Birefringent Action of Strained Glass. 335 



with particular care. The fall per cent, of effect from air to 

 water oscillated as formerly about the value 30, the agree- 

 ment of successive results being rather better than in the 

 former experiments, but not yet satisfactory. The average 

 of Jill the determinations, carried to four figures, was 31*0c5 ; 

 and this is pretty close to the average of the four per- 

 centages noted above, which is 30*9. 



Upon the whole, and in the meantime, the decrease in 

 question must be taken, in round numbers, as 31 per cent. 



Proposition VIII. 



Of the two strain-generated absolute retardations compared in 

 prop. IV., one half of the greater is the only part which is pro- 

 duced by change of refringent power of the glass, the other half 

 of the greater, and the whole of the less, being due to change of 

 thickness of the plate. 



14. Returning to the final equations of art. 12, and sub- 

 stituting for m, m 1 , a—d, their values given or measured in 

 art. 13, namely, 1*53, 1'33, 'ola, we find 



(»_l)«.-y.^« =-498a, 



eSm =a — *498a = '502a, 

 eS'm = '502a — 'ha = '002a. 



These quantities are very nearly equal to '5a, '5a, 0, which are 

 the values that satisfy the proposition. The common value 

 of the differences, which is '002a, is a quantity too small to 

 be detected with certainty in the experiments, being less than 

 the 2500th part of a wave-length ; and this important pro- 

 position may therefore be accepted in the meantime, as true 

 exactly or approximately. 



Cor. Returning to eq. (6) of art. 12, we see that b f =0 

 when nt' = m, and that V <0 when id >m. The latter case 

 has been realized with the preceding winged plate immersed 

 in carbon disulphide. 



Inferences equally obvious, but more important, are stated 

 in the next two propositions. 



Proposition IX. 



The action of directionally strained glass in the common 

 polariscope, upon light crossing the line of strain at rigid angles, 

 M due exclusively to strain-generated change of velocity of the 

 component ray which has its plane of polarization parallel to 

 the line of strain. 



Suppose the plate vertical, and the line of strain also 



