298 Prof. R. W. Wood on the Dispersion of Sodium 
little adjustment it was possible to get the fringes formed by 
helium light distinct nm one telescope, and those formed 
by the light from the monochromatic illuminator distinct 
in the other. The helium tube standing a little to one 
side of the slit, very little overlapping of the two systems 
occurred. 
The sodium was contained in a seamless steel tube, covered 
with asbestos paper and wrapped with a helix of iron wire 
through which a heavy current was passed. The ends of the 
tube were closed with plates of glass, which were of such a 
thickness that they served in place of the compensating plate 
which is usually placed parallel to the transparent mirror. 
The tube was exhausted by means of a mercurial pump, with 
which it was kept in connexion throughout the experiments 
in order to remove the occluded hydrogen which was given 
off as soon as the sodium was heated. This occluded gas 
gave a good deal of trouble, and the behaviour of the fringes 
seemed to indicate that a small amount of the gas became 
entangled in the sodium vapour, no amount of pumping 
appearing to remove it entirely. Its effect on the fringe-shilt 
will be discussed presently. 
The Ds line of helium was so strong in the tube used that 
no colour-screens were necessary, the fringes being as distinct 
as those obtained with sodium light. Owing to its proximity 
to the absorption-band, very large fringe-displacements were 
obtained with a small amount of comparatively rare sodium 
vapour. 
In the electrically-heated tubes the sodium vapour showed 
no tendency to form the non-homogeneous equivalent of a 
prism, and the fringes consequently remained straight, even 
after very great displacements. 
Two observers were necessary ; one counting fia helium 
fringes as they passed the cross-hair of the telescope, and 
the other noting the transit of the fringes formed by the light 
from the monochromatic illuminator. 
If the light used was on the same side of the D lines as the 
helium line, the two sets of fringes were shifted in the same 
direction ; if on the other side, the shifts were in opposite 
directions. In addition to comparing light from the mono- 
chromatic illuminator with the helium light, a very careful 
comparison was made with the green mercury line, since 
this light was to be used in the absolute determinations. 
At first very discordant results were obtained, but the source 
of error was soon discovered to lie in the heating and conse- 
quent expansion of the base-bar of the interferometer by the 
radiation from the sodium tube. The instrument was then 
