Vapour in the Visible and Ultra-violet Regions. 311 
fringes being taken as the standard. A wave-length, for 
which the relative shift with respect to helium light of the 
interferometer-fringes was found to be 1: 4, is entered in 
the table as 25. 
The shift ratio for the green line of mercury with respect 
to helium was found to be 1: 25. The wave-length of this 
light is, therefore, represented in the table by the numeral 4, 
which means that a prism of sodium vapour which gives a 
deviation 4 for green Hg light gives a deviation 100 for 
helium light. The results obtained by the methods of crossed 
prisms (both visual and photographic) were reduced to the 
same scale, being compared with the value found for the 
green mercury line. This was necessary, since if a vapour of 
sufficient density is used to give a measurable deviation in 
the blue and violet region, it absorbs almost entirely that 
portion of the spectrum occupied by the helium line. The 
values obtained by means of the Zeeman doublet and the two 
yellow lines of mercury, by the method of coincidences, are 
not given in the table. As these values were purely rela- 
tive, 2. e. not compared with helium or any other standard 
wave-length, it was necessary to assign to one of the wave- 
lengths a shift taken from the curve determined by the other 
methods, and then calculate the shift of the other wave-length. 
For example, in the case of the yellow mercury lines, of 
wave-lengths 577 and 579, the ratio as obtained by the co- 
incidence method was 5: 6. The shift of 577 on the curve 
was found to be 14, i. e. 14 of these fringes to 100 of helium 
5. The shift of wave-length 579 with respect to helium is 
given by the proportion 5: 6=14: 2, from which we get 
“=16°8, a value which fell exactly on the curve obtained 
with the interferometer. 
The same thing was done in the case of the values obtained 
by breaking the helium line up into a Zeeman doublet, 11 
fringes for 1=587487, 11:5 fringes for X=587513. The 
values for the range of wave-lengths comprised between the 
helium line and D, were determined by the method of crossed 
prisms, the measurements being made with the reticulated 
mesh in the focus of the eyepiece. For light of wave-length 
midway between D; and D, the deviation is twice as great 
as for D;, consequently we enter this in the table as 200. 
The Ultra-violet Dispersion. 
A qualitative study of the dispersion in the ultra-violet 
was first made by the method of crossed prisms. | 
A tube of hard Jena glass, 4) cms. long and 1:5 cms. 
Y 2 
