Vapour in the Visible and Ultra-violet Regions. 321. 
Unfortunately sufficient time did not remain for the re- 
petition of the experiment with a tube permanently in con- 
nexion with the pump, but the omission is not serious, for I 
feel certain that the explanation given above is the correct one. 
An inspection of the formula shows us that, according as 
we are on the red or blue side of the absorption-band, the 
refractive index is given by adding to or subtracting from 
unity the value of the constant m=0:000055 multiplied by 
the fraction x2 : : In the case of ali other substances 
showing anomalous dispersion, aniline dyes for example, to 
the dispersion of which the formula has been applied, the 
value of this fraction never exceeds 3 owing to the impossi- 
bility of applying it to wave-lengths very close to the centre 
of the band. For example, in the case of the dispersion of 
nitroso-dimethyl-aniline, with its strong absorption-band at 
X=43, we cannot obtain accurate data nearer than X=50. 
In this case 
In the case of sodium vapour the value of the fraction may 
be several hundred or even thousand. In the case of 
X= 5882 the fraction is 367, and yet the observed and cal- 
culated values agree closely. For X=58884 the fraction is 
1940, and for X\=5889°6 we have a value as high as 3044. 
The product of these very large numbers and the small 
fraction 0:°000054 give, however, values of the index which 
are in close agreement with the observed values. 
Discrepancies occur in the immediate vicinity of the D 
lines which can be explained in the following way. To get 
values in any way consistent with the observed values, it was 
necessary to assign to X,, the value of the D, line, the mean 
value 5893 being too far removed from the wave-lengths in 
question to give the requisite steepness to the curve. The 
calculated values, therefore, apply to a medium with a single 
band at D,, and with a constant m=0°00055. This gives us 
a pretty good approximation to the observed curve, but the 
latter is due to the combined effects of the bands D, and D,, 
the presence of the D, band tending to make the observed 
curve flatter than the calculated. A more correct approxi- 
mation could be obtained by assigning to A, a value inter- 
mediate between D, and 5893. The proper method of 
procedure would of course be to make use of two members 
in the dispersion formula, one for D, and the other for D,. 
vs my md? 
. Ae XE 
ne! 
ne=]+ 
