190 



Table 181 . 

 INDEX OF REFRACTION. 



Indices of Refraction of Qases 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 »t — i =: °4- aJ 'fin' ^^^""^ 

 «e is the index of refraction for temperature i, «o for temperature zero, o the coefficient of expansion of the gas 

 with temperature, and / the pressure of the gas in millimetres of mercury. Talung the mean value, for Jiir and 

 white light, of «<, — i as 0.0002936 and a as 0.00367 the formula becomes 



.0002936 P .0002895 ^ 



I + .00367 / 1.0136 X lo^ I + .00367 10"' 

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



Smithsonian Tables. 



