190 TABLE 181. 



INDEX OF REFRACTION. 



Indices of Refraction of Gases and Vapors. 



A formula was given by Biot and Arago expressing the dependence of the index of refraction of a gas on pressure and 

 temperature. More recent experiments confirm their conclusions. The formula is n ti = n 7^\-4- t where 

 n t is the index of refraction for temperature f, for temperature zero, a the coefficient of expansion of the gas 

 with temperature, and> the pressure of the gas in millimetres of mercury. Taking the mean value, for air and 

 white light, of n i as 0.0002936 and a as 0.00367 the formula becomes 



n i .0002936 . P . 



i -f- -00367* 1.0136 X io 8 



.0002 _ 



i -f- .00367 io e ' 



where P is the pressure in dynes per square centimetre, and t the temperature in degrees Centigrade. 



