the ^Ether cause Double Refraction ? G&'d 



is 1*2 x 10" 10 . The effect to be expected is of the order 

 10 -8 , so that there is nearly 100 times to spare. The above 

 relates to the bisulphide of carbon. With the water the 

 delicacy of the test was somewhat less. 



When it is attempted to replace the liquid by solid matter, 

 the difficulties of experiment are greatly increased. The 

 best results that I have been able to obtain were with built 

 up thicknesses of plate-glass. A sufficient thickness in one 

 piece is liable to exhibit too much double refraction from the 

 effect of internal strains. A number of triangular pieces of 

 plate-glass, no larger than necessary, and about 6 mm. thick, 

 were put together in a trough to a total thickness of about 

 110 mm. The interstices between the faces being filled 

 up with bisulphide of carbon, the internal reflexions were 

 sufficiently reduced. One difficulty is to get quit of motes 

 and threads which adhere to the glass and become extra- 

 ordinarily conspicuous. Advantage was thought to be derived 

 from shaking up the bisulphide of carbon with strong sulphuric 

 acid. At the best the residual motes and specks in the glass 

 interfere very seriously with the observation, and the loss of 

 light due to imperfect transparency operates in the same 

 direction. The least load upon the upright strip that could 

 be detected with certainty was now 100 grms., so that as 

 compared with the observations upon liquid there was a loss 

 of delicacy of four times. In addition to this, the effect to 

 be expected is reduced in the proportion of 7 : 1, that being 

 the ratio of lengths traversed by the light. Thus in all we 

 lose 2$ times as compared with the liquid. In the latter 

 case we calculated a margin of 100 times, so that here there 

 would remain a margin of about 3 times . 



A subsequent attempt was made to increase the total 

 thickness of the combined glasses to about 220 mm., but no 

 real advantage was gained. The loss of light and increase 

 of disturbance from motes and residual double refraction 

 prejudiced the delicacy in about the same proportion as the 

 length of path was increased. 



But although the results of the observations upon solids 

 are very much less satisfactory than in the case of liquids, 

 enough remains to justify us in concluding that even here 

 there is no double refraction (of the order to be expected) 

 due to motion through the rether. 



Terlma- Place, "SVitham. 



