68 



THE ROYAL SOCIETY OF CANADA 



The atmospheric pressure was 750 mm. and the temperature 

 21°C. 



The saturation vapour pressure at 21°C. is 19 mm. 



If L is the length of the tube and Xi, X2, are the number of 

 wave-lengths in the gas and in the air respectively then we have 



Lt =^ Xi Ai = X2 \2 



where Xi, Xv, are the wave-lengths of the light on the two sides. Let 

 Wi, W2, be the refractive indices of the gas and the air respectively and 

 Xo the wave-length in vacuo of the light used. Then 



JLi — ^1 



Xo Xo X\ — X2 



— =X2 — and .". = Xo 



Wl W2 Ui — W2 



or Wi — «2= — (:>!^i — :'C2)- 



Xi — :V2 is the number of fringes that would cross the field if the gas 

 were replaced by air, or what comes to very nearly the same thing 

 if the pressure of the oxygen were lowered to atmospheric pressure. 

 The index for air for X = 5210 Â.U. is takpn as 1.000294. 



Selecting at random 3 states of the gas indicated by A. B.C. in 

 the table, we have calculated the refractive index of the gas in those 

 states using the formula 



w — 1.000294 = ~y~ (number of fringes from the given state down to 

 atmospheric pressure). 



