WOOD. — ANOMALOUS DISPERSION OF SODIUM VAPOn. 891 



For X = X the value of n is calculated to be 1.0000275, while for 

 A = 4167 it is 0.999975, the deviations being the same in both cases, but 

 in opposite directions. 



Al'PLTCATION OF THE ReSITLTS TO THE DISPERSION FORMULA. 



The simplest form of the dispersion formula, developed from electro- 

 magnetic considerations for a medium with a single absorption band, is 



m X' 

 n= 1 4- 



X2 - A„ 



in which m is a constant, A the wave-length of the light employed, and 

 A,„ the wave-length at the centre of the absorption band. 



The vapor of sodium has, of course, a pair of very close absorption 

 bands (the I) lines), which are chiefly effective in modifying the refrac- 

 tivity of the medium. As I have said, the ultra-violet bauds affect the 

 dispersion in their immediate vicinity, but their effect can be neglected 

 in comparison with the stronger band, except for wave-lengths very close 

 to them. 



The first question to consider is whether we are justified in consider- 

 ing the D lines as a single absorption band in the case of the very dense 

 vapor. If we consider the medium as having a single band, and assign 

 to A,„ the value 5893 (a point midway between the D lines), we shall 

 find that the observed and calculated values of the refractive index agree 



