1887 - albert michelson and edward morley - on the relative motion of the earth and the luminiferous...

8/6/2019 1887 - Albert Michelson and Edward Morley - On the Relative Motion of the Earth and the Luminiferous Ether

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No. 203. VOL. XXXIV. NOVEMBElt, 1887.,

Bltablilhedhy lIBlil A_Ill SnLI.AII in 1818.

THE

AMERICAN

JO U RN A L O FSOIENOE..GUITORS

JAMES D. AND EDWA.RD S. DANA.

A.81!100lkTB lCDITOBS

PBO:rB880BS ABA GRAY. JOSIAH P. COOKE, AND

JOHN TROWBRIDGE, OE CA,llBRIDG:B,

PaOPB8S0B8 H. A. NEWTON .LND A. E. VERRILL, 0 ....

NBW HAVEN,

PaoJ'SB8o:a GEORGE F. BARKER, OE PUlLA..Ji)BJ;.PUIA.

'rEIBD BBBIES.

V OL . X XX IV.-[W HO LE N U.M BE R. O XX XIV.J

WIm PL,'&'n:s II mIX.

No. 203-NOYEMBER, 1887.

NEW HAVEN, OONN.: J. D. &' E.S. DANA.1887,.

Tu" rTLB • .u:OB II IOOI lB . . TAYLO R. J l> BU ITBI t8 . t )1 STA -TII : STRlC iB . .. .

THE

A M E R IC A N JO U R N ,A L O F S C IE N C

[TH IR D BERIES.]

ART. X:XXVI.-On t he ~ lat tve Motion of th e E arth. and theLuminiferou8 Elher,. by ,A,LBERT A. MICHELSON andED'WARD W. MORt.!!!y.*

T'HE discovery of th.e aberration of light was 800n followed!by an explanati on according to the emiesion theory. Toe:effectwas attributed to II simple composition of the veloeit v of Hghtwith the velocity of the earth in its orbit. The di~ultie9 inthis apparently Bllfficient explanation were over looked untilafter ail eXJPlanation on the undulatory theory of light Yi'1l.8

proposed. This new eXJ?lanation was at first almost as simpleas the former.. But it railed to account for the fact proved, byexper iment that the aberration waS unchanged wben observa-tionll were made with a telescope filled with water; For if tiletangent of the angle of aberraaion is the ratio of the veIoo:ityof the earth to the veloclty of light, then, since the latterve]ooi ty ' in wa. ter i: 8 three-fourths its velocity ill a. vacuum, tbeaberration observed, with a water teleeeopeehould bafocr-thirds '0 1 ita true value.t

• T hi ll re se Bl 'Q h w as c ar ri ed ! o ut w it h t h. e a i d. o r t he Ba ch e Fu nd .t It may be nmiced tiul t, most wrlt orll admit the l Iu lf ic ie u ey 0 1Uto erpJ:.natioD

aC«lrdiD;g, to the. , e .m i M .i O U w o o . " .Y. IlIf liglilt; wbile Iu fact h ~.UIlClllty' is Q ve ngreater thall iIOOo,tdiug:to fJu, und.Ulator"yt.heory: Fo.rOIl t he B lQ ll le lo n t h eo ry t hevelocity of l lght IDUet be gre&te~ ill t he wa te r telellOOpe', and ilIe~~ the IWJIIIQof abirntioD lIh.ould be 1_; h C DO O , I D o r de r to r\&duce it $0 i t a ~ru l! ln i l !l . e ,. "emOlt blUO theabaurd hypothe,sill tbat the !notion of the water iD t he teieaeopecanI .." ihe r& Tof ligllt w Ule opp08ite drn.clioD!

All. J01l1l. SoL-TIURD SBBIZB,Vor. XXXIV. No. 1l0il.-Nov., 188'.2:1



884: XicAilAOtiand Moruy-Motion of the Earth, etc.

