TABLE 198. 



INDEX OF REFRACTION. 



Indices of Detraction 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 t i =r - - "- where 



i + 0/760 



nt is the index of refraction for temperature t, n a 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 i as 0.0002936 and a as 0.00367 the formula becomes 



.0002936 _ P .0002895 P_ 



i -j- .00367 t 1.0136 X io 6 i + .00367 io' 



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



SMITHSONIAN TABLES. 



IQO 



