in the Velocity of Propagation of Light in Water, 355 

 ingly A = l and B = 0. Then from (I.) and (II.) we get 



A 2 -l 



D 



= M, (III.) 



J=N (IV.) 



Schrauf calls M the " specific refractive power" (specifisches 

 Brechungsvermogen), and he calls N the "specific dispersive 

 power" {specifisches Dispersionsvermogen). 



Although Schrauf s views concerning the cause of dispersion 

 and the effect of the material molecules on light contain much 

 truth, and the consistent carrying out of his hypothesis must be 

 called very able, it is impossible to refraiu from applying the 

 term obscure, if not confused, to the commencement of his work. 



In order to establish the constancy of the magnitude M and 

 N, he takes the experiments of Gladstone and Dale (1858), and 

 uses also whatever other materials are at his disposal, such as 

 the observations of Deville, Cahours, Handl and Weiss, De Roux, 

 and others. 



It appeared that — ~r — was approximately constant, but dimi- 

 nished continually as the temperature increased. The constancy 

 of B does not appear to me to be even approximately proved by 

 the numerical data. Schrauf endeavoured to account for the 

 diminution of the magnitude M by assuming that the angle of 

 the prism had changed by increase of temperature — a supposi- 

 tion which Gladstone and Dale* have rejected as totally un- 

 founded. 



All the elegant subsequent deductions, founded upon the fun- 

 damental assumption that M and N are independent of the 

 temperature, fall to the ground as soon as it is shown that these 

 magnitudes are not constant, but vary considerably with the 

 temperature. We cannot, therefore, regard it proved in this 

 manner that the refractive power is constant, or a multiple of 

 the simplest factors of the series of natural numbers, 



"(A'-IJ -.-r 



nor, further, that only the variations of density and not the elas- 

 ticity is of considerable influence upon the velocity of propaga- 

 tion of light, nor that the latter is only a function of the den- 

 sity, &c. 



* Phil. Trans. 1863, p. 343. 



