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64
MODERN MAGIC LANTERNS.
The function of the objective can be seen from Fig. 53A,
where the paths of three of the rays from the lowest part
of the slide only are shown, to avoid confusion, coming to a
focus on the screen at G. With many lenses the rays which
pass through the centre of the slide would not under such
circumstances come to a focus also upon the screen, but at
some point further off than it, say at FL The further the
rays from the centre, the nearer to the lantern would be the
point at which they will come to a focus; in fact, to obtain a perfectly
sharp picture with such a lens, the screen itself would
have to be concave, like the inside of a saucer. Such a lens
would be said to have a curved field, and a lens free from
this defect a flat field. In selecting a lens for use in the
lantern it should always be tried in the lantern itself, and a
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Fig. A. DIAGRAM TO ILLUSTRATE THE FUNCTION OF THE OBJECTIVE.
slide should be used of such a kind as to give a good idea
of the defining power of the lens or its capability of reproducing
on the screen the details of the slide with sharpness.
As good a test slide as any for the purpose is made
by enclosing a piece of open muslin or fine net between a
couple of glasses 31 by 31 and binding it up like an ordinary
lantern slide. Such a subject put in the carrier and
focussed as sharply as possible will give an excellent idea of
the powers of the objective.
Another matter of great importance is the focal length of
the objective, since upon this depends the position of the
THE OPTICAL SYSTEM. 65
lantern and screen for a given size of disc. Perhaps the most
useful length is 6 in., which has the advantage of being that
of a large number of the portrait lenses which are so suitable
for the purpose. The effect of the focal length of the lens
on the size of the disc is best expressed by saying that, with the
lantern and screen in any one position, the shorter the focus of
the objective the bigger the disc, and vice versa, the difference
in diameter being in exact proportion to their focus ; thus a
lens of 12-in, focus gives a disc just half the size of that
obtained with a lens of 6-in, focus at the same distance.
It follows, of course, from this that to obtain always the
same size of disc on the screen, the further the lantern is
from the screen the longer the focus of the lens necessary.
The 12-in, lens above mentioned would give, with a distance
of 24 feet between the lantern and screen, a disc the same
size as would be obtained with the 6-in, objective at 12 feet
distance.
To ascertain the focus of a lens in inches required to
get a given size of disc at a given distance off, the
distance in feet must be multiplied by three* and divided
by the diameter of the disc in feet. ThusWhat lens is
required to give a 15-ft. disc at a distance of 40 feet ?
Multiplying 40 by 3 we get 120, which, divided by 15
gives us 8. The lens, required is therefore one of 8-in.
focus.
This rule may be reversed to find out the size of disc which
would be obtainedthat is to say, by multiplying the distance
in feet by 3* and dividing by the focus of the lens in inches.
For example : we have a6-in. lens, how large will the disc be
at a distance of 50 feet Multiplying 50 by 3 we get 150,
and dividing by 6 the result tells us the disc will be 25 feet in
diameter.
In the same way, to discover the distance at which the
lantern must be placed to give a disc of a given size with a
given lens, the diameter of the disc in feet is multiplied by
the focus of the lens in inches and divided by three.* We
need hardly give another example.
"Three is taken as the effective diameter of the ordinary slide. If slides
are used of any other size the diameter of their opening in inches must be
substituted for " three "in the calculations.