fluoresce, but if a small quantity of rhoda-

mine is added to some transparent varnish and the paper colored

red by a heavy layer of this varnish it fluoresces brightly red.

To show you the fluorescent spectrum, I have here a mercury

lamp

surrounded

by

a

very

diluted

solution

of

rhodamine

6

G ;

and some rhodamine R, contained between two concentric glass

cylinders. As you see, through the spectroscope a broad band

appears in the red and the green light has faded considerably.
You also notice that the light of this lamp, while still different

from white light, does not give anything like the ghastly effect of

human faces, as the plain mercury lamp, but contains considerable

red rays, though not yet enough. I also show you a mercury

lamp surrounded by a screen of a very dilute solution of uranine: you see, its light is bright greenish yellow, but much less ghastly

than the plain mercury light and the spectroscope shows the

mercury lines on a fluorescent spectrum, which extends as a con-

tinuous luminous band from the green to and beyond the red.

You also see that with this uranine screen the mercury lamp

gives more light than without it: considerable of its ultra-violet and violet light is converted to yellow and thereby made visible
or more effective.

LECTURE V.

TEMPERATURE RADIATION.

34. The most common method of producing radiation is by
impressing heat energy upon a body and thereby raising its tem-
perature. Up to a short time ago this was the only method available for the production of artificial light. The temperature is
raised by heating a body by the transformation of chemical energy, that is, by combustion, and in later years by the transformation of electric energy, as in the arc and incandescent

lamp.
With increasing temperature of a body the radiation from the
body increases. Thus, also, the power which is required to maintain the body at constant temperature increases with increase of
temperature. In a vacuum (as approximately in the incandescent lamp), where heat conduction and heat convection from the
radiating body is excluded, all the power input into the body is radiated from it, and in this case the power input measures the

power of the radiation. The total power or rate at which energy is radiated by a heated
black or grey body varies with the fourth power of its absolute

temperature, that is,

A If

=

surface

area,

T l

=

absolute

temperature

of

the

radia-

tor and

T = absolute 2

temperature of

the surrounding objects

on which the radiation impinges : the total power radiated by the

body is (Stefan's Law) :

Pr - kA (T* - ?Y),

(1)

P where for a black body, as the carbon filament with r given
in watts per square cm., k is probably between

5 x 10-12

and

x 6 10~12 ;

(2)

T 2

is

usually

atmospheric

temperature

or

about

300

degrees

abs.

"if

T t

does

not

differ

much

from

T 2,

that

is,

when

considering

the radiation of a body raised slightly above the surround-

70

TEMPERATURE RADIATION.

71

ing temperature, as an electric machine, equation (1) can be

written:

+ P + r = kA (2\ - T2) (T*

T2

2

T, 2 -h T,T2

ra );

or, approximately,

Pr - 4 kAT* (T, - T),

(3)

where T is the room temperature (Tl T) the temperature rise
of the radiator above room temperature; that is, for moderate

temperature differences the radiation power is proportional to

the temperature rise.

This equation (3) gives the law generally used for calculating

temperature rise in electric machinery and other cases where the

temperature rise is moderate. Obviously, in air the power given
P off by the heated body, P, is greater than the power radiated, r,

due to heat convection by air currents, etc., but as heat conduc-

tion and convection also are approximately proportional to the

temperature rise, as long as the latter is moderate, equation (3)

can still be used, but with the numerical value of k increased to

& so as to include the heat conduction and convection: in

= t

stationary

air

& x

reaches

values

as

high

as

k^

25 X 10~12 to

X 50

1(T 13 .

As soon, however, as the temperature rise (Z\

T7 )

becomes

comparable with the absolute temperature T, the equation (3)

can no longer be used, but the complete equation (1) must be

used, and when the temperature of the radiator, Tv is very much

greater

than

the

surrounding temperature

!T2 ,

T4 3

becomes

negli-

gible compared with T* and equation (1) can, for high tempera-

tures, thus be approximated by:

Pr - fcATV;

(4)

That is, the radiation power, as function of the temperature, gradually changes from proportionality with the temperature rise, at low temperature rise, to proportionality with the fourth
power of the temperature for high temperature rises. Inversely then, with increasing power input into the radiator
and thus increasing radiation power, its temperature first rises proportional to the power input and then slower and ultimately approaches proportionality with the fourth root of the power

output:

TJl

=

VV

-
kA

72

RADIATION, LIGHT, AND ILLUMINATION.