On the undula tory theory, according to Fresnel, fin,t, the etheri s ~ m pp o e e dlobe a t re st, except in the interior o f t ra D B pa re l ll tmedia, in wbich secondly, i tis supposed to move with a velociy

1e811than the velocity of th e m ed iu m in the ratio n ' : - \wheren

n is t.he index of refraction. These two hypotheses gives com-pl et e and s:n.t.isf:actory expl anation of aberration. The secondhypothesis, notwit bstanding its seeming improbabilit y, must beconsidered as fully proved, firat, by the celebrated experimentafFizesll,* and secondly, by tile ample confirmation of our ownwork.t The experimental trial of the first hypothesis formathe suhject oftbe present paper.

If tile earth were a transparent body, it might perhaps beconceded, in view of the experiments just cited, that the inter-moleeular ether was at rest in space, notwithetanding the mo-tion of the earth in ita orbit i but we have no right to ex-~nd tbe conclusion from these experiments to opaque bodies.But, there 000 hardly be question that tile ether can and doespails through metals. Lorentz cites the illuatrntion of a metallicbarometer tube. When the tube isinclined the ether in thespaee above tbe merCllry is certainly forced out, for' it is im-eompressible.f But again we have no rigbt to assume thllt itmakes its escape with perfect f reedom, and if there btl aoy resist-anee, however slight, we, c ertainly could not assume an opaquebody sueh as the whole earth to offer free passage through itsentire mass, But as Lorentz aptly remarka: aquoi qui'] ensoit , on "lera bien, a monavis, de ne pas se laisser guider, dansuna question aussi importante, par des con8iderations sut' .\edegre de probabilite ou de aimplicitd de l'une au de l'autrehypothese, mais de s 'addresser a l 'exper ieo.ce . pour apprendre i\eonnai tee 1' 6tnt, de repos OQ . de mouvement, dans lequel setrouve l 'ather a la surface terr,estre."§

In April, 1881, a method was pr()f. osed and carried Ollt forteBtin~ the lJuestion experimentally. ..

In deducing the formula for t'li.e quantlty to be measured,the e ffe c t o f tbe motion of the earth through the ether on thepath of the rny .o.t rigbt angles to this motion WB.S overlooked.,

• Compt98 Rendiu!!, :uz:lii, 3 (9 , 1 8111 j Pogg. A . . I : i D .E1'gB.nizu!IJgsband!,,ii, 45T,1811a. ADD. Ch im. Pb ya. , III. l vi i, 38 5, 1 86 9.

f JlDflll9nce of Motion of (he Medi um on the Velocity of ·Light. Thill Journal,In; xxxi, :8n., \886._t It :may be objected fbat it· may escape by the space between the , mercury aD d

the waill!!; bl!~ tb.iB could b" prev,ented by ama.lgamatiug the walle.§ Arehiv811 N60rlandeiaeB" :ai, 2 " '· l iv : r.(The rel ll lt ive mot ion of the earib and the ll!lminiferous ether, by Albert A. .

KichelaoD, thl ll Jour . . In, %:l:i~120.,. n ma,y be mentioned here lbat the error was poluted out.-klthe au$hor of the

former paper by .M .A.. Potoisr, of Paris, in the winter of 1881.

The diSClJ6S8iono f this oversight and of tbe enti re e!perimentforms the subject of a very searching ana.lysis by H. A. Lo -relltz,* who finds that this e ff ec t c an by D Q m e a I U Jbe disregarded.In cOnsequence. the qua.nti ty to be measured. bad in foot butone-half the value supposed, and as it W,IUI already barely be-yond the. limits of errore of e~perime~t, the ooeolueion ~lI'awnfrom the result of the ezpenmene might weH be quest ioned;sines, however, the main portion of the theory remains un-questioned, it was decided to repeat the experiment with suchmodifioat.ionS 88 wOl;lld insure a theoretical result much toolarge, to be masked by experimental errors. The theory of themethod may be briefly stated as follows .

Let 8a, fig. 1, be a ray of light which is partly reflectedill o b, and partly transmitted in ao , being returned by the mi r-rors band 0, along ba and en . b a 1 8par tly t ransmit ted along' ad,

1 .

and ila is part ly reflect ed along ad. If then the paths ab and acare equal, the two rays interfere along ad. Suppose now, theether being at rest, tbat the whole apparatu moves in the di-rection .te, with the velocity of the earth. in i l .B orbit,. the diree-

* De l'InflueDou du KouvemeDi du 1a Terre Bur lea Ph.n. Lum. ArchiTfis N6er-l~ :n:i, 2- liw., 1886.



