Provide two small pieces of window-glass and a lump of wax. Soften and
mould the wax, stick the two pieces of glass upon it, so that they
meet, as in the cut, where _w_ is the wax, _g_ and _g_ the glasses
stuck to it, (Fig. 1.) The end view (Fig. 2) will show the angle, _a_,
at which the pieces of glass meet; into which angle put a drop of water.
[Illustration: Fig. 1.]
[Illustration: Fig. 2.]
To use the instrument thus made, make a small hole, or a narrow
horizontal slit, so that you can see the sky through it, when you stand
at some distance from it in the room. Or a piece of pasteboard placed
in the upper part of the window-sash, with a slit cut in it, will
serve the purpose of the hole in the shutter. The slit should be about
one-tenth of an inch wide, and an inch or two long, with even edges.
Then hold the prism in your hand, place it close to your eye, and look
through the drop of water, when you will see a beautiful train of
colours, called a spectrum; at one end red, at the other violet, and in
the middle yellowish green.
[Illustration: Fig. 3.]
The annexed figure will better explain the direction in which to
look: here, _e_, is the eye of the spectator, _p_, is the prism,
_h_, the hole in the shutter or pasteboard, _s_, the spectrum. By a
little practice, you will soon become accustomed to look in the right
direction, and will see the colours very bright and distinct.
By means of this simple contrivance, white light may be analysed and
proved to consist of coloured rays, and several of its properties be
beautifully illustrated.