In Fig. 27 is shown the radiation curve, with the temperatures
T as ordinates and the radiated power Pr as abscissas, the upper

curve with 100 times the scale of abscissas.

Thus, to double the temperature rise, from 10 deg. cent, to 20

deg. cent., requires doubling the power input. To double, how-

ever, the temperature rise, from 1000 deg. cent, to 2000 deg. cent.,

requires

an

increase

of

the

power

input

from

12734

to

22734 ,

or

more than ten fold. At high temperature the power input, there-

fore, increase enormously with the increase of temperature.

FIG. 27.
With bodies in a vacuum, the radiation power is the power
input and this above law can be used to calculate the temperature of the radiator from the power input. In air, however, a large part of the energy is carried away by air currents, and this part of the power does not strictly follow the temperature law of radiation, equation (1). For radiators in stationary air (that is, not exposed to a forced blast, as the centrifugal blast of revolving machinery), the total power input for high temperature (as expended by radiation and heat convection) varies with a high power of the temperature, so that the radiation law equation (1) can still be used to get a rough approximation of the
relative values of temperatures.
It, therefore, is not permissible to assume the temperature rise as proportional to the power input as soon as the temperature

TEMPERATURE RADIATION.

73

rise is considerable and even in electrical apparatus of fire-proof
construction as some rheostats, etc., where a higher temperature
rise is permitted, the calculation of this temperature rise must be approximated by the general law (1) and not the law of proportionality (3), as the latter would give entirely wrong results. For instance, assuming a temperature rise of 50 deg. cent, per
watt per sq. in. a cast silicon rod, which at bright incandescence can dissipate 200 watts per sq. in. would give by (3), a
temperature rise of 10,000 deg. cent. This obviously is impossible, as silicon melts at about 1400 deg. cent.
35. With increasing temperature of the radiator, the intensity of the radiation increases, and at the same time the average
frequency of radiation also increases, that is, the higher frequen-
cies increase more rapidly than the lower frequencies and higher
and higher frequencies appear, until ultimately frequencies are reached where the radiation becomes visible to the eye, as light.
When with increasing temperature the radiation just begins to be
visible, it appears as a faint colorless grey, "gespenster grau" exhibiting the same weird and indistinct appearance as are seen at higher intensities in the monochrome blue and violet radia-
tions : that is, we see a faint grey light, but when we look at it, it
has disappeared : the reason is that the sensitivity of the sensitive spot of the eye for very faint light is less than that of the surrounding retina and the first glimmer of light thus disappears as soon
as we focus it on .the sensitive spot. With increasing temperature, first the lowest of the visible frequencies appear and become
visible as red light, and with still further increase of temperature gradually orange, yellow, green, blue, violet and ultra-violet rays appear and the color thus changes from red to orange, yellow, yellowish white and then white, the latter at that temperature where all the visible radiations are present in the same propor-
tion as in daylight. With still further increase of temperature, the violet end of the spectrum would increase faster than the red end and the light thus shift to bluish white, blue and violet.
The invisibility of the radiation of low temperature is not due to low intensity. I have here an incandescent lamp at normal brilliancy. If I decrease the power input and thereby the radiated power to fa it becomes invisible, but if we move away from the lamp to 10 times the previous distance, we get only Tta the radiation reaching our eyes and still the light is very plainly

74

RADIATION, LIGHT, AND ILLUMINATION.