836

t iona and di stances traversed! by the rays will be altered tbus:-'I'be ray sa 18 reflected along ab , fig. 2; the angle W, beingequa] to the aberrat ion =a, i s returned along ball (aba,=2a). andgoes to the ~OCu.sof the t el escope, ~hose di recti on i s unalt ered.The transmitted ray goes along ac, IS returned along , c a" and I.B

ref lected at a" making ca,e equal 90-a, and therefore' still coin-ciding with the first ray. It may be remarked that the rays 00 1

and cal' do :not now meet exac~ly in tbe same point a" thoughthe difference is of the aeeond order j this does not affect thevalidit:y of the reasoning. Let it now be required. to find thedifference in the two paths aba" and aca,.Let V=velocity of light.

v=veJocity of the earth in its orbit.Deedistanee ob or ac , fig. 1.T=time light occupies to pass from a to c.T =time light occupies 10 retam frOID c to ai' (fig. 2.)

Then T =vD, T'I=v

D, The wbole time of going and com-

--11, . + tI

ing is T+T,=2D v~v.' and the distance traveled in this time

is 2DV~:I= 2D(1+;) neglecting terms of the fourth order,

The length of the other path is evidently 2D¥ 1+~.' r to the

same degree of accuracy, 2D( 1+ 2 ~ ' ) 'The difference is there-",'

fore: DV.' If now the whole appars.tus be turned through 90 0,

the difference: wil l be in the opposite direction,bence the dis-v'

placement of the interference fringes should be 2D V '- 000.

sidering only tbe velocity of the earth in its orbi t, this wouldbe 2DXlO- l • If, as was the case' in the :first experiment,D=2Xl0' waves of yellow ligbt, the displacement to beexpected would be 0' 04 of the dist ance between the int erferencefringes.

In the firat experiment one of the principal difficulties en -countered was that of r evolving tbe apparates without predu-oing distortion; and another was its extreme sens:itivenesa tovibration. 'Thi8 was so great that it was impossible to see theinterference fringes eJicept at brief intervals when working inthe city, even at two o'clock in the morning. Finally, as be.fore remarked, the quantity to be observed. namely, a dieplace-ment of something less than a twentieth of the diatan,ee be -tween tbe interference fringes may have been 'too small to bedetected when masked by 6:a;per imElntal error&.

The: f irat named diff icult ies were ent irely overcome by mount-ing the ap~aratl1s on a massive stone floating on mercury j: andtheaecona by increasing, by repeated reflection, the path of theligbt to about ten times its former value.

Tbe apparatus i s represented in pel 'Specti ve in fig, 3, in pl an in

fig. 4, and in vertical sootion in fig. 5.. The stone a (eg. 5)is about: 1 ' l>meter square and ,0'3 meter thick. It rests on an annularwooden f loat bb, 1'5 meter outside diameter, 0-7 meter insidediameter, and 0'25 meter thick, The float rests on mercurycontained io the cast-iron trough cc, 1·5 centimeter thick, andofsuch dimensions as to leave a clearance of abeus one centi·meter around the float. A pin d, guided by arms gggg, flts intoa socket e at tached to the float, Tbe pin may be pushed intothe socket or be withdrawn, by Ii lever pivoted at f Tbis pinkeeps the float concentric with tbe trough, but does not bearaD y part of the weight of the stone. The annular iron troughrests on 11 bed of cement 011 a low brick pier built iu the formof a holl ow octagon.

