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No. 203. VoL. XXXIV.

TOVEMBER, 1887.

B1tabli1hed by BBJIJ'AKIJI SILLIKAJI in 1818.

THE
AMERICAN
JOURNAL OF SCIENCE.
EDITORS
JAMES D. AND EDWARD S. DANA.
ABSOOUTB BDITOBS
Paoussou ASA GRAY, JOSIAH P. COOKE, AND JOHN TROWBRIDGE, OP OAllBR1DGB,
PaoHsso:as H. A. NEWTON AND A. E. VERRILL, OF
NBW HAVBN,
Pao.-1H&OB GEORGE F. BARKER, 011 PutLADBLPHU..
TJlIBD BBBIBS,
VOL. XXXIV. -[WHOLE NUMBER, OXXXIV.J
l't'ITR PL.U'll:8 II to IX,
No. 203-NO'VEMBER, 1887. NEW HAVEN, OONN.: J. D. & E. 8. DANA.
1887.
TUTTLB, IIOBSBOU&B • TA.Tl.OB, PBllfTBB8, 1'71 &TATII 8TR:S:ST,
az dollan per JW1 (poaap prepaid). '6,'41 &o foreign 1ubaoribera of oountrlea in
._ Poeal Union. Rem.l~oe• 1hould be made either b7 mone7 orden, r91i1wred '-'wft, or bulk oheoka.

THE
AMERICAN JOURNAL OF SCIENCE.
[THIRD SERIES.]

ART. XXXVI.-On the Relative Motion of the Earth and the

w. Luminiferous Ether; by ALBERT A. MICHELSON aod

EDWARD

MORLEY.*

THE discovery of the aberration of light was soon followed
by an explanation according to the emission theory. The effect was attributed t.o a simple composition of the velocity of light with the velocity of the earth in its orbit. The difficulties in this apparently sufficient explanation were overlooked until after an explanation on the undulatory theory of light was proposed. This new explanation was at first almost as simple as the former. But it failed to account for the fact proved by experiment that the aberration was unchanged when observations were made with a telescope filled with water. For if the tangent of the angle of aberration is the ratio of the velocity of the earth to the velocity of light, then, since the lotter velocity in water is three-fourths its velocity in a vacuum, the aberration observed with a water telescope should be fourthirds of its true value.t
t* Thill reaearch was carried out with the aid of the Bache Fund. It may be noticed that most writers admit the eulllciency 01 Ule erplanation according to the emiMiou theory of light; while io fact the dU!lculty ia even greater than according to the undulatory theory. For on the emiulon lheory the velocity of light must be greater io the water teleBCOpe, and therefore the an«ll! of 1berratioo lhould be •-; henoe, in order to reduce it to its I.rue nlue,-we muat mue the abaurd hypothesia that the motioo of the water in the teleeoope carriee the rr.7 of light in the oppoeile direc&ion !
All. JoDB. Sot-TBUU> 8011111, Vor. XX.XIV, No. 20!1.-Nov., 188T. 22

.Micli~uon and Morky-Motion of the Earth, etc.

On the undulatory theory, according to Fresnel, first, the ether

is supposed to be at rest except in tbe interior of transparent

media, in which secondly, it is supposed to move with a velociy

1, less

than

the

velocity of

t.he medium

in

the rat.io

n':-
n

where

n is the index of refraction. These two hypotheses give a com•

plete and Bll.tisfactory expl1tnation of aberration. The second

hypothesis, notwithstanding its seeming improbability, must be

considered as fully proved, first, by the celebrated experiment of

Fizeau,* and secondly, by the ample confirmation of our own

work.t The experimental trial of the first hypothesis forms

the suhject of the preseut paper.

If tl,e earth were a transparent body, it might perhaps be

conceded, in view or the experiments just cited, that the inter-

molecular ether was at rest in space, notwithstanding the mo-

tion of the earth in its orbit; but we have no right to ex-

t.end the conclusion from these experiments to opaque bodies.

