124 The Use of the Hygro-Spectroscope
of the instrument used by the author. This spectroscope
contains a battery of five prisms : 1, 3, and 5 are made of
crown-glass, and 2 and 4 of flint-glass. It is found that by
this arrangement a greater dispersion of the coloured rays is
produced, and consequently a better view of the dark, or
"Fraunhofer's," lines obtained. The prisms are contained
in a cylinder of brass, which slides in another tube to allow
of adjustment, and the slit, stops, and various other adjuncts
make up an instrument called the "Direct-vision Spectro-
scope." The course of the rays of light is shown by the
converging, diverging, parallel and again diverging lines.
The light entering the end of the instrument has to pass
through a flat piece of microscopical glass to a slit which is
adjustable to as fine a degree as is necessary by means of the
milled wheel outside (Fig. 2). The rays forming the image
Fig. 2.—Grace's Spectroscope, as made by Mr. Browning, about two-
thirils actual size.
of the slit are rendered parallel by passing through the plano-
convex lens ; these again travel through the prisms, and are
split up into the various colours forming the prismatic
spectrum, an extended view being visible when the eye is
placed at the opening at the other end of the instrument. In
the more perfect forms of Grace's Hygro-spectroscopes an
adjustable photographic micrometer, with prism of comparison,
is fitted to the outside of the instrument.—Ed.]
The use of the spectroscope in the study of the weather is
of comparatively recent date. At the present time it is not
yet generally recognised by scientific men as an addition
worthy to be placed amongst the instruments forming the
equipment of the meteorologist, but happily, I am pleased to
state, the evidence in favour of it has considerably augmented
during the last twelve months; in fact I am almost daily
receiving testimony from various sources that something
must be expected from the analysis of daylight as an aid in
forecasting weather, and that the indications of the hygro-