8,

At each corner of th,e stone were placed four mirrors ddeefig. 4. Near the center of the stone was a plans-parallel glMS b.These were so disposed that light from IU! ergand burner (I,

passing through a : .lens, fell on b so lIS ~ ,?e in J;>artreBectedto tid the two peacil s followed the paths indicated lD the fi.gure,bdedll jaod bd,B1d,bjrespect i.vely,and were obseered by tb(1 tele-scope/. Both f and a revolved with the stone, The mirrorswere of speculum metal carefully worked to optically planeI!udaces five centimeters in diameter, and the gluaee band cwere pl ane-paral lel and of the same thickness.. 1'25 centimeter;



838 Michel80n and MO'I'ley-Relaeiv8 Motion o f the 889

tbeir 8 u rf ac e s m e a su re d .5'0 by 7'5 centimeters. The second ofthese WII.B placed in the path of one of tbe pencils to compen-sate for the paseage of the other through tbe same t.hickness ofglass. The whole o f the optical portion o C the apparatus W 88kept covered with a wooden cover to prevent a i r C l lr r ente and

rapid changes of temperature.T he adjustment WtUI effected aB . follows: The mirrors hav-ing been adjusted by screWB in the castings wbiQh held tbe

4.

/

al,

but Oner; it also bad an adjustment in the direction of theipoident ray, el iding forward or backward, but keeping veryaoollrately parallel to its former plana The three adjuetmentsof this mirror could be made with the wooden cover ill position,

Tbe paths being DO W approximately equa], the two images

of the source of light or of some well-defined object placed infront of the condensing lens, were made to coincide, the reles-cope was DQW adj usted for distinct vision of the expected inter-ference bands, and sodium light was sub8~.tuted for white light,when the interference bands appeared. These were DOW made8:8 olear as possible b y adjllstiug the mirror 6, ; then white lightwas restored, t.he screw altering the length of/atb was veryslowly moved (one turn of a screw of one hundre threads to the

5. mch altering the pathDearly1000 wave-lengtbs)till tl]e colored interfer-ence fringes reappearedin white light. These

c c were now given a eon-venient w idth and posi-tion, andtbe apparatuswas ready for observa-tion.

Tile observations wereconducted as follows:Al'ound the cast-iron

trough were sixteen equidistant marks. The apparatus wasrevolved very slowly [one turn in six miuntes] and after afew minutes the cross wire of tbe micrometer was set on theclearest of the interference fringes at the instant of passingone of the marks. The motion was so elow that this could bedone readily and accurately, Tbe reading of the screw-headon the micrometer W88 Doted, and II very slight and gradualimpulse was given to keep up the mot ion of the stone; onPlU!8iDgbe second mark, the same process was repeated, and!this was continued till the apparatus bad completed six revolu-tions. It wa.s found that by keeping the apparatus in slowuniform motion, the results were much more uniform and con-sistent than when tbe stone w as brought to rest for every ob-8flrvation j for t he effects of atraina could be noted for at leasthalf a minute after the srone eame to rest, and during tbis timeeffectB of change of temperature esme into action,

The following' tables give the means of the s ix readings i theii.rat, for observationa made near noon, tbe second, those nearsix o'clock in the evening. The ~ing8 are divisions of tbescrew-heads. The widlth of the friDges varied from 40 to 60~ivisioDS, the mean value being near 60, so that one division

G

e

Ib

Ieg d

,D b

I9 i'

mirrors, against which t' bey we.re pressed by springs, 'till lightfrom both pencils could he seen in the teleeeope, the lengths oftbe two paths were measured by a light wooden rod reacbingdiagonally from mirror ' to mirror, the distance being read from& small steel seale to t.enlbB of millimeters, The di1feren~ intbe lengths of the two paths was then ann ulled by moving themirror s,. This mirror bad three adjustments t it had an adjust-

meD' in altit.ude and one in aaimutb, like aU the other mirrol'8,



340

means 0'02 wave-length. The rotation in the observations atnoon was contrary to ,and in the evening observations, with,that of the hands of a . watch,

:-IO{)N OllSBIlVATIOSS.

• " '. I I 1 :1 .----I.::_ _ _ : _ . - = - ~ , . ! . : _ _-= -2 ..'.2 :_ ~ - = - - .~I~ ~ ~l~ ~ 11'

Ju ly :8 . .. .. .• t .n ' "''''. (3\,) 39'1 a.'i'~ 30 ; } loa !~fI ~"j! ~1!. ~'a ' z:I '2 21) '3 18"'l'117'5 161~ IB',July 11.. _ .. , . . 6T '~ [ il' ;} 1 ;8 '2 .' ;9 '2 i]8j 00'2 0 0 '8 ' 6 2 11Br,., Glll Grrll' 6jil~1 70'7: 7 3'0 "ro'~ ill'!!