visible. The invisibility in the former case, thus , is not due to low

intensity, but to low frequency. The fraction of the total radiation, which is visible to the eye

as light, thus increases with the increasing temperature, from zero at low temperature where the radiator does not give sufficiently high frequencies to be visible and very low values
when it just begins to be visible as red light, to a maximum at that
temperature where the average frequency of the radiation is in the visible range, and it would decrease again for still higher

temperature by the average frequency of radiation shifting beyond the visible into the ultra-violet. The efficiency of light
production by incandescence thus rises with increasing temperature to a maximum, and then decreases again. If the total

radiation varies with the fourth power of the temperature, it thus

follows that the visible radiation first varies with a higher power

of the temperature than the fourth, up to the maximum efficiency
point, and beyond that increases with less than the fourth power
of the temperature. The temperature at which the maximum

efficiency of light production by incandescence occurs, that is, where the average frequency of temperature radiation is in the visible range, probably is between 5000 and 8000 cleg. cent, and as the most refractory body, carbon, boils at 3750 cleg, cent., this

temperature thus is unattainable with any solid or liquid radiator. Most bodies give approximately the same temperature radia-
tion, that is, follow the temperature law (1), differing only by the

numerical value of the constant k; that is, with increase of

temperature the radiation intensity increases and the average frequency of radiation increases in the same manner with most solid and liquid bodies, so that at the same temperature all the
bodies of normal temperature radiation give the same radiation

curve; that is, the same distribution of intensity as function of

the frequency and thus the same fraction of visible to total radia-

tion, that is, the same efficiency of light production.
If T is the absolute temperature in deg. cent, and lw the wave

length of radiation, the power radiated at wave length lw and

temperature

T l

by

normal

temperature

radiation

is:

r V a
P (lw) - c,AlJ

, (Wien's law) ;

or,

/
P (k) - c.AlJ {

V - &_

j-i

1j

(Planck's law) ;

TEMPERATURE RADIATION.

T5

where a = 5 for normal temperature radiation or black body

= A = radiation; b 1.42, and

surface area of the radiator.

Integrating the formula of Wien's law over lw from to >,

gives the total radiation :

"
thus, for a 5;

P = cAT*;

or, Stefan's law, as discussed above.
The maximum energy rate at temperature T occurs at the wave length lw - lm given by :
dP (lw) __
'
dlw
which gives :

lmT -1-0.284;

or,

_ 0.284

I'm

m>

lm = 50 X 10"8 thus gives:

T

=

284-
^p-

=

5680

deg.

abs.

With normal temperature radiation the efficiency of light production is thus merely a function of the temperature and does not depend upon the material of the radiating body, provided
that the material is such as to withstand the temperature.
As the efficiency maximum of normal temperature radiation is
far beyond the attainable, within the range of temperature available up to the boiling*point of carbon, the efficiency of light production by incandescence continuously increases, but even then the octave of visible radiation is at the far upper end of the radia-
tion curve, and thus the problem of efficient light production is to operate the radiator at the highest possible temperature.
The efficiency of light production is rather low even at the
maximum efficiency point, that is, with the average frequency of
radiation in the visible range, since this visible range is less than one octave; under these most favorable conditions the visible

76

RADIATION, LIGHT, AND ILLUMINATION.

energy probably does not much exceed 10 per cent of the total
radiation, the rest falls below and above the visible frequencies. 36. At the highest attainable temperature, the boiling point
of carbon, the efficiency is much lower, probably below 10 per cent and this would be the highest efficiency attainable by normal
temperature radiation. It is utilized for light production in the
carbon arc lamp. The carbon arc flame gives practically no light, but all the light comes from the incandescent tips of the carbon electrodes, mainly the positive, which are at the boiling point of carbon and thus give the most efficient temperature
radiation.
Obviously, in the carbon arc lamp a very large part of the energy is wasted by heat conduction through the carbons, heat convection by air currents, etc,, and the total efficiency of the carbon arc lamp, that is, the ratio of the power of the visible radiation to the total electric power input into the lamp, thus is much lower than the radiation efficiency, that is, the ratio of the
power of the visible to the total radiation. Thus the efficiency of the carbon arc is considerably increased
by reducing the loss by heat conduction, by the use of smaller
carbons the life of the carbons, however, is greatly reduced
thereby, due to their more rapid combustion. The carbon arc lamp thus gives the most efficient incandescent
light, as it operates at the highest temperature, the boiling point
of carbon. But by doing so the radiator is continuously consumed and has to be fed into the arc. This requires an operating
mechanism and becomes feasible only with large units of light.
To attain the highest possible efficiency of light production by
temperature radiation with a permanent radiator, thus requires the use of extremely refractory bodies, since the efficiency increases with the increase of the temperature, and is still very
low at the melting point of platinum.
To exclude all the losses of energy by heat conduction and
heat convection, the radiator is enclosed in a vacuum, so that all the power input is converted into radiation. Even in this case
the efficiency of light production is still relatively low.
The vacuum used in the incandescent lamp, thus, is not only for the purpose of protecting the filament from combustion. Filling the globe with some gas which does not attack the carbon would do this and yet it would very greatly lower the efficiency,

