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IX.
The Use of the Hygro-Spectroscope (or Rain-band
Spectroscope) in Meteorology.
By F. W. Cory, M.R.C.S., F.R.Met.S., &c.
[Read 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 shutting-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 .