the jEther cause Doable Refraction ? 679 



aether in the laboratory shares the earth's motion. But other 

 phenomena, especially stellar aberration, favour the opposite 

 theory of a stationary rether. The difficulty thus arising has 

 been met by the at first sight startling hypothesis o£ FitzGerald 

 and Lorentz that solid bodies, such as the stone platform of 

 Michelson's apparatus, alter their relative dimensions, when 

 rotated, in such a way as to compensate the optical change 

 that might naturally be looked for. Larmor (' iEther and 

 Matter/ Cambridge, 1900) has shown that a good case may 

 be made out for this view. 



It occurred to me that such a deformation of matter when 

 moving through the aether might be accompanied by a sensible 

 double refraction ; and as the beginning of double refraction 

 can be tested with extraordinary delicacy, I thought that even 

 a small chance of arriving at a positive result justified a careful 

 experiment. Whether the result were positive or negative, 

 it might at least afford further guidance for speculation upon 

 this important and delicate subject. 



So far as liquids are concerned, the experiment is of no 

 great difficulty, and the conclusion may be stated that there 

 is no double refraction of the order to be expected, that is 

 comparable with 10~ s of the single refraction *. But the 

 question arises whether experiments upon liquids really 

 settle the matter. Probably no complete answer can be given, 

 unless in the light of some particular theory of these relations. 

 But it may be remarked that the liquid condition is no 

 obstacle to the development of double refraction under electric 

 stress, as is shown in Dr. Kerr's experiments. 



The apparatus was mounted upon the same revolving board 

 as was employed for somewhat analogous experiments upon 

 the rotation in quartz (Phil. Mag. vol. iv. p. 215, 1902). Light, 

 at first from the electric arc but later and preferably from lime 

 heated by an oxyhydrogen jet, after passing a spectacle-lens so 

 held as to form an image of the source upon the analysing nicol, 

 was polarized by the first nicol in a plane inclined to the hori- 

 zontal at 4:5°. The liquid, held in a horizontal tube closed at 

 the ends by plates of thin glass, was placed, of course, between 

 the nicols. When at 12 o'clock the board stands north and 

 south, the earth's motion is transverse and the situation is 

 such as to exhibit any double refraction which may ensue. 

 It might be supposed, for instance, that luminous vibra- 

 tions parallel to the earth/ s motion, i. e. east and west, 

 are propagated a little differently from those whose direction 

 is transverse to the earth's motion, i. e. vertical. But if the 



* 10 -8 =(10~ 4 ) 2 , where 10~ 4 is the ratio of the velocity of the earth 

 in its orbit to the velocitv of light. 