But there can hardly be question that the ether can and does

p888 through met~s. Lorentz cites the ill ustrntion of a metallic

barometer tube. When the tube is inciined the ether in the

space above the mercury is certainly forced out, for it is im-

compressible.t But again we bnve no right to assume that it

makes its escape with perfect freedom, and if there bti any resist-

ance, however slight, we certainly could not assume an opaque

l,ody such as the whole earth to offer free passage through its

entire ma11t1. But as Lorentz aptly remarks: "quoi qui'I en
soit, on 'fera bien, amoo avis, Je ne pas se laisser guider, dans

une question aussi import.ante, par des consid.Srations sur le

degre de probabilit.S ou de simplicit.S de l'une ou de l'nutre
hypothese, mais de s'addrPsser a 1'exp6rience. pour apprendre a

connaitre 1'6tat, de repos ou de mouvement, dans lequel se
trouve l'ether ala surface terrestre."§

In April, 1881, a method was pror osed and carried out for

testing the 9.uestion experimentally.

In deducmg the formula for tne quantity to be measured,

the effect of the molion of the earth through the eth1,r on the

path of the ray at right angles to this motion was overlooked,1

• Comptes B.endu1, :nnli. 349, 1861 ; Pogg. Ahn. Ergil.nzungaband, ,ii, -ill'r,

1863 ; Ann. Chim. Phys., Ill, !vii, 886, 1859.

mt, t

Influence of llotion of
xu:i, an, \886.
It may be objected that

the Medium on the Velocity of 'Light. Thie Journal, it·may escape by the space between the mercury and

the walls i but ills oould be prevented by amalgamatiug the walls.

§ Arehn·es Neerl.andaiaea, nl, 2• • livr.

(The relative motion of the earth and the luminiferoue et.her, by Albert A.

Klcheleon, this Jour.. Ill, z:ii:11, 120.
,. n mAy be mentioned here that the el'l'Or was pointed o ~ the author of the

former paper by K. A. Potier, of Paris, iu the winter of 1881,

ltlichelaon and .Morley-Relatilv8 .Motwn, of tM 835

The disca888ion of this oversight and of the entire experiment forms the subject of a very searching analysis by H. A. Lorentz,* who finds that this effect can by no means be disregarded. In consequence, the quantity to be measured had in fact but one-half the value supposed, and as it was already barely beyond the limits. of errors of experiment, the conclusion drawn from the result of the experiment might well be questioned; since, however, the main portion of the theory remains un-
ques~ione~, it was deoid~ to repeat the ~xperiment with such mod1ficat1ons as would msure a theoretical result much too large to be masked b.r experimental errors. The theory of the
method may be briefly stated as follows : Let sa, fig. 1, be a ray of light which is partly reflected
in ab, and partly transmitted in ac, being returned by the mirrors b and c, along ba and ca. ba is partly transmitted along ad,

b,I b

I

b

I

I

I

I

I

I

I

8

I

d

8

1.
C
C

2.

and c:a is partly reflected along ad. If then the paths ab and ac are equal, the two rays interfere along ad. Suppose now, the ether being at reat, that the whole apparatus moves in the di-
rection ac, with the velocity of the earth in its orbit, the direc-

• De l'lnfluenoe
JNldaiw, m, 2-

du Jloavemeni
uvr., 1886.

de

la

Terre

1111r

lee Pheu.

Lum.

A.rehiYee

N4er-

836

./!,a,rt and tM .LumwfM'Om Ethw.

tions and distances traversed by the rays will be altered thus:­

ThP. ray sa equal to the goes to the

faoibscuerrsrefoaltIeicottnheed=taa,elloeissncgroeptauebr, , nwfeihdgo. as2elo;dngitrhebecat,,aion(nagbilaes,

u=ban2ha,altb), eeraiennddg.

caTrtheindhfedleiendcctgaitrf,aefw,ednrdsieatmonht cianttetho1, ietemds fnaorirokafswyit ntrghgmeoaycee.aess,teeacIeleootxnmnaqdugcaatlyoaly9rcbd, 0ieein-srrtraieh,meettahuasnirrsdknameedtddehoeeatprshleoafoinnnt ogorttethacesaa,f,t,irfl,tleaahcyocntsdutobgihniaehs•,

validity of the reasoning. Let it now be required to find the

difference in the two paths aba,, and aca,.

= Let V velocity of light.

v=velocity of the earth in its orbit.

D=distance ab or ac, fig. 1.

T=time light occupies to pass from a to c.

T =time light occupies to return from c to a,, (fig. 2.)