Ju ~y 1 1.. .. . .2 7" 3 2 3,'5 . 22 '0J 9 111 ' .]9 ,-t 19'3 18:,.119'8 1 11 '2 U ' ll 1 wa : 1 2'S is-a 1 2'S , 1 0" 2 ,.7 '3 . ii'SMean .. -. .. .<b3T 4Hi' 4i' ~ 00'4 (17"; 38'1 37"01 31'S ;)5'33!:6' 1 U " 3 :34'4 31,'4 lIS": ila'6 art 00'8:Mean in w. 1. '8Ii:! 's:!t ' 8 2 1 ! "I!8 ' l"5-I ,'7 62 '7 58 ' 7 0 0 .";00 ' o re1 ' 6 8 6 :'6811 '1lIlII1'6i8 ' 1 11 '~ . .~ '616

',100' ' O O ! ! 'I l0 0 '6 BR '6 11 8'!r.8 ' 6721 ''628 'G 16 i IFiDa.l QlelUl- '784~ '162! '1M ''jQ(l "j~ '7W -n s '~9l!' ' 661 I

r. y, OIlSERV ArION's.

Ju~y s.... . .. !I I'!!!11 3 ' 3 1US':) 1IB, '21 67 ' ' ' : , 119'370<'3 OO'lj 1J9'(I, '?l'3 n o a ,70 ,'I; '11'2'nill 70';5 'Ii.2"5 ' . .in .July 9 ". ;?<I-QI26'~1 28~2,~'~1 31'S ~-I) 31-S 31-j 311'(1 : M o l 60 0' 5 3 7' 3 '38'S n'o, ~ '7 43 "'i: 4 4" 0-Ju~y l2 00-8 00'5: 0 0'0 M3, m' 2 61' 0 61' S 0 0'7 58' 2 M," , 5 3' , M l 55'0 58~1 Mo 07i1, 5611:MSiin si-a 1> 1'9' 52'5 53'9 53'S &4'11 M 'Sr.s '" 511'4 iii'll O O ', S, M "2 , 55 '0 00 '8 ' 15 ";'2 5 '1 '" 1 M "

:M88n,' in "oj. l"@6,l 'OO8"!l' (I~) 1,"078.l '00"61'l1S21'lJ86 ..1 ' O T ' : j1'Q66,,1:tl8O1'>07·61 : 0 8 4 1'100 nOOIH441'Wl'17Z1 ' "0 0 I ! 1 1 " 0 8 1 1; 1 '0 7 6'11ll4 1'100 11 ' 1 0 01'1« 1"1M it'172 .

Fillal meil. Il , l 'Gnlr.I)Il2i100·1"0B11'088!'1091'11Jj l~n4. il'12O~~~~~~~~~--~~~--~~~~~-

The results of the observations aile expressed graphically infig" 6, [The upper is the: curve for the observations at noon,~nd tbe ~o wer that for the evening observatiecs, The dottedcurves ~present one'~ighlli of the theoretical displacements, I tseems fau to coseludetrom the figure that if there is any dis-

6,

-----_ - !I'"",J...... <,

. . . . . . . . . . . . . . . . . . .-

'<,-Q ,rJ,S" . . . . . .. . . ,#. ., . . . ....~. ., . . . .. . . . _ .__.,;

placement due to the relative motion of the earth and tlileluml~lfer()U8 ether, this cannot be mucb greater than 0'01 'ofthe dlst.a.nc~ between tile fringes.

ConsJdelilDg the motion of 'he earth in it.! orbit only, this

diaplacementsbould be 2D;I=2D Xl0~', The distance D was

aoollt eleven meters, or 2.XlO' wave-lengths of yellow light;hencatbe dlsplacemect to be expected was 0'4 fringe. Theacto.a], displacement was ce'rta:inly less than the twen.~ieth part

of thIs, and probably less than the fortieth part, But since thedisplacement is proportional to the square of the velocity, the:relative velocity of tbe earth and the ether is probably less thano n e f li xt b tbe earth 's orbital v e l o e itY I an d certainly less thanODe-fourth,

In wba.t precedes, only the orbital motion of the earth ia eon-sidered. If this is combinedwitb tbe motion of the BOlarays·tem,cQDcerning which but little is known with certainty, theresult would have to be modified; and it is just possible tha,tthe resuhant velocity at the time of the observations was swa]lthoughthe chances are mucbagainst it . Tbe experiment willtberefore be repeated at intervals of three months, and thus alluneertainty will be avoided.

