66 Prof. Quincke on the Change in the Volumes and Refractive 



liquid was increased by a certain amount measured by means 

 of a mercury-manometer. 



The liquids, contained in glass or metal tubes of 230millim. 

 length, were introduced into an interference-apparatus pre- 

 viously employed*, which I have already described. The 

 liquid was traversed by one of the two interfering pencils 

 of rays, and the number of interference-bands measured which 

 corresponded to a given Fraunhofer line in a pure spectrum. 

 The changes of pressure were produced by means of an air- 

 pump. 



The compressibility of the liquid may be calculated from 

 these optical experiments, under certain assumptions as to the 

 constancy of the so-called specific refractive power or of the 

 refraction- constant. If we call 



y the number of interference-bands which pass through the 

 fixed Fraunhofer line for an increase in pressure of p 

 millim. mercury in a layer of liquid of D millim. length; 



Y the number of interference-bands which would pass 

 through the same line of the spectrum for an increase in 

 pressure of one atmosphere, and a layer of liquid of 1000 

 millim. length.; 

 n, n x the refractive index 1 of the liquid before and after the 

 «j, 0*! the specific gravity J compression; 



X the wave-length of the reference-line of the spectrum in 

 air; 



fi the compressibility of the liquid; 



then, since change of volume is proportional to increase of 

 pressure, 



?= 1+ "7fo> ••■•••• (1) 



The difference in the refractive indices may be calculated by 

 the aid of equation (1) in different ways, according to the 

 assumption we make as to the so-called refractive power, or 

 some analogous quantity. 



If we assume with Dale and Gladstone!, Landoltf , and 

 others that the so-called specific refraction is constant, then 



72 — 1 



* Quincke, Pogg. Ann. cxxxii. p. 53, and Table II. fig, 13, 1867. 



t Phil. Trans. 1863, p. 377. 



J Pogg. Ann. cxxiii. p. 596 (1864) j JBerL Sitzungsber.ix. 1. 1882, p. 64. 



