[mclennan-elworthy] JAMIN INTERFEROMETER 21 



the standard gas and the gas whose refractive index was required. 

 The pressure on the gas was changed by varying amounts up to ten 

 atmospheres, the corresponding shift of bands being measured and 

 the refractivity calculated from the relation just given. Burton found 

 for argon, that 300 bands passed the cross line in the spectroscope 

 for a change of pressure of two-thirds of an atmosphere, while for 

 helium a shift of the same number of bands required five atmospheres 

 change in pressure. 



Method Adopted 



In the method described in this paper estimations had to be 

 made every few minutes. Burton's procedure proved unsuitable and 

 was modified, as it was found it could be, by using a much smaller 

 pressure change. The greater refractivity of air, and of low percentage 

 helium-air mixtures, gave a correspondingly greater shift of bands 

 than pure helium would, for the smaller pressure change and therefore 

 measurements of the same accuracy could be made. By the use of a 

 micrometer eyepiece in the observing telescope, the shift of bands 

 could be measured to one hundredth of the distance between two 

 bands. 



From a knowledge of the shift of bands caused by a definite 

 pressure change, the refractivity of a mixture of helium and air or 

 hydrogen and air was found by means of the formula just given. The 

 percentage of hydrogen or helium was then calculated from the re- 

 fractivity as described later. 



During preliminary experiments with this method it was observed 

 that the displacement of air by the gas mixture in one tube of the 

 interferometer gave a sufficiently large shift of bands for accurate 

 measurement, without making a pressure change. 



From the relation (Na— Nx) L= f A where 



Na = refractive index of dry air at Pcms. and t°C 

 N X = refractive index of a 1 % gas mixture at the 



pressure and temperature 

 L = length of interferometer tubes 

 f = shift of bands past the cross line 

 A = wave length of the standard line. 



Curves were plotted for various pressures and temperatures 

 showing the relation between the difference in refractive index of air 

 and of a 1% gas mixture and the shift of bands. 



Na and N^ were calculated from the relation 



