123 
IX 
The Use of the Hygro-Spectroscope (ok Rain-band 
Spectroscope) in Meteorology. 
By F. W. Cory, M.R.C.S., F.R.Met. S., &c 
[Bead November 24th, 1883.] 
[Before proceeding to the main subject of his paper, Mr. 
Cory gave an explanation of the well-known property of the 
prism, by which it is enabled to split up a beam of light into 
the component coloured rays which form white light, the 
resulting band being termed a spectrum. He also demon¬ 
strated, by diagrams, the fact that the violet rays are most 
bent out of the straight course (or refracted), and the red the 
least so, these colours forming the extremes of the tinted 
band or spectrum visible to human eyes. He then showed 
the effect of allowing the light to flow through a slit in a 
metal plate, whereby the image of the slit is repeated very many 
times (owing to the different refrangibilities of the component 
Fig. 1.—Section of Rain-band Spectroscope, size of original, the eye¬ 
piece being on the right-hand side of the spectator. 
parts of a ray of white light), and a coloured band is formed, 
each kind of light always occupying a certain relative position 
in the spectrum. He showed that the dark lines (“Fraun¬ 
hofer’s lines ”) which are then seen in various parts of the 
spectrum are due to the absorption or slmtting-off of certain 
rays of light by various vapours in the sun’s atmosphere 
(solar lines) or in our own (telluric lines), and consequent 
absence of any image of the slit in those particular positions; 
the result being that dark lines of greater or less intensity 
are seen crossing the spectrum. The application of these 
facts to the construction of the direct-vision spectroscope was 
then explained, and at Fig. 1 is a drawing showing a section 
