Vapour tn the Visible and Ultra-violet Regions. 307 
80 times the deviation at the helium line. If we number the 
wave-lengths at the lines of the mesh between D, and D, 1 to 
9, the observations were as follows :—The light very close to 
D, (1/4 of a square distant) was seen to be “distinctly pulled 
down through a distance of seven squares with a vapour-density 
sufficient to deviate the spectrum at the first line (=5888°3) 
through a distance of one square. The temperature was 
then raised a trifle. The light very close to D, now dis- 
appeared, partly as a result of absorption and partly through 
being deviated out of the field. The spectrum at the first 
line was deviated through three squares, that at the second line 
one square. The temperature was now raised until the spec- 
trum at the 9th line was deviated through one square. The 
deviation at the 2nd was now four, that at the 3rd three 
squares, and that at the 5th two squares. 
These results for the different wave-lengths between D, 
and D, are given in the following table :— 
; Deviation. 
oss ae re Ee AK Cert 
Bie Baas cs a halajecd Sieis & 2 
JS. Sh Seer a 
EIS SLES 5 a eae ) 
re Shae olism ones 12 
Bt TI Me cS caweaS + duns 54 
(DES SEU nee — 
These values are of course only approximations, the errors 
becoming greater as we approach D,. The same experiment 
was repeated, using a filar micrometer in the eyepiece in 
place of the reticulated mesh, and values obtained which 
agreed fairly well with those given above. This method, 
how ever, did not admit of such a near approach to D, as the 
other. 
From these results it is possible to calculate the values of 
the refractive index within this range for the very dense 
vapour used in the absolute determinations. These will be 
called observed values, and they will be compared with the 
values calculated from the dispersion formula for the corre- 
sponding wave-lengths. 
I have already spoken of the enormous dispersion at the 
helium line, a dispersion great enough to separate a Zeeman 
doublet by an amount equal to nearly the width of the entire 
visible spectrum produced by a glass prism of 60°, provided 
we were able to construct a prism of sodium vapour giving 
the same mean deviation, and get the light in question through 
it. Great as is this dispersion it is ag nothing when compared 