Then T=v-D-v, , T, =vD+ti . The whole time of going and com· ing is T+T,=2D V;-v ., and the distance traveled in this time
v· ( is 2Dv•-v• = 2D 1 + vti'• ), neglecting terms of the fourth order.
✓ The length of the other path is evidently 2D 1+<2B>,, or to the

same degree of accuracy, 2n(1+ 2<>1). The difference is there• fore Dyv',· If now the whole apparatus be turned through 90°, the difference will be in the opposite direction, hence the dis­

placement of the interference fringes should be 2n;:. Oon•

sidering only the velocity of the earth in its orbit, this would

be 2Dx10-•. If, as was the case in the first experimens,

D=2Xl0• waves of yellow light, the displacement to be

expected would be 0·04 of the distance between the interference

fringes.

In the first experiment one of the principal difficulties en­

counte<EFBFBD> was that of revolving the apparatus without produ­

cing distortion; and another was its extre.me seneitiveneea to

vibration. This was so great that it was impossible to see the

interference fringes eJEcept at brief intervals when working in

the city, even at two o'clock in the morning. Finally, as be­

fore remarked, the quantity to ment of something less than

be observed, a twentieth

nofamtheelyd, iastdainscpelabcee­•

tween the interference fringea may have been too small to be

detected when masked by experimental errors.

Michelaon and JlO'l'ky-Belativd .Motwn of the 38'7

atodwic1klmftnihihofi·eggos5aeegenTeT.hostomtpssm4tudthheecsoerhka,ceeeiteacncteheonattkafneoetrhrnaidpeaddeo,rrfeedptbpsiluisopmptfaiaonnb.aonlnaurtnoqerdtyutvdraatanabaatettltmisctutrh>ree0nhihucbsoeeet,ec·neswiid2cnaordscf1n5ostienaacltdr·olian5dmhteeasassmitodnfspeiat·f-em0tni.riecsrtcgr·emarttth3eouiisweoi,coetAenrtlatsbmnnnselwfriys,tetet fleipoiaihornostvtebovradieiurhnwyceuetfmrte.ikipgegns.dtaseaetirht.,hdrhpae5olTgecetiecnev.cTelevucheerdntdak, eiheratdlTsoi.pf1rauerirpelurhoae·meeepd5ci.nfaegfnIltileclytvhectoibstivece,atmyonorentetgtoreoibvnea,taosifdeiryueotnnmr0se,rmtanacb-afesb1ottsidtm(toegsohmn/fgome.ueipregogee3tasrup.gen,ctbnostgoaihuynhT,nnt)meirairehoclyesmhpinedkttio;iacsrnlno,safecbbisuiuaannupnonteinnnldthrauatiitntdiydnooeerr•t•

arenysts

part on

of the weight of the a bed of cement on a

lsotwonbe. rickThpeiear nnbuuillatr

iinronthetrofuorgmh

of a hollow octagon.

a.

sstfTpwbwiocudageehroesAdr.rfepdsee,as4bie;tncef. poeg/etals.wfahaNnceentdfshhrieeptBvrea-wb peceoorcoosuatdtr,ucoerhghnlpea,duehdene/l1lmnritbecsia/camloeprinnloafmleetsdenstsetenedefphradtosrea<65>os,loclrlfftfosetihetivwtcnvltoaho1henetealdedelrveoysiflnesa,iuttwgdahomlaebmlnehneyrwetsedetpoiewatfwtrphwtrhaa,oohlseisasmrcatrkchaknteieeenodndopaddesnlbsbiafttseoscthon.eaunoaeert1irer-vnep.ggoe·d2ampald5apnritsiTndiaarcabclrerehlayttoesnelhblrlrttyuesebihmgmrfftdllini,apM egerdencrtltSuraeotedlrrneereaebsed;oe­,.,

838

FArtl,, and tM Lt,,mimf"ou, EtMr.

their surfaces measured 5·0 by 7·5 centimeters. The second of these waa plac)ed in the path of one of the pencils to compensate for the passage of the other through the same thickness of glaes. The whole of the optical portion of the apparatus was kept covered with a wooden cover to prevent air correots and ra~d changes of temperature.
The adjustment was effected as follows: The mirrors haviog been adjusted by screws in the castings which held the
4.

mirrors, against which they were pressed by springs, till light from both pencils oonld be seen in the telescope, the lengths of the two paths were measured by a light wooden rod reaching diagonally from mirror to mirror, the distance being read from
a small steel scale to tenths of millimeters. The difference in
the lengths of the two paths was then annulled by moving the mirror e,. This mirror had three adj ustmeots; it bad an adj 118tment in alLit.ude and one in azimuth, like all the other mirrors,

.Miche"8on and Morley-Rdative N.otiM of the 889

but ftner; it also had an adjustment in the direction of the

incident ray, sliding forward or backward, but keeping very

accura~Iy parallel to its former plane. The three adjustments

of this mirror could be made with the wooden cover in position.