TEMPERATURE RADIATION.

77

as can be seen by admitting air into the lamp bulb, when the filament drops down to dull red heat, before it burns through.
However, the presence of an indifferent gas of low heat capacity
may lower the evaporation of the filament and so permit operation
at higher temperature, and the gain in efficiency more than makes
up for the increased losses, as in the gas filled tungsten lamps.
A search, thus, has been made and is still being made, through-
out the entire range of existing bodies, for very refractory mate-
rials. Such materials may be chemical elements or compounds.
However, the combination of a refractory element with one of
very much lower melting point lowers its melting point, and very
refractory compounds, thus, may be expected only amongst the
combinations of very refractory elements with each other.
The chemical elements, arranged in order of their atomic weight, exhibit a periodicity in their properties which permits

FIG. 28.
a systematic study of their properties. In diagram Fig, 28 the elements are arranged in order of their atomic weight in the
77
"periodic system.
The height of their melting point is indicated by the darkness of the background. That is, the most refractory elements, wolfram and carbon, are shown on black background. The elements of somewhat lower melting point are shown on cross
shaded background. Inversely, the elements of the lowest melting point, mercury under the metals and helium under metal-

78

RADIATION, LIGHT, AND ILLUMINATION.

loids, are shown on white background, and the easily fusible metals and gaseous metalloids on lightly shaded background.
As seen, there are two peaks of refractoriness, one amongst the metalloids, in carbon, and one under the metals in wolfram (or tungsten), and around these two peaks all the refractory elements are grouped. Inversely, there are also two depressions, or points of minimum melting point, in helium under the metalloids, around which all the gaseous elements are grouped, and in mer-
cury under the metals, around which all the easily fusible metals
are grouped. It is interesting to note that the melting point rises towards
wolfram from both sides, as diagrammatically illustrated at the
top of Fig. 28, in such a manner that the maximum point
should be expected in the space between wolfram and osmium and the unknown element, which belongs in this space of the
periodic system, thus should be expected to have still a higher melting point than wolfram, and thus give a higher efficiency of

light production.
As metal alloys almost always have lower melting points than

their most refractory element, very refractory compounds thus

may be expected only in the compounds between the very refrac-

tory elements, in which at least one is a metalloid, that is, amongst

the carbides and borides and possibly silicides and titanides.

37. Some of the earliest work on incandescent lamps was

carried out with metal filaments. Platinum and iridium, how-

ever, were not sufficiently refractory to give good efficiencies, and

the very refractory metals were not yet available in sufficient

A purity.

small percentage of impurities, however, very greatly

lowers the melting point, especially with metals of very high

atomic weight. For instance, wolfram carbide contains only

3 per cent of carbon and 97 per cent of wolfram and even 0.1 per

cent of carbon in wolfram metal thus means that over 3 per cent

of the metal consists of the easily fusible carbide.
Very soon, therefore, metal filaments were abandoned and carbon used as lamp filament. While carbon is the most refractory body, remaining solid up to 3750 deg. cent., it was found that the
carbon filament could nbt be operated much above 1800 deg. cent,
without shortening the life of the lamp below economic limits by the evaporation of the carbon and the resulting blackening of the lamp globes. All bodies evaporate below their melting point.

TEMPERATURE RADIATION.

