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84 MODERN MAGIC LANTERNS.
required is the removal of the tubes carrying the objective,
so that objects much thicker than the usual slides can be
inserted, and the provision in consequence of some other
kind of support for the front lens. A method of effecting
this is shown in Fig. 60.
Such a lantern will admit of the insertion of cells of
liquid, test tubes, galvanometers, and many other pieces of
apparatus. The number of scientific experiments which can
be shown in a lantern of this class is legion, but we can only
give as examples one or two that can be performed.
The development and fixation of a photographic plate can
be shown by the use of a tank, as shown in Fig. 61. A
chloride or lantern plate should be employed, both on account
of its superior transparency and of its lower sensitiveness
to light, the developer
being ferrous ox alate.
The plate is exposed under
a negative in the
ordinary way, and is
then placed in the tank,
care being taken that the
image on the plate is upside
down. The plate
should be protected from
actinic light by the insertion
of a piece of ruby
glass between the condenser and the tank before the experiment
commences. When the plate is in position it can be
focussed, a strip of wood being inserted so as to hold the
back of the plate in contact with that glass of the cell which
is furthest from the lantern. Unless the wood is weighted
it will probably float up when the developer is poured in,
and the success of the experiment will be marred. When
the plate is thus held in position and focussed, the developer
must be carefully poured in with the help of a funnel, no
splashing being allowed to take place. When all the plate
is protected by the developer the ruby glass may be removed,
the deep orange tint of the solution being a sufficient
protection with a slow plate. The image will be seen
gradually to grow up on the originally plain opalescent
LANTERN EXPERIMENTS. 85
plate. When development is complete, the ruby glass
should be again inserted, the tank withdrawn and emptied,
the plate rinsed in slightly acidulated water, then in plain
water, and restored to the empty tank replaced in the
lantern. This is now filled with fixing solution, which will
be seen to dissolve gradually the unaltered silver salts in
the film, leaving the finished transparency, if all has gone
well, in full brilliancy on the screen, when the ruby glass
can be finally withdrawn. This experiment is a very
striking one, and is not difficult to perform, but it should be
rehearsed once or twice before being attempted In public, a
remark which applies to all demonstrations of a like nature.
It is often convenient to be able to show upon the screen
the presence of currents of electricity set up by one means
or another ; this is easily done. At any of the shops which
supply working jewellers, such as are to be found in the
neighbourhood of Clerkenwell, little compasses with glass
Fig. 62. LANTERN GALVANOMETER.
sides, which when mounted are often worn on watch
chains, etc., can be purchased for a, few pence. One of
these can be easily made into a fairly sensitive galvanometer
by mounting it in a wooden block, as shown in
Fig. 62. Round the block should be wound some turns of
silk-covered copper wire, B B, Fig. 62; twenty or thirty
turns of No. 30 B W G wire will answer most requirements,
though more can be used if necessary, the two
ends of the wire being brought out to two terminals, D D,
on the end of the block. A little bar magnet, c, which can