The paths being now approximately equal, the two images

of the source of light or of some well-de6ned object pl11ced in

front of the condensing lens, were made to coincide, the teles-

cope was now adjusted for distinct vision of the expected inter•

fereoce bands, and sodium light was substituted for white light,

when the interference bands appeared. These were now made

as clear as possible by adjusting the mirror e,; then white light

was restored, the scl'ew altering the length of patb was very

slowly moved (one turn of a £'crew of one hundred threads to the

fi.

inch altering tbe path

nearly 1000 wave-lengths)

0

till the colored interfer-

ence fringes reappeared

~

in white light. These

al I e b

g

IC

g
CI

b

c were now given a convenient width and posi-

I g °I"°'

tion, and the apparatas was ready for observa•

I

I tioo.

I

The observations were

I

I conducted as follows:

I

Aroand the cast-iron

troagh were sixt.eeo 89Uidistant marks. The apparatus was

revolved very slowly (one turn in six minutes) and after a

few minutes the cross wire of the micrometer was set on the

clearest of the interference fringes at the instant of passing

one of the marks. The motion was so slow that this could be

done readily and accurately. Tbe reading of the screw-head

on the micrometer was noted, and a very slight and gradual

impulse was given to keep up the motion of the stone; on

passing the second mark, the same process was repeated, and

this was continued till the apparatus b11d completed aix revolu•

tions. It was found that by keeping the apparatus in slow

uniform motion, the results were much more uniform and con-

sistent than when the stone was brought to rest for every ob-

servation; for the effects of strains could be noted for at least

half a minute after the atone came to rest, and ~uring this time

effects of change of temperature came into action.

The following tables give the means of the six readings; the

first, for observations made near noon, the second, those near

six o'clock in the evening. The reapings are divisions of the

screw-head& The width of the fringes varied from 40 to 60

divisions, the mean value being near 60, so that one division

340

Earth and tli.6 LwminiferO'U8 .E't,her.

means 0·02 wave-length. The rotation in the observations at noon was contrary to, and in the evening observations, with, that of th e hands of a watch.

~OON OBSERV ATI0SS.

-=-- I _ _ __ , 1&. ' 1. _::_

u. ~ 5. e. ,I 1. a. 9. 10. 11. 12. 11. 1I

ll!. 11·-

Ju ly8 . . . . .. . in 44 ·o 43·5 a1n iW2 an :K·a! 112-r. 28'2 20-2 23·a 23'2 20·a 1s·1l 1,·5 10-~ 18'7

July 9. ,- ... . .;1·4 5; -a 68"2 n9·2 55·; 00·2 oo·s' 62'0 61"5 0011 e.,·81 o; -s eo-1 .0·7: 73'0 '10':! 12-2

July ll .... . Mean .... ... . H eanln w. I.

24~ 13·•13· ~ 24:1J·•sfi ·• 2421··20 ~ l1l0!1H1111

1w2 19·3 19·; 1 18·8
3~ 7·; ~ 311·1 - a;·o ~ 37·8

rn-2
3~5•3

u 11 1a·3: 12·8 1a·a 12·s, 1011
~~ -6 u~ -a, ~ a.·t ~ 34,-l d 83'9! a~a·6

7'3
S~N

6-5
o~o·s

I -.oo· ·61121 ·6811 ·688 ·688 ·678 •11721 ·628 ·616

•

i Fina.I m ean . "78( ·762, '755 '738 '721 '720 ·715 ·092 •ff6t

P. M. OBSERVATIONS.

July 8 ...... 61'21 63•81 118'3 68"2 07·; oo·s ;o·a 69·s oo·o n·a 71•3 70·5 n·2, 71'2j ro•i; -r.i-s 75'7

July 9 .. .. ... July 12 ... ...