79

Thus water evaporates considerably below the boiling point and even below the freezing point : ice and snow gradually disappear by evaporation even if the temperature never rises above the melting point. Considerable differences, however, exist between
different bodies regarding their rate of evaporation. Thus water and benzine have practically the same boiling point, but at the same distance below the boiling point, benzine evaporates much faster than water; that is, has a much higher vapor tension. Carbon has a very high vapor tension, that is, shows a very rapid evaporation far below the boiling point, and since in the incandescent lamp the carbon vapor condenses and is deposited on the globe and carbon is black, it blackens the globe and obstructs the
light. Also, the decrease of the filament section by evaporation increases its resistance and thereby decreases the power consumption and so still further lowers the efficiency. While, therefore, carbon remains solid up to 3750 deg. cent., at about 1800 deg. cent, its rate of evaporation is such as to lower the candle power of the lamp by 20 per cent in 500 hr. life, and at this temperature it gives only an output of one candle power for 3.1 watts
input. Operating the carbon filament at higher temperature would increase the efficiency and thus reduce the cost of energy
for the same amount of light, but would decrease the useful life
of the lamp and, therefore, increase the cost of lamp renewals, and the most economical operation, as determined by balancing the cost of lamp renewals against the cost of energy, is reached by operating at such temperatures that the candle power of the lamp decreases by 20 per cent within 500 hr. life. The life of a lamp down to a decrease of candle power by 20 per cent, thus, is called the useful life, and when comparing the efficiencies of incandescent lamps it is essential to compare them on the basis of the same length of useful life: 500 hours with the carbon filament, since obviously by shortening the life higher efficiencies could be reached in any incandescent lamp. The operating temperature of the carbon filament lamp, thus, was limited by the vapor tension of carbon and not by its boiling poi^itl
This limitation of carbon lead to the revival of the metal fila-
ment lamps in recent years. First arrived the osmium lamp, with 1.5 watts per candle power. The melting point of osmium is very high, but still very much below that of carbon, but the vapor tension of osmium is very low even close to its melting point, so

80

RADIATION, LIGHT, AND ILLUMINATION.

that osmium could be operated at temperatures far closer to its

melting point without appreciable evaporation; that is, without

blackening

and

falling

off

of

candle

power,

or ;

in

other

words,

could be run at a temperature from which carbon was excluded

by its too rapid evaporation. Osmium, however, is a very rare

metal of the platinum group, and found only in very limited

quantities in very few places and is one of those substances of

which no search could very greatly increase the supply, and while

one pound of osmium is sufficient for some 60,000 filaments, the

total amount of osmium which has ever been found on earth

would not be sufficient for one year's supply of incandescent lamps. Osmium, therefore, was excluded from general use by its

limited supply.
The metal tantalum does not seem to have quite as high a melt-

ing point as osmium, as it can be operated only at 2 watts per candle power. Tantalum also is a very rare metal, but, unlike
osmium, it is found in very many places, though in small quan-
tities, but it is one of those substances, like the rare earth metals used in the Welsbach mantle, of which it seems that the supply
could be indefinitely increased when required by the industries and the prices thus would go down with the demand, just as has been the case with the rare earths of the Welsbach mantle.

Last of allj however, was made available the most refractory of
all metals, wolfram or tungsten, and permitted to lower the specific consumption to 1 to 1.25 watts and finally, in the gas filled lamp, to less than 0.5 watts per candle power. Wolfram melts far lower

than carbon, probably at about 3200 deg. cent., but far above the

temperature to which the carbon filament is limited by evaporation, and having practically no vapor tension below its melting
point, it can be operated far above the temperature of the carbon
filament, and thus gives a much higher efficiency. Tungsten (or rather wolfram, as the metal is called, tungsten is the name of its ore) is a fairly common metal, its salts are industrially used to a very large extent for fire-proofing fabrics and its supply practically

unlimited.

These metal filaments thus differ from the carbon filament in

that their temperature is limited by their melting point and not by evaporation, as is the case with the carbon filament, and thus their useful life is usually ended by the destruction of the filament by melting through at some weak spot, but not by blackening.

TEMPERATURE RADIATION.

