184
THE BOOK OF THE LANTERN.
placed upon a stand that they can each be turned in any
required direction. The spectrum is allowed to fall upon
this system of mirrors, and each one is so turned upon its
axis that the particular colour which it reflects is thrown
upon one spot. The collective images of the various
colours then appears as a white disc.
The methods thus detailed are all good, but cannot
readily be applied to the lantern. A way of demonstrating
the recomposition of light with that instrument lias
recently been published in America, by Mr. G. M. Hop¬
kins, and the following remarks are borrowed from him.
After detailing the various known methods of recomposing
light, he says :—“Besides these methods, the spectrum has
been recombined by whirling or rocking a prism ; the move¬
ment of the spectrum being so rapid as to be beyond the
power of the eye to follow, the retina receiving the impres¬
sion merely as a band of white light, the colours being
united by the superposing of the rapidly succeeding
impressions, which are retained for an appreciable length
of time. The engraving shows a device to be used in
place of the ordinary rocking prism. It is perfectly
simple, and involves no mechanism. It consists of an
inexpensive prism, having attached to a knob on either
end a rubber band. In the present case the bands are
attached by making in each a short slit, and insert¬
ing the knobs of the prism in the slit. The rubber
bands can be held by inserting two fingers in each and
drawing them taut. The prism can then be held in a
beam of sunlight, and with one finger the prism is given
an oscillating motion. The band of light thus elongated