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92 MODERN MAGIC LANTERNS.
and render parallel the rays of light, D is the cardboard
with the slit, E the objective, and F the prism, the screen
being beyond the latter in the direction indicated by the
lines of light. The slit being placed in position, it must be
focussed upon the screen in the usual way ; too great a
degree of enlargement should not be attempted, or the
illumination will be feeble. When focussed, the lantern
must be turned round bodily until its position with reference
to the screen is as it would be if the latter were placed
across the right-hand bottom corner in Fig. 68, and the
prism placed in front of the objective as shown. A little
Fig. 68. ARRANGEMENT OF APPARATUS FOR PROJECTING THE SPECTRUM.
adjustment will be necessary to get the best result ; for
this reason the arrangements should all be made, and the
prism put into its place before the audience are present.
The best position for the prism to occupy is that in which
it bends aside the beam of light the least, and this should
be fo and as nearly as possible by experiment. If all has
been properly arranged, the beam of light from the lantern
will be found to be split up in its passage through the
prism, and to be widened out into a band of coloured
light, one end of which is red, and the other violet, with
the other colours in between these two extremes. This
band is the spectrum.
By inserting in the path of the beam' as it emerges from
the objective, coloured glasses or stained gelatine fili5as, the
THE LANTERN-SPECTROSCOPE,
POLARISCOPE, ETC. 93
absorption of such media can roughly be shown. Solutions
of various substances can also be employed, using for the
purpose a cell similar to that shown in Fig. 61. Solutions
of potassium permanganate, of potassium bichromate, of
many of the aniline dyes, etc., give interesting results when
shown in this manner. With glass or stained films the
absorption is better shown if the coloured material is in
contact with the slit itself, covering, say, the lower half of
it. A sharp line upon the screen will then be seen to
divide the spectrum into two distinct parts, the upper one
being the spectrum of the light employed after it has passed
through the coloured film, the lower one simply the spectrum
of the light itself.
An interesting experiment consists in saturating a sheet
of white paper with a solution of quinine sulphate in water
rendered slightly acid with sulphuric acid. The paper
should be allowed to dry, and then be mounted on a sheet
of card side by side with a similar piece of paper which has
not undergone the treatment with quinine, in such a way
that the two papers are separated by a straight line running
right across the card. If now a very bright spectrum be
thrown upon such a card, so that the upper half of the band
of colours falls on the plain paper, and the other on that
treated with quinine, it will be seen that a greater length of
spectrum at the violet end is visible on the latter than on the
former. It will most likely be necessary, in order to show
this distinctly, to cover up all the spectrum between the
blue and the red, as otherwise
the brilliancy of that portion
will drown the darker violet, and the experiment be less
striking. This is shown better with the electric arc than
with limelight, although with care it can be very plainly
demonstrated even with the latter.
The experiment can be elaborated by interposing, in the
path of the beam of light, a glass cell containing the solution
of quinine. This will at once alter the appearance of the
spectrum, which will then appear no longer on the one paper
than on the other, but on the other hand the solution itself
will exhibit a very beautiful blue fluorescence. Various
other substances can be used in place of the quinine with
varying results.