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4
MODERN MAGIC LANTERNS.
employed to bring the light proceeding through the condenser
and slide to a focus upon the screen, as will be
explained hereafter ; and the brightness of the picture
depends also upon the proportion of the light passing
through the slide which the objective, or front lens as it is
often called, allows to pass. This will be best seen by
reference to Fig. 2, which represents the optical system of a
lantern, and the course taken by the light in its passage to
the screen. The letters are the same as in Fig. 1, F being
the slide. Photographers will have no difficulty in understanding
that if the lens, E, is one which would be described
as a " slow " one, it would not allow so great a proportion
of the light to pass as a " faster " lens.
All these points have to be taken into consideration in
deciding what instrument to employ for any given work.
With good condenser and objective, probably the following
summary will be a sufficient guide :
For discs not exceeding eight feet in diameter, or for
enlarging on bromide papera good petroleum lamp.
For discs not exceeding twelve feetan oxy-calcium or
" blow-through" jet, which is besides preferable to the foregoing
for enlarging purposes, when circumstances permit.
For larger discsthe mixed jet or the electric arc light.
A lantern for all-round work, such as an amateur photographer
would desire, is best fitted with a blow-through
jet, or if this is impracticable, a three- or four-wick petroleum
lamp.
CHAPTER II.
Cif /Lamps.
NEARLY all the old-fashioned lanterns in use before the
introduction of petroleum as an illuminant depended upon
sperm or colza burnt in an Argand lamp, the construction
of which can be gathered from Fig 3.
The wick in these lamps is cylindrical,
with an air passage in the
centre as well as outside ; the tank
for the oil is seen at the back of the
figure, carrying in front of it the
silvered reflector.
The glass chimney for such lamps
should taper towards the top, and
the wick must be of loose cotton,
the compact wicks employed for petroleum
being of no use whatever for
sperm or colza. Petroleum cannot
be employed in this form of lamp ;
sperm is preferable to colza, and
much longer time must be given for
the wick to get saturated with oil
Fig. 3.
OLD TYPE OF SPERM than in the case of petroleum. We
OR Col,. LAMP.
might add here, that it is not advisable
with any lamp to saturate
the wick before inserting it, as it is impossible to avoid
messing up the outside with oil by so doing, a thing
which should always be guarded against.