81

These filament lamps do not blacken the globe, except when the vacuum is defective or becomes defective, and by the residual gases in the lamp globe volatile compounds are formed, as tungsten oxides, which then deposit on the globe and terminate the life of the lamp. Even then their blackening is characteristically different from that of the carbon filament, in that it occurs very rapidly, and the lamp, after running possibly for hundreds or
thousands of hours without blackening, suddenl}7 blackens within a few days and thereby becomes inoperative, while with the carbon filament the blackening is gradual throughout the life.
38. By the use of these refractory metals the efficiency of
light production by temperature radiation has been greatly increased, by permitting the use of higher temperatures in the radiator than were permissible with the carbon filament clue to
its evaporation. However, regarding the rate of evaporation, different modifications of carbon show very different characteristics. The carbon filaments first used in incandescent lamps were
made by carbonizing vegetable fiber, as bamboo, or by squirting
a solution of cellulose through a small hole into a hardening solution and carbonizing this structureless horn-like fiber. These filaments had a very high vapor tension, thus could not be run as hot as the modern carbon filament and so gave a lower effi-
ciency. They are now used only as base filaments, that is, as
core on which a more stable form of carbon is deposited. Such a form of carbon was found in carbon deposited on the filament by heating it in the vapor of gasolene or other hydrocarbons. This carbon deposit is of much lower electric instance than the base on which it was deposited, its negative temperature coefficient of electric resistance is lower and its vapor tension so much lower as to make it possible to operate the lamp at a specific consumption of 3.1 watts per candle power. Of late years a still more stable form of carbon has been found in the so-called "me-
tallic carbon," produced from the gasolene deposited carbon shell of the filament, by exposing it for several minutes to a temperature at the boiling point of carbon; that is, the highest attainable temperature in an electric carbon tube furnace. Hereby the
gasolene deposited carbon of the filament shell the inner base
does not appreciably change its characteristics acquires metallic characteristics: a low electric resistance, a positive tempera-

82

RADIATION, LIGHT, AND ILLUMINATION.

ture coefficient of electric resistance, metallic luster and elasticity
and very low vapor tension, so that it can be run at higher temperature corresponding to a specific consumption of 2.5 to 2.6 watts per candle power, with very little blackening. These metallized carbon filament lamps exhibit characteristics similar to the
metal filament lamps; their life is largely limited by 'breakage
and not by blackening. Whether hereby the possibilities of carbon are exhausted or
still more stable forms of carbon will be found, which permit
raising the filament temperature as near to the boiling point of carbon as the temperature of the wolfram filament is to its melting point * and thereby reach an efficiency superior to that of the tungsten lamp, remains to be seen, but does not appear entirely
impossible. Carbon exists in a number of "allotropic" modifi-
cations of very different characteristics (similar to phosphorus in "yellow phosphorus," "red phosphorus" and "metallic phosphorus") to a greater extent than any other element, probably
due to the tendency of the carbon atom to join with other carbon atoms into chains and rings, which tendency is the case of the infinite number of carbon compounds. These form two main groups: the chain carbon derivates (methane-derivates) and the ring carbon derivates (benzol derivates). The latter are far more stable at high temperatures, since the breakage of the molecule by temperature vibration is less liable in a ring structure than a chain : a single break splits the molecule in a chain forma-
tion, while with a ring formation it still holds together until the
break closes again. Chain hydrocarbons at higher temperatures usually convert to ring hydrocarbons. It is, therefore, reasonable
to assume that the carbon skeleton left by the carbonization of
the hydrocarbons also -may exist in either of the two characteristic atomic groupings : as chain carbon and as ring carbon, and
that the latter exhibits a much greater stability at high tempera-
ture than the former, that is, a lower vapor tension. Cellulose
is a chain hydrocarbon, and as in carbonization it never passes through a fluid state, the molecular structure of its carbon atom probably remains essentially unchanged. Thus the base fila-
* As carbon boils, at atmospheric pressure, below its melting point, and the limiting temperature is that at which the filament ceases to be solid, with carbon the limit is the boiling point temperature, while with tungsten it is
the melting point.