( 825 ) 



that those parts which are dark wliile the syslcm is at rest, will remain 

 so after it has been put in motion. It will therefore be impossible 

 to detect an intlnence of the Earth's motion on any optical experi- 

 ment, made with a terrestrial sonrce of light, in which the geome- 

 trical distribntion of light and darkness is observed. JNIaiiy experi- 

 ments on interference and ditFraction belong to this class. 



In the second place, if in two points of a system, rays of light 

 of the same state of polarization are propagated in the same direction, 

 the ratio between the amplitudes in these })oints may be shown not 

 to be altered by a translation. The latter remark applies to those 

 experiments in which the intensities in adjacent parts of the Held 

 of view are compared. 



The above conclusions confirm the results 1 ha\'e formerly obtained 

 by a similar train of reasoning, in which hosvever the terms of the 

 second order were neglected. They also contain an explanation of 

 Michelson's negative result, more general and of somewhat different 

 form than the one previously oiven, and they show why Rayleigh 

 and Brace could find no signs of donble refraction produced by 

 the motion of the Earth. 



As to the experiments of Trouton and Noble, their negative result 

 becomes at once clear, if we admit the hypotheses of § 8. It may be 

 inferred from these and from our last assumption (§10) that the only 

 effect of the translation must have been a contraction of the whole 

 system of electi'ons and other particles constituting the charged 

 condenser and the beam and thread of the torsion-balance. Such a 

 contraction does not give rise to a sensible change of direction. 



It need hardly be said that the present theory is put forward with 

 all due reserve. Though it seems to me that it can account for all 

 well established facts, it leads to some consequences that cannot as 

 yet be put to the test of experiment. One of these is that the result 

 of Michelson's experiment must remain negative, if the interfering- 

 rays of light are made to tra\'el through some ponderable transparent 

 body. 



Our assumption about the contraction of the electrons cannot in 

 itself be pronounced to be either plausible or inadmissible. What 

 we know about the luiture of electrons is very little and the only 

 means of pushing our ^^ay farther will be to test such In'potheses 

 as 1 have here made. Of course, there will be difïiculties, e.g. as soon 

 as we come to consider the rotation of electrons. Perha[)S we shall 

 have to suppose that in those phenomena in which, if there is no 

 translation, spherical electrons rotate about a diameter, the points of 

 the electrons in the moxing system ^vill describe elliptic paths, 



