Vapour in the Visible and Ultra-violet Regions. 319 
numbers representing the ordinates and abscissee being outside 
of the large rectangle. 
Refractive Indices-in the vicinity of the D lines. 
r. w Cale. av Obs.. 
5875 0:9958 0°9954 
5882 0°:9890 0°9908 
5885 0:9830 0°9860 
58866 0:9750 0:9770 
58884 0°9450 0°9443 
58896 0°697 0-614 
5991 1:0046 
5904 1-0092 
5901 1:0138 
58994 1°0184 
58976 1:0557 
5897 1-094 
58964 1°386 
Application of the Results to the Dispersion Formula. 
The simplest form of the dispersion formula, developed 
from electromagnetic considerations for a medium with a 
single absorption-band, is 
2 
mr 
n=1+ Suen: 
m 
in which m is a constant, A the wave-length of the light em- 
ployed, and X,, the wave-length at the centre of the absorption- 
band. 
The vapour of sodium has, of course, a pair of very close 
absorption-bands {the D lines) which are chiefly effective in 
modifying the refractivity of the medium. As I have said, 
the ultra-violet bands 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 considering the D lines as a single absorption-band in the 
case of the very dense vapour. If we consider the medium 
as having a single band, and assign to 2,, the value 5893 (a 
point midway between the D lines), we shall find that the 
observed and calculated values of the refractive index agree 
very closely, up to within a distance of the band about equal 
to the distance of the D; line of helium. If we attempt to 
go closer than this, we immediately find discrepancies, which 
become larger as we approach the D lines. 