I t a pp e ar s,from all that p r e ce d e s, r e as o n ab l ycertain th!lt ifthere o e ! li nyrelat ive motion betweenthe earth and 'tbe luminif·e l l ' O U Setber, it must be small ; q u i t e sraall enough entirely torefute Fresnel 's explanation of aberration. Stokes bas given atheory ofaoorration wbich assumes the ether at the,earth's sur-face to be at r e st w it h regard to the latter, and only requiresin addition that the relative velocity have a pooontiaI j butLoremtz shows tbat. theaeconditioms are lneomperible, Lorentzthen proposes & mcdiflcation whioh combines some ideas ofStokes and FresmeI, and assumes the existence of a potential ,together with Fresnel 's coefficient. If DOW it were legitimateto oonclude from tbe preaent work that the ether is at rest withregard to the earth's surface, according to Lorentz there couldnot be a : veloeity potential, aud! his own theory also fails .

Supplemen.t.

It is obvious from what has gone before that it w()uld behopeless to atte,mpt to solve the question of the motion of theBOlar system by o?~rvati()!3B of o,P,ticalphenomena. ar the 8urff1;ceof~ earth. But It 18 not imposalble tbat at even moderate dis-tances above the level of, the sea, at the top of an isolated moun-tain .peak, for in8tan~e,.the. re la . ,tiv'~ m.otion.mig~t be pereepti-ble man apparntu8 like that used In these experiments, Per-haps if the experiment should ever be tried. in these circum.stances, tbe cover should be of glass, or should be removed.

It may be worthwhile to notice another method for multi-plying the square of the aberration sufficiently to bring it within

the range of observation I whicb bas presented itself during the



342

. ./

prep.aratioll of ,thi, pap~r. T~is is f~und,ed o~ the fa.ct tha.t re-RectlOn from surfaces m monon varia,s from the ordInary lawsof reflection.

Let ab (fig. 1) be a plane wave falling on the mirrormn at anincidence of"45°. If the mirror is at rest, the wave front after

reflection will 'b e ac .Now suppose the mi rror to move ina directien which makeaanangle a with its normal, with a velocity ld. Let V be thevelocity of light in the ether supposed stationary, and let cd b~the increase in the distance the Ugh' bas to travel to reach d.

In this- time the mirror will have moved a distance t19 cd _.- cos a

, cd ttl 412 cos a. , d ac ,We bave- - - V which put; = r" an -d = 1- r.a d - a

In order to -find the new wave front, draw the sreJg with basa center' and ad as radius: the tangent to this arc from d winbe the mew wave front, and the normal to the tangent from bwill be the new d~rection. Tbis will differ from the direction~- by t he angle () which it is requi.red to find. From the equal-l~y of the triangles adb and edb It fol lows that (}=2tp, ab=ac,

{J-_ 1- tao-

(o ( j' ) :2 a c 1

tan adb= tan 45 - 2- =_ 8 = ad -- ~T.

l+tan-.2

,

or neglecting terms of' the order r',

(J .+ r" t' 2w (lOS (1 +al. _ •,= r '2 = V . V' oo s a.

Now let the light fall on a pamllel mirror facing the fi.rst, we. - -y"2 ro cosa w'

should then have fJ,= V + V" cos'e, and the total de-

viation would be 0+0, = 2p~' costa where p is tbe angle ,ofaberration, if only the orbit al motion. of t he earth is considered.'The maximum diaplaeement obtained by revolving the wholeappsratua tbrough 90° would be d=2p'=O·004". Withfifty, I J U o hcouples the displ acement, waul ld be 0·2". Bil t astronomi-cal observations in circumstances far less favorable tlran thoseill which these may be taken nave been made tohundredths ofa seeond ; so that this new method ibids fair to be at least assensitive Il.8 the (o,rmer.

