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CHAPTER X.
ZLbe Optical %pstem.
WE have considered the light and the mechanical portions
of a lantern, and have intentionally reserved the most
important part of alli.e., the lenses by which the image of
the slide is thrown upon the screen, until last. The optical
system consists of the condenser, the function of which is
not to condense the light upon the slide, but to divert the rays
from the lamp so that the margins of the slide as well as
the centre shall be illuminated by rays which can reach
the objective, and the latter itself which is used to secure a
sharp and brilliant picture.
That the purpose of the condenser is really what we
have said can be seen on reference to Fig. 49, in which are
Fig. 49. DIAGRAM SHOWING THE LANTERN, BUT WITHOUT CONDENSERS.
sketched the arrangements of a lantern and screen minus
the condenser. It is evident, since light under ordinary
conditions travels in straight lines, that the only direct light
which can fall on the screen D from the light A, will be that
THE OPTICAL SYSTEM.
represented by the lines EE and F F passing through the centre
of the slide B. The rays GG HH which reach the edge of
the slide would, if allowed by the tube of the lens, travel as
indicated by the dotted lines. In Fig. 50 we have the same
Fig. 50. DIAGRAM SHOWING THE FUNCTION OF THE CONDENSERS.
arrangement again, but in this case the condenser, j, has been
introduced. The effect is not to condense any greater
amount of light upon the centre of the slide, but to bend
down, as it were, the rays G G H II, so that they not only pass
through the slide but reach the objective. It will also be
obvious from this that it is important that the focus of the
Fig. 61. CONDENSERS.
condenser for the rays from the light shall be somewhere
about the position c of the objective.
Condensers in the simplest form are merely double convex
lenses, as shown in Fig. 51,A.* These are never to be
met with now except in toy lanterns, and have been sup-
The curves in this figure, as indeed in most, are intentionally a little
exaggerated, the aim being to give a general idea of their arrangement
rather than an illustration which is strictly accurate but obscure.
61