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94
MODERN MAGIC LANTERNS.
A more expensive arrangement for the projection of
spectra, but one which possesses the great advantagethat
the whole of the apparatus is in one straight line with the
screen, consists of the employment of a group of prisms, as in
a direct vision spectroscope. Such a series of prisms, when
interposed in the path of a beam of light, disperse the rays
passing through them, without refracting them. The whole
Fig. 69. LENS AND PRISMS FORMING A DIRECT VISION SPECTROSCOPE.
apparatus then takes, in plan, the form shown in Fig. 68,
but with the lens and prisms E and F, Fig. 69, taking the
place of E and F in Fig. 68.
While on the subject of the spectrum, we might mention
a form of Newton's disc for use in the lantern, which shows
in a very distinct manner how, by the admixture of light of
the various colours of the spectrum, we once more get white
light. It consists of a circular disc of glass painted as
1,1111111111 linw 111111111 1/11
11111111,
hg. 70.
NEWTON'S DISC FOR LANTERN USE.
brilliantly as possible with
colours resembling those of
the spectrum, the different
tints being laid on in ra
diating wedge-shaped sectors;
the disc is provided
with a pulley and band by
which it can be rapidly
rotated (Fig. 70). To ex-
hibit it effectively, the disc must be inserted as a slide in
the ordinary lantern, and its image projected on the screen
and focussed. The room should be perfectly dark, and the
illumination of the disc the brightest possible. While
stationary the image of the disc on the screen presents, of
course, a magnified image of the original disc with all its
THE LANTERN-SPECTROSCOPE, POLARISCOPE, ETC. 95
colours brightly shown ; but as soon as it is rapidly rotated
the colours blend, with the result, if the disc be a good one,
that the image on the screen is a simple white circular patch
of light. By covering over some one or more of the colours
on the disc with black paper, and then rotating it, it can be
shown that the absence of any of the colours gives the
blended images a colour, the white image only resulting
when all the tints are mingled.
The lantern polariscope is, as its name implies, an instrument
designed to exhibit objects on the screen by means
of polarised light. To do this the beam of light, before it
falls upon the screen, has to pass through two pieces of
Fig. 71. POLARISCOPE ATTACHMENT (ELBOW Fowl).
apparatus known respectively as an analyser and a polariser,
Fig. 71. These may be similar in consl ru3tion, since they
could be used the one for the other indiscriminately were it
not that, for mechanical reasons, the polariser is generally
larger than the analyser ; but, as a general rule, in the more
expensive instruments the polariser is constructed of glass,
and the analyser of Iceland spar, in the form known as a
nicol prism. With the aid of this apparatus many experiments
can be performed, one or two of which we mention.
Slides made of selenite and other suitable substances,
frequently quite colourless in themselves, yield magnificently
coloured images when projected by means of the polariscope.