Indices of Fluids produced by Hydrostatic Pressure. 67 



Whilst there is no theoretical ground for this hypothesis, the 

 theory of light requires that the specific refractive power 

 should be constant, or 



n 2 — l n\ — 1 



= — = constant (3b) 



cr a v y 



M. H. A. Lorentz*, of Amsterdam, and M. L. Lorenzf, of 

 Copenhagen, have recently drawn the conclusion, from dif- 

 ferent theoretical grounds, that another quantity, the so-called 

 refraction-constant, is constant, whence 



n 2 _l 1 nf— 1 1 



~~ ^2^2 ' 7 X = constant - • • ( 3c ) 



i 



From the three equations (3a), (3b), (3c) the ratio of the 



specific gravities is obtained, observing that n x —n is a very 



small quantity, 



0"i -• n,—n ,. x 



-=1+- J — T -, (4a) 



cr n—l y v 7 



0i - 2n(n-,—n) /A 



0" 7T — 1 ; v ' 



cr" ±+ (n a -lXw 2 + 2y ' ' ' K ' 



If in these equations we put for n x — n its value from equation 

 (2), and for — its value from equation (1), we have 



X 



1000 



1 



n-V 



X 



1000 



2n 

 n 2 —V 



X 



6n 



^=Y. TP : 7 ^.-^— T , ..... (5b) 



v a, vn ,~ x 



^"" 1, iooo , (n 2 ~ix^ 2 +2y " ' ( ; 



the suffix a, &, or c denoting whether the value of /<& has been 

 obtained by calculation from formula (3a), (3b), or (3c). 



The values of the compressibility of water calculated from 

 these equations (5) are compared in the following Table with 

 those obtained by direct observation, and the numbers of inter- 

 ference-bands Y for the particular Fraunhofer line. 



* Wied. Ann. ix. p. 642 (1880). 



t Wied. Ann. xi. p. 77 (1880). 



F2 



