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66 MODERN MAGIC LANTERNS.
For those who do not care to calculate out for themselves,
the following table, abridged from Mr. Lewis Wright's
standard work on optical projection, is given. The table is
calculated for slides of the usual size.
Diameter of disc
in feet.
Focus
4/ inches.
Focus
6 inches.
Focus
8 inches.
Focus
10 inches.
Focus
11 inches.
ft. in. ft. in. ft. in. ft. in. ft. in.
9 136 180 240 300 360
12 180 240 320 400 480
15 226 300 400 500 600
18 27 0 36 0 48 0 60 0 72 0
20 30 0 40 0 53 4 66 8 80 0
Before leaving the subject of the lenses, a word on their
proper preservation can hardly be out of place. They are
best kept out of the lantern itself except when required for
use. The condenser should immediately before use be
dusted with a clean, soft handkerchief, as also should the
objective. In unscrewing the lenses of the latter, care
should be used to take only one out at a time, and to make
sure it is screwed into its proper place before removing the
other. In dusting them, they should be dusted, and not
breathed on, scrubbed, and polished as if they were the knob
of a door. It is a good plan, whenever possible, to get the
condensers warmed a little before putting them into the
lantern for use.
CHAPTER XI.
Cbe %men anb General Elrrangement6.
GIVEN a lantern of first-class power, good slides, and plenty
of gas, the brilliancy of the display can be made or marred
by the nature of the screen itself upon which the pictures
are thrown.
One arrangement of lantern screen and audience is that
shown in Fig. 54, the dots representing the audience. In
this case the screen should be as opaque as possible ; since
it is wanted to reflect as much as possible of the light falling
upon it ; in fact, the finest screen imaginable under these
circumstances consists of a smooth plaster wall well whitewashed.
This is not often obtainable, and the Ianternist has
to put up with a substitute, in the form generally of some
kind of sheeting. When this is the case the stouter and
whiter the sheet the better, since it will reflect more light.
If it must have any seams in it they should run horizontally
rather than vertically, and will probably show their
presence, and, incidentally, the advantage of an opaque
screen, by appearing whiter and more brilliant than the rest.
The size of the screen and its position depend largely upon
circumstances. It should on no account be hung too high ;
nothing is more unpleasant than craning back the neck for
some time to gaze at a sheet right above one's head ; for this
reason it should be as far in front of the audience as possible.
Screens are also made of canvas faced with a glossy-white,
surfaced paper. These cannot be folded, and consequently
are not so portable in large sizes, and are more expensive.
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