550 



NA TURE 



[April 7, 1892 



of parts of the medium, as when dense matter is artificially 

 moved. The latter kind of motion may produce many effects 

 which the former cannot. 



A summary of this part of the discussion is as follows : — 



Source alune moving produces a real and apparent change of 

 colour ; a real but not apparent error in direction ; no lag of 

 phase, except that appropriate to altered wave-length ; a change 

 of intensity corresponding to different wave-lengths. 



Medium alone moving, or source and receiver moving to- 

 gether, gives no change of colour ; no change of direction ; a 

 real lag of phase, but undetectable without control over the 

 medium ; a change of intensity corresponding to different 

 distances but compensated by change of radiating power. 



Receiver alone moving gives an apparent change of colour ; 

 an apparent change of direction ; no change of phase, except 

 that appropriate to extra virtual speed of light ; change of 

 intensity corresponding to different virtual velocity of light. 



The probable absence of a first order effect of any kind, due 

 to ethereal drift or relative motion between earth and ether, 

 makes it necessary to attend to second order effects. 



The principle of least time is applied, after the manner of 

 Lorentz, to define a ray rigorously, and to display the effect of 

 existence or non-existence of a velocity potential. Fresnel's 

 law is seen to be equivalent to extending the velocity potential 

 throughout all transparent mattar. 



It is shown that a ray traversing space or transparent sub- 

 stances will retain its shape, whatever the motion of the medium, 

 so long as that motion is irrotational, and that in that case the 

 apparent direction of objects depends simply on motion of ob- 

 server ; but, on the other hand, that if the earth drags with it 

 some of the ether in its neighbourhood, stellar rays will be 

 curved, and astronomical aberration will be a function of latitude 

 and time of day. 



The experiment of Boscovich, Airy, and Hoek, as to the effect 

 of filling a telescope-tube with water, does not discriminate 

 between these theories. For if the ether is entirely non-viscous 

 and has a velocity potential, stellar rays continue straight, in 

 spite of change of medium (or at oblique incidence are refracted 

 in the simple manner), and there will be no fresh effect due to 

 change of medium ; while, if, on the contrary, the ether is all 

 carried along near the earth, then it is stationary in a telescope 

 tube, whether that be filled with water or air, and likewise no 

 effect is to be expected. In the case of a viscous ether, all the 

 difficulty of aberration must be attacked in the upper layers 

 above the earth ; all the bending is over by the time the surface 

 is reached. It is difficult to see how an ethereal drift will not 

 tend to cause an aberration in the wrong direction. 



Of the experiments hitherto made by Arago, Babinet, Max- 

 well, Mascart, Hoek, and perhaps others, though all necessary 

 to be tried, not one really discriminates between the rival hypo- 

 theses. All are consistent either with absolute quiescence of 

 ether near moving bodies, or with relative quiescence near the 

 earth's surface. They may be said, perhaps, to be inconsistent 

 with any intermediate position. 



Two others, however, do appear to discriminate, viz. an old 

 and difficult polarization experiment of Fizeau {An7t. de Chim. 

 £t de Phys., 1859), which has not been repeated since, and the 

 recent lamous experiment of Michelson {Phil. Mag., 1887) with 

 rays made to interfere after traversing and retraversing paths at 

 right angles. 



The conclusions deducible from these two experiments are 

 antagonistic. Fizeau's appears to uphold absolute rest of ether ; 

 Michelson's upholds relative rest, i.e. drag by the earth. 



The author now attempts a direct experiment as to the effect 

 of moving matter on the velocity of light in its neighbourhood ; 

 assuming that a positive or negative- result with regard to the 

 effect of motion on the velocity of light will be accepted as 

 equivalent to a positive or negative result with respect to the 

 motion of the ether. 



He gives a detailed account of the experiment, the result of 

 which is to show that such a mass as a pair of circular saws 

 clamped together does not whirl the ether between the plates to 

 any appreciable amount, not so much, for instance, as a i/500th 

 part of their speed. He concludes, therefore, that the ether is 

 not appreciably viscous. But, nevertheless, it may perhaps be 

 argued that enormous masses may act upon it gravitationally, 

 straining it so as perhaps to produce the same sort of effect as if 

 they dragged it with them. He proposes to try the effect of a 

 larger mass. Also to see if, when subject to a strong magnetic 

 field, ether can be dragged by matter. 



