﻿"30 Messrs. W. J. Jones and J. R. Partington on the 

 If we denote the expression 



+ ]28' 7r ' r(1 ~ 6T2) ( 8 ) 



by cf) we have from (7) and (5) 



*£?-#.B.T. . .... (9) 



For a low pressure p , the factor $ obviously approaches 

 unity, and, if /x denote the refractive index of the gas at 

 the pressure p , we have 



*£?=*■* do) 



Dividing (9) by (10) we obtain 

 P Mo — 1 



*>-i -*■:- ■ < u > 



If for a second gas, for which the value of 



at the same standard pressure, p, and temperature, T, 

 denoted by <£', the values of the refractive indices are //,' and 

 /jlq at p and p respectively, then 



is 



p /V— 1 ,, 



^ >'-l 



*' (12; 



Whence dividing (12) by (11) we eliminate pjpo, and obtain 



^r^S^rr* < 13 > 



Mo — J- Mo J- 



The experimental determinations of the refractive indices 

 of gases, at normal temperature and pressure, have usually 

 been made by interference methods involving the counting 

 of the number of bands which cross the field of view on 

 subjecting the gas to small changes of pressure. Strictly 

 speaking each of these determinations would require cor- 

 rection for the changes of pressure from the normal value. 

 Since, however, these pressure-changes were inconsiderable, 

 it is quite legitimate to assume that the values of the re- 

 fractive indices, as given by the several investigators, repre- 

 sent the values at normal temperature and pressure. These 