The arrangement of apparatus might be as in fig. 2; IJ in thefocus of the lens a, is a slit; bb cc are two glass mirrors Qpti-, e ;a l lypl ane and so si lvered as _to all ow say one-tweutieth of t .he·!lghli ~ pasa through, and reflect ing say ninety per cent. TheIDteD!llty of the light falling on the observing telescope dt

II

6c

8,

B



ywould .be about ooe.mi !liontb of the ori~inal i nte:~eity, 80' tbatIf sunlight or the electric arc 'were used It could st ill be r eadilyseen. The mirrors b b . , and CC I would differ from parallelismsufficiently to. separate the successive images. Finally, 'heapparatus need not be mounted so as to revolve, as the earth'srotat ion would be sufficient.

If ii i were possible to measure with sufficient accuracy thevelocity of light without returning the ra y to its s'tarting pointtbe problem of measuring the a,rst power of the relativevelocit:of the earth with respect to the ether would be solved. Thfl;may TIO't be as hopeless as might appear at first sight., since thedifficulties are entirely mechanical and may possibly be sur-mounted in the course of time.

For example, suppose (fig. 3) m and m l two mirrors revolvingwith equal veloci ty in opposite di rections. It s evident 'that lightfrom :> will form astat.ionary image at 3, and similarly lightfrom 8, will form a stationary image at 8. If now the velocityof the mirrors be increased safficiently, their phases still beingexactly the same, both images will be deflected from 6 and 6,

i~ i nv e rs e w o po rt io n _to,the, velocities 0, f h g,bt in the. two ~i,r,ec.twos j or, If the two deflections are made equal, and the differ-ence of phase of the mirrors be simultaneously measured, thiswill evidently be proportional to the difference of velocity inthe two direotione, The only I ' E l B Idifficillty l ies in this measure-meat, 'I'he following is perhaps a possible solution: 99, (fig.,4) are two gratings on whichsunligbt is ooaeentrated, Theseare placed so that after faUil1lfi';on the revolving mirrors m and171" the light forms images of the grating'S at 8 ands , two verysensiti ve 9~lenium cella in eircuit with a ba.ttery and a'telephone.neverytbmg be symmetr ica l; the sound in the telephone will bea maximu.m. If no ... one of ~he slits ~be displaced throughh~lf the distance between the Image of the gratmg bars, thereWill be silence, Suppose now that the two deflections having~eetl made 6:X8ct1y equal, the siit is adjusted for silence. Thend the experiment be repeated when the eartb's rotation hasturned. the whole apparatus through 180", and the deflecticnsare a ,g am .made equal, there will no longer be silence, and theangular distance through which s must be moved to restoreSi lence wil1lJl~asure the required difference in phase.

T~ere remain three other methods, all astronomical, for at-tackmg the problem ofthe motion ofthe solar sy,stem throughspace.

1. Tbe ~elescopi~ observation.; Of. the p.roper. motion,. 8 ~.f thestars." T ~ l l Shas g~ven 11 - "a hIghly probably determinatiou ofthe direorion of thIS motioe, but only a guess as to its amount.

2 .. :rhe speotroacopie observation of the motion of :ata,ra inthe Iine of sigh Thu,cQuid furni~h data, lor tbe relative

mQtoions001,.. though it .seems likely tbat by the immense im-provements II!) the photography of stellar spectra, the informa-t ion thus obta ined will be far more accurate tha.n any otber.. 3. Fillallyther~ remains the ~e'terlDinatio? of tbevelocity of

bgh t by observl l. tlOns of tbe eel ipsea of JupI ter 's satel li tes. Ifthe improved photometric methods practiced at the Harvardo'bserntory make it possible to observe these with sufficienta,oouraoy, the d~ffere~ce ia the results found for the velocity oflight wben.1uploor IS nearest to and farthest from the Iiue ofm?tion will give, not merely the motion of 'the .8018or.systemWIth reference to the stars, but with reference to the lumio-iferotlllether itself, '

1887 - albert michelson and edward morley - on the relative motion of the earth and the luminiferous...

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