64 THE ROYAL SOCIETY OF CANADA 



the corresponding densities of the gas, and L = length of inter- 

 ferometer tubes 

 then X/= (np-n) L (1) 



Also, if Gladstone and Dale's law is true, 



np—l 71—1 Hp — n np — n 

 = and .*. = =■ 



Pp P Pp 



Cfo - 



Hp-n X/760 

 whence n-1 = ^ = ^(p_760) ^2) 



V760 V 

 so that n may readily be found. A temperature correction can be 

 applied if necessary. 



It is obvious from (1) that if the gas in one tube is allowed to 

 drop from a pressure Pi to a pressure P2 the number of fringes cross- 

 ing the centre of the field is proportional to the change of refractive 

 index from pressure Pi to pressure P2. 



Gladstone and Dale's law may be tested in two ways: 



(1) By changing the density of the gas by the same amount at 

 different pressures and fielding the corresponding change of refractive 

 index. If the number of fringes crossing the centre remains the 



dn 



same, the change of index is constant, i.e. ~ is constant hence 



dp 



n-1 



— — = a constant. 

 P 



(2) By finding the absolute values of the refractivities of the 



gas at different densities and testing the relationship 

 Wj— 1 «2 — 1 



etc. 



Pi P2 



The oxygen was supplied from cylinders, 99.2% pure, the prob- 

 able impurity being nitroged. The oxygen tube (Fig. 1 shows one end 

 of tube) was of steel 28 cms. long, 3 cms. in diameter, threaded at both 

 ends. Glass windows G (9 mm. thick) were firmly waxed into steel ends 

 5 and these could be screwed on to the tube making gas-tight joints. 



F/G. I . 



