“apour in the Visible and Ultra-violet Regions. 295 
altering the appearance of the fringes. By combining the 
data obtained with the interferometer with the relative values 
obtained by prismatic deviation, it has been possible to obtain 
absolute values for a very wide range of wave-lengths. We 
will begin with a brief discussion of the properties of the 
medium. 
Physical and Optical Properties of the Vapour of Metallic 
Sodium. 
One of the chief difficulties which has stood in the way of 
measuring the optical constants of sodium vapour is the 
impossibility of confining the vapour in vessels bounded by 
flat surfaces of glass. As soon as the temperature is raised 
to such a point that vapour of any considerable density forms, 
the glass is attacked and rendered opaque by the reduction of 
silica. The vapour, however, has a most remarkable viscosity, 
which I am at the present time investigating, which makes 
it possible to form a mass of great density separated from the 
glass plates which close the ends of the tube by a high vacuum. 
If we place a number of pieces of clean sodium in a tube of 
hard Jena glass, the ends of which are closed with small pieces 
of thin plate-glass, and exhaust the tube on a mercurial pump, 
on heating the under side strongly with small Bunsen flames, 
the sodium vapour shows very little inclination to distil to 
the cold parts of the tube. It condenses to be sure on the 
upper side of the tube, but is given cff so much more rapidly 
from the surface of the molten metal than it can diffuse to 
the upper portion, that the density gradient is very steep. 
Observations on the deviation produced by the non-homo- 
geneous cylinder show that the equivalent prism has a form 
similar to that shown in fig. 1, the density gradient 
Jenene 
being steeper near the bottom of the tube. To secure 
good definition it is therefore necessary to place in front of the 
tube an opaque screen, perforated with a wide horizontal 
slit. The tube thus prepared, used in the manner to be 
presently described, shows the strong anomalous dispersion 
in the vicinity of the D lines with great distinctness. If the 
flames are made to play upon the upper surface of the tube, 
a 