26·01' 26·0, 28"2, 29'2 oo·s 00·5: oo·o 61·s

31·5 112·2

32·0 a1 ·o

a1 ·a arn

3Vi
oo·,

aa·o oa·2

35·s 115"7

311·5 53·;

37·3 r.&·7

38·8 u·o, 42'7 M·O 58'2i 118"5

i3"'i 57'0

« il
56'0

Mean ....... s1·a 51'9' 52•5 53·0 53·8 5'"1 M·S 53'7 53·4 114,8 53·8 114'2 55·0 66"81 57·2 57'7 58'6

Meanlnw. l. 1·000 1-oosl1·000 1·07s 1·ara 1·082 t·OSG t il'I'• lil68 l 'Oe6 lil'i6 1"064 1'100 1·1Jl6r ·1« J·lM l'17Z

] ·008 1·08611'076 l "fflol 1'100 1'100 1'1« l·lM 1"172

Fioal mean. 1'047 1'06211'063°1'081 1'088 1'109 l'll6 1'114 1·120

The reizults of the observations are expressed graphically in fig. 6. The upper is the curve for the observations at noon ~nd the lower that for the eveniug observations. The dotted curves r~present one-eighth of the theoretical displacement!!. It seems fair to conclude from the figure that if there is any dis•

6.

- - ,,,,,------·---....... ....

-

.,,"''

.......

,,

........, .

/1-/)$'A,

',. - OOS' "", .........'-. ... ______ .,. __ ,,,,,

N G

r.lac~~ent due to the relative motion of the earth and the
um1~1ferous ether, this cannot be mucli greater than 0·01 of the d•s~nc? between th_e fringes.
Cons1dermg the motion of the earth in ite orbit only, this

Michel,son and .Morl,ey-Relawve Motion of th6 841
diaplacemen t should be 2Dvy',= 2D X10- •. The distance D was
about eleven meters, or 2xl01 wave-lengths of yellow light; hence the displacement to be expected was 0·4 fringe. The actual displacemen, was certainly less than the twen~ieth part of this, and probably less than the fortieth part. But since the displacement is proportional to the square of the velocity, the relative velocity of the earth and the ether is probably less than one 11ixtb the earth's orbital velocity, and certainly less than one-fourth.
In what precedes, only the orbital motion of the earth is considered. If this is comhined with tbe motion of the solar system, concerning which but little is known with certainty, the result would have to be modified; and it is just possible that the resultant velocity at the time of the observations wrui small though the chances are much against it. The experiment will therefore be repeated at intervals of three months, and thus all uncertainty will be avoided.
It appears, from all that precedes, reasonably certain that if there be any relative motion between the earth and the luminiferous ether, it must be small ; quite small enough entirely to refute Fresnel's explanation of aberration. Stokes has given a theory of aberration which assumes the ether at the earth's surface to be at rest with regard to the latter, and only requires in addition that the relative velocity have a P.otential; but Lorentz shows that these conditions are incompatible. Lorentz then proposes a modification which combines some ideas of Stokes and Fresnel, and assumes the existence of a potential, together with Fresnel's coefficienf.. If now it were legitimate to conclude from the preaent work that the ether is at rest with regard to the earth's surface, according to Lorentz there could not be a velocity potential, and his own theory also fails.
Supp'frment.
It is obvious from what has gone before that it would be hopeleee to attempt to solve the question of the motion of the solar system by observations of optical phenomena at -ehe aurface ojt'M e-art.h. But it is not impossible that at even moderate distances above the level of-the sea, at the top of an isolated mountain peak, for instance, the relative motion might be perceptible in an apparatus like that used in these experiments. Perhaps if the experiment should ever be tried in these circumsla.ncee, the cover should be of glass, or should be removed.
It may be worth while to notice another method for multiplying the square of the aberration sufficiently to bring it within the range of observation, which has presented it.self during the

342

Earth and tM .Luminif8'1YYU,f EeMr.

preparation of this paper. This is founded on the ~ct that re-

flection from surfaces in motion varies from the ordinary laws

of reflection. Let ab (fig. 1) be a plane wave falling on the mirror mn at an

incidence of 45°. If the mirror is at rest, the wave front after

reflection will be ac. Now suppose tbe mirror to move in a Jirection which makes

an angle a with its normal., with a velocity OJ. Let V be the

velocity of light in the ether supposed stationary, and let cd be the increase in tbe distance tbe light bas to travel to reach d.

cd
In this-time the mirror will have moved a distance ..,. 2 cos a .

We have a-cdd = a, .y'2Vcos a whi.ch put= r, and aa-cd = 1- r.

