394 



SCIENCE 



[N. S. Vol. XXXVIII. No. 977 



is that the time taken by light to travel to 

 and fro on a measured interval fixed on a 

 rigid block of matter is independent of the 

 aspect of that block with respect to any 

 motion of the earth through space. A defi- 

 nite and most interesting result : but it may 

 be, and often is, interpreted loosely and too 

 widely. 



It is interpreted too widely, as I think, 

 when Professor Einstein goes on to assume 

 that no non-relative motion of matter can 

 be ever observed even when light is brought 

 into consideration. The relation of light 

 to matter is very curious. The wave front 

 of a progressive wave simulates many of 

 the properties of matter. It has energy, 

 it has momentum, it exerts force, it sus- 

 tains reaction. It has been described as a 

 portion of the mass of a radiating body — 

 which gives it a curiously and unexpect- 

 edly corpuscular "feel." But it has a 

 definite velocity. Its velocity in space 

 relative to the ether is an absolute constant 

 independent of the motion of the source. 

 This would not be true for corpuscular 

 light. 



Hence I hold that here is something with 

 which our own motion may theoretically be 

 compared; and I predict that our motion 

 through the ether will some day be de- 

 tected by help of this very fact — by com- 

 paring our speed with that of light: 

 though the old astronomical aberration, 

 which seemed to make the comparison easy, 

 failed to do so quite simply, because it is 

 complicated by the necessity of observing 

 the position of a distant source, in relation 

 to which the earth is moving. If the source 

 and observer are moving together there is 

 no possibility of observing aberration. 

 Nevertheless I maintain that when matter 

 is moving near a beam of light we may be 

 able to detect the motion. For the velocity 

 of light in space is no function of the 

 velocity of the source, nor of matter near 



it; it is quite unaffected by source or re- 

 ceiver. Once launched it travels in its own 

 way. If we are traveling to meet it, it will 

 be arriving at us more quickly ; if we travel 

 away from it, it will reach us with some 

 lag. And observation of the acceleration 

 or retardation is made by aid of Jupiter's 

 satellites. "We have there the dial of a 

 clock, to or from which we advance or re- 

 cede periodically. It gains while we ap- 

 proach it, it loses while we recede from it, 

 it keeps right time when we are stationary 

 or only moving across the line of sight. 



But then of course it does not matter 

 whether Jupiter is standing still and we 

 are moving, or vice versa: it is a case of 

 relative motion of matter again. So it is 

 if we observe a Doppler effect from the 

 right- and left-hand limbs of the rotating 

 sun. True, and if we are to permit no 

 relative motion of matter we must use a 

 terrestrial source, clamped to the earth as 

 our receiver is. And now we shall .observe 

 nothing. 



But not because there is nothing to ob- 

 serve. Lag must really occur if we are 

 running away from the light, even though 

 the source is running after us at the same 

 pace, unless we make the assumption — true 

 only for corpuscular light — that the ve- 

 locity of light is not an absolute thing, but 

 is dependent on the speed of the source. 

 With corpuscular light there is nothing to 

 observe; with wave light there is some- 

 thing, but we can not observe it. 



But if the whole solar system is moving 

 through the ether I see no reason why the 

 relative ether drift should not be observed 

 by a differential residual effect in connec- 

 tion with Jupiter's satellites or the right 

 and left limbs of the sun. The effect must 

 be too small to observe without extreme 

 precision, but theoretically it ought to be 

 there. Inasmuch, however, as relative mo- 

 tion of matter with respect to the observer 



