Vapour in the Visible and Ultra-violet Regions. 305 
does not give light enough. In the second place, as is well 
known, the D lines do not break up into polarized triplets in the 
magnetic field, but into a quadruplet and sextuplet. At the 
time of trying the experiment this circumstance was not 
thought of, and the failure may have been due in part to the 
fact that only D, was utilized, which is obviously less suited to 
the purpose, for the reason that it gives us a double line on 
each side of the absorption-band when the central components 
are cut out by the nicol. I am of the opinion, however, that 
the finite breadth of the normal line is chiefly responsible for 
the failure. Some experiments were made with vacuum- 
tubes containing sodium vapour, which yield highly homo- 
geneous light, but they were found troublesome to manage. 
It is quite possible that by the use of these tubes, results 
could be obtained in the case of D,, and utilizing the light 
emitted in the direction of the lines of force (Zeeman 
doublet) we might get data for both lines. This experiment 
would have been tried were it not for the fact that almost 
as satisfactory results can be obtained by the method of 
prismatic deviation, with a much less expenditure of time. 
As a discussion of the modifications of the appearance of 
the fringes formed by sodium light, by the introduction of the 
vapour of sodium into the path of one of the interfering beams 
of light, has no especial bearing upon the determination of 
the dispersion of the vapour, it will be postponed for the 
present. A fuller discussion will be given in a subsequent 
paper on “The Achromatization of Fringes formed by 
approximately homogeneous light by highly dispersing 
media.” 
Prismatic Deternunation of the Dispersion in the immediate 
vicinity of the D Lines. 
As I have already shown, prisms of excellent defining power 
can be formed by heating metallic sodium in highly-exhausted 
tubes of hard glass. In my earlier experiments, in which the 
metal was heated in an atmosphere of hydrogen, the definition 
was not good enough to obtain a smooth curve close to and 
between the D lines by the method of crossed prisms. The 
photographs illustrating my first paper, in which the dis- 
persion between the lines was shown, were taken by employing 
a prismatic flame of hydrogen burned at a platinum jet, and 
fed with the vapour of sodium. By using vacuum-tubes 
heated by very small flames far better results can be secured, 
and perfectly smooth sharp curves obtained. ‘Two different 
methods were used in the determination of the deviation pro- 
duced by the prism for wave-lengths close to the D lines. 
