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96 MODERN MAGIC LANTERNS.
Geometrical designs, butterflies, etc., are built up of pieces
of the substance in question, mounted up as slides, and
are obtainable through the dealers. A more satisfactory
method with many people will be to make them themselves,
the operation not being a very difficult one,
consisting, as it does, in a building up of the design on
a glass slide with pieces of mica cemented together with
Canada balsam. Crystallised benzoic acid, salicine, etc.,
also give very beautiful projected images with polari3ed
light.
Slabs of glass held in a clamp in such a way that, while
the image of the glass is on the screen, stress can be set up
with the glass by means of a screw, exhibit in a very clear
manner the changes set up by the stress which the polariscope
revealschanges which in no other way can be
rendered visible.
Anderton's stereoscopic lantern, introduced about two
years ago, is an example of an ingenious application of
polarised light, with a view to enabling an audience to see
the image on the screen stereoscopically or in relief. In
order to effect this it is essential that two distinct images
shall be seen by the two eyes, the two slides for the purpose
being made from two negatives taken from different standpoints,
side by side, so that one differs from the other by
seeing a little more round one side or the other of the object.
The two slides, which in all other respects are identical, are
shown simultaneously upon a screen, the surface of which
is composed of metallic foil, by means of a biunial lantern.
After leaving the objective, the light from each of these
lanterns passes through a polariser, these polarisers in the
two lanterns being so arranged that the planes of polarisation
are at right angles to one another. Each observer is
provided with a little pair of analysers mounted like operaglasses,
but in each of which the analysers are arranged
with reference to each other as are the polarisers in the
lanterns, the effect of which is that only one of the blended
pictures on the screen is seen with one eye, and the other
picture with the other eye. The result, on viewing the two
images on the screen through such an apparatus, is to
eliminate from the field of view of one eye the picture from
THE LANTERNSPECTROSCOPE, POLARISCOPE, ETC. 97
one lantern, and from the other eye that from the other ; the
brain combining the two pictures so as to give the impression
of the objects standing out in relief, as in nature.
All optical lanterns are virtually lantern microscopes, as a
moment's consideration will make clear. The slide, whatever
it may be, is the object, an enlarged image of which is
projected upon the screen. Hence there are many things
which, needing only a small degree of magnification to
render their details plain, can be exhibited without the aid
of any further apparatus whatever. The writer had an
Eg.
MICROSCOPE FOR TABLE
LANTERN USE.
AND
opportunity a little time since
of looking over a large number
of what appeared at first sight
to be very carefully drawn and
beautifully coloured lantern
slides of sections of various
animal tissues, healthy and
diseased, interesting more especially
to veterinary surgeons.
These turned out, on more careful
examination, to be the actual
preparations themselves, the
vessels of which had been injected with various colouring
matters, the whole tissues stained to show as far as possible
their structure, and then mounted up as slides ; and very
beautiful they were, while their value for educational purposes
was almost immeasurable. Such slides in the lantern were
magnified thirty or forty diameters without difficulty.
This is the simplest form of lantern microscope, but it is
of course limited in its powers, and for greater magnifical