322 = Prof. R. W. Wood on the Dispersion of Sodium 
If m and m’ were each assigned the same value obtained 
by dividing our original value by 2, in all probability a very 
close approximation would be obtained in the region in 
question. This has not been done for two reasons. In the 
first place, it does not appear as if much would be learned by 
the procedure; and in the second place, m and m’ are not 
equal, as is shown by the stronger dispersion near D,, and 
until the relative values have been determined we are not in 
a position to write the two-member formula accurately. It 
is doubtful whether anything new would come out of such a 
determination, and it was on that account not attempted. 
Another matter of considerable interest is the question of 
the indices represented by the square root of a negative 
quantity in the immediate vicinity of the absorption-band on 
the blue side. Lord Kelvin interprets this as indicating that 
no light of such wave-lengths enters the medium; in other 
words, it is metallically reflected. It is in this way that he 
has explained the apparent greater broadening of the D lines 
on the more refrangible side in some of Becquerel’s photo- 
graphs. In the case with which we are dealing, the second 
term of our original formula does not become less than unity 
until we reach wave-length 58898, which we get by equating 
2 
we to unity and solving for 2. 
nr 
This shows us that, even with a vapour so dense that both 
D lines run together and broaden out into a wide band, we 
do not get values of the,index which are imaginary until 
we are within 0°2 of an Angstrém unit of the D line, or, in 
other words, until we are within a distance of D. equal to 
1/30 of the distance between D, and D,. In the case of the 
comparatively rare vapour employed by Becquerel, we should 
have to approach much closer than this to get the imaginary 
values. This makes it appear certain that the greater 
broadening on the more refrangible side, if it exists, must 
be assigned to some other cause than imaginary values of 
the refractive index. 
The medium is exceptionally interesting in that its dis- 
persion can be represented throughout the entire range of 
wave-lengths without taking the extinction coefficient into 
account, as is always necessary in the case of solids and 
liquids when in the vicinity of the absorption-band. 
The Question of the Selective Reflexion of Sodium Vapour. 
The vapour of sodium should exhibit strong selective re- 
flexion in the vicinity of the D lines, just as the aniline dyes 