NO. II 7 I, VOL. 45] 



The aberrational effect of slabs of moving transparent matter 

 is considered, also the effect of a differently refractive medium. 



Motion of medium, though incompetent to produce any aber- 

 rational or Doppler effect, is shown to be able to slightly modify 

 them if otherwise produced. 



The Doppler effect is then entered into. The question is dis- 

 cussed as to what the deviation produced by a prism or a grating 

 really depends on : whether on frequency or wave-length. It is 

 shown that whereas the effect of a grating must be independent 

 of its motion and depend on wave-length alone, yet that the 

 effect observed with a moving grating by a moving observer 

 depends on frequency, because the motion of the observer super- 

 poses an aberrational effect on the true effect of the grating. 

 This suggests a means of discriminating motion of source from 

 motion of observer ; in other words, of detecting absolute 

 motion through ether ; but the smallness of the difference is not 

 hopeful. 



Michelson's experiment is then discussed in detail, as a case 

 of normal reflection from a moving mirror or from a mirror in a 

 drifting medium. No error in its theory is discovered. 



The subjects of change of phase, of energy, of reflection in 

 a moving medium, work done on a moving mirror, and the 

 laws of reflection and refraction as modified by motion, are 

 considered. 



It is found that the law of reflection is not really obeyed in a 

 relatively moving medium, though to an observer stationary 

 with respect to the mirror it appears to be obeyed, so far as the 

 first order of aberration magnitude is concerned ; but that there 

 is a residual discrepancy involving even powers of aberration 

 magnitude, of an amount possibly capable of being detected by 

 very delicate observation. 



The following statements are made and justified : — 



(1) The planes of incidence and reflection are always the 

 sarne. 



(2) The angles of incidence and reflection, measured between 

 ray and normal to surface, usually differ. 



(3) If the mirror is stationary and medium moving, they 

 differ by a quantity depending on the square of aberration mag- 

 nitude, i.e. by I part in 100,000,000 ; and a stationary tele- 

 scope, if delicate enough, might show the effect. 



(4) If the medium is moving and mirror stationary, the 

 angles differ by a quantity depending on the first power of 

 aberration magnitude (i part in 10,000), but a telescope moving 

 with the mirror will not be able to observe it ; for the common- 

 place aberration caused by motion of receiver will obliterate 

 the odd powers and leave only the even ones ; the same as in 

 case (3). 



(5) As regards the angles which the incident and reflected 

 waves make with the surface, they differ in case (3) by a first 

 order magnitude, in case (4) by a second order magnitude. 



(6) At grazing incidence the ordinary laws are accurately 

 obeyed. At normal incidence the error is a maximum. 



(7) The ordinary laws are obeyed when the direction of drift 

 is either tangential or normal to the mirror, and is disobeyed 

 most when the drift is at 45°. 



(8) In general, the shape of the incident wave is not precisely 

 preserved after reflection in a moving medium. To a parallel 

 beam the mirror acts as if slightly tilted ; to a conical beam as 

 if slightly carved. But either effect, as observable in the result, 

 is almost hopelessly small. 



(9) Similar statements are true for refraction, assuming 

 Fresnel's law. 



The possibility of obtaining first order effects from general 

 ethereal motion by means of electrical observations is considered. 



Chemical Society, March 17.— Dr. W. J. Russell, F.R.S. 

 Vice-President, in the chair. — The following papers were 

 read : — A study of the conditions which determine combination 

 between the cyanides of zinc and mercury, and of the composi- 

 tion and properties of the resulting double salt, by W. R. 

 Dunstan. When a solution of zinc sulphate is added to one of 

 mercuric potassium cyanide, HgK2(CN)4, or when mercuric 

 chloride is added to a solution of zinc potassium cyanide, 

 ZnK2(CN)4, a white precipitate is formed, which has been stated, 

 on the authority ofGmelin, to consist of a double cyanide of zinc 

 and mercury of the formula ZnHg(CN)4. This, however, is not 

 the case. The maximum amount of mercuric cyanide that can 

 be retained by the precipitate is only 38*5 per cent., and is 

 dependent on the amount of water present during precipitation 

 as well as on the pi-oportions in which the salts interact. When 



