Secrion III., 1910. [ 45 Trans. R. 8. C. 
III.—The Variation of Water Vapour Lines in the Solar Spectrum. 
By N. R. Gris, M.Se., Lecturer in Physics, 
McGill University, Montreal. 
Communicated by Dr. H. T. Barnus. 
(Read Sept. 28th, 1910.) 
HISTORICAL. 
As early as 1833 certain lines and bands of the sun’s spectrum 
were found to vary with the proximity of the sun to the horizon’. 
These consisted of a large group near the D line, the lines A, B and C, 
which were greatly widened, and other lines which were developed 
between A and B and between C and D. The source of these lines 
was suggested to be the absorptive action of the earth’s atmosphere. 
A very extensive study of the variation of such lines was carried 
out by Brewster and Gladstone’. A map showing the lines studied 
is appended, together with smaller maps of selected portions of the 
spectrum. 
These observations were verified by the eminent French physicist 
Janssen.? He was able by means of the greater dispersive power of 
his spectroscope to render these bands into lines. By noting the dif- 
ference in intensity of the lines at various altitudes, he showed that 
the agency by which the alteration was produced was the absorption 
of the atmosphere. He observed the absorption spectrum of water 
vapour’ and found that the bands therein corresponded’ with those 
of the solar spectrum that increased in blackness as the sun approached 
the horizon. He also mapped the spectrum of the sun between C and 
D, as it appeared at noon and near sunset; by a comparison of these, 
the water vapour lines could be detected, viz., they consisted of all 
lines in the second spectrum which were not found in the first. In 
this manner the lines were concluded to occupy the region near C, that 
in the neighbourhood of D and a region almost midway between these 
two. 
Cornu® carried out a very careful investigation upon the lines 
that are due to atmospheric absorption. He succeeded in distiñguish- 
1 Sir D. Brewster: Phil. Mag. (3), 8, 384. 
? Phil. Trans., 150, 149 (1861). 
# Annales de Chimie et de Physique (4), 23, 274-300 (1871). 
* Annales de Chimie et de Physique (4), 24, 215-217 (1871). 
5 Annales de Chimie et de Physique (6), 7, 1-102 (1886). 