In order to find the new wave front, draw the arc/g with bas

a center and ad as radius; the tangent to this arc from d will

be the new wave front, and the normal to the tangent from b will be the new direction. This will differ from the direction

ba by the angle 8 which it is required to find . From the equal-

ity of the triangles adb and edlJ it follows that 8=2tp, ab=ac,

tan adlJ= tan ( 450 -

8) 1- tan -8

2 =

= = 82 ac 1

ad

-r,

1+tan 2

or negl ecting terms of the order r1,

a+ = O =

r

+

2r'

.y' 2a, cos V

va,••

,
cos a.

Now let the light fall on a parallel mirror facing the first, we
should then have 8,= -y2wVcosa + vw•• cos'a, and the total de-

viation would be 8+8, = 2p' cos'a where p is the angle of
aberration, if only the orbital motion of the earth is considered.
The maximum displacemeat obtained by revolving the whole apparatus ibrougb 90° would be J=2p"=O·OO4". With fifty such couples the displacement would be 0·2". But astronomical observations in ciroumst.ances far less favorable than those
in which these may be taken have been made to hundredths of a sec-0nd; so that this new method bids fair to be at least ae sensitive as the former.
The arrangement of apparatus might be as in fig. 2; a in the focus of the lens a, is a slit ; bb cc are two glass mirrors optically plane and so silvered ae to allow say one-twentieth of the· !ight ~ pass throu_gh, and ~eflecting say ninety per cent. The intene1ty of the hght falhng on the observing telescope df

Michela<>n and Morl8'!J-Relative Motion of the 348

8

'
I

a

4.

6

C

a.

_f
s,

844

Eatrth and t~ Lumvniferou8 Ether.

would be about one-millionth of the original intensity, so that if sunlight or the electric arc were used it could still be readily seen. The mirrors bb, and cc, would differ from parallelism sufficiently to separate the successive images. Finally, Lbe appamtus need not be mounted so as to revolve, as the earth's
rotation would be sufficient. If it were possible to measure with sufficient accuracy the
velocity of light without returning the ray to its starting poin~ the problem of measuring the 6rst power of the relative velocitv of the earth with respect to the ether would be solved. Thts may not be as hopeless as might appear at 6rst sight, since the difficulties are entirely mechanical and may possibly be sur-
mounted in the course of time. For example, suppose (6g. 3) m and m, two mirrors revolving
with equal velocity in opposite directions. It is evident that light from s will form a stationary image at s, and similarly light from s, will form a stationary image at s. If now the velocity of the mirrorB be increased sufficiently, their phases still being exactly the same, both images will be deflected from s and s, in inverse P.roportion to the velocities of light in the two directions; or, 1f the two deflections are made equal, and the differ• ence of phase of the mirrors be simultaneously measured, this will evidently be proportional to the difference of velocity in the two directions. The only real difficulty lies in this measurement. The following is perhaps a possible solution : gg, (fig._ 4) are two gratings on which sunlight is concentrated. These are placed so that after falling on the revolving mirrors m and m,, the light forms images of the sratings at s and s11 two very semiiLive s~lenium cells in circuit with a battery and a telephone. If everythmg be symmetrical, the sound in the telephone will be a maximum. If now one of the slits s be displaced through b~lf the ~istance between the image of the grating bars, there will be silence. Suppose now that the two deflections having
~een made e~actly equal, the slit is adjusted for silence. Then If the experiment be repeated when the earth's rotation bas turned .the whole apparatus tl!rough 180°, and the deflections
to are agam made equal, there will no lonaer be silence and the
a_ngular ~istance through which s must be moved restore stlence will ~~asure the required difference in phase.
T~ere remarn three other methods, all astronomical, for attackmg the problem of the motion of the solar system through space.
1. The ~elescopi~ observatio':1 of the proper motions of the stars.. T~1s bas g~ven u_s a highly probably determination of the d1rect1on of this mol.100, bot only a guess aa to its amount.
2. '.J'he spectroscopic observation of the motion of stare in the hne of sight. This could furnish data for the relative

Hwhiu<>n a;nd M<>rley-Motwn of the .Eatrth, etc. 345
motions onl,, though it seems likely that b_y the immense improvements m the photography of stellar spectra, the information thus obtained will be far more accurate than any other.
8. Fioally there remains the determination of the velocity of light by observations of the eclipses of Jupiter's satellites. If the improved photometric methods practiced at the Harvard obeervatory make it poBBible to observe these with sufficient f:OOUracy, the d~ffere_nce in the results found for the velocity of light when Jupiter 1s nearest to and farthest from the line of m~tion will give, not merely the _motion of ·the solar system with reference to the stars, but with reference to the luminiferoua ether itself.