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- 
thirds 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 liygro- 
