Untitled

10 MODERN MAGIC LANTERNS.
The best petroleum is cheap, and such lamps are the most
economical in use ; a three-wick lamp does not consume so
much oil as a four-wick for the same amount of light, but
the difference is immaterial.
It is almost unnecessary to add that the glass at one end
of the combustion chamber and the mirror at the other
must be kept clean and bright, or much light will be lost.
The mirror is generally hinged at the top, as shown in Fig. 5,
and can be raised to inspect the lamp ; it is also often
provided with a coloured glass window, but this, from its
small size, we have not found to be of much use.
The method of adjusting the position of the lamp in the
lantern is usually the same as that of the jet tray for limelight,
and will be dealt with under that heading. In the
better form of lantern, the lamp slides into the body in
grooves, which will also carry a jet, if at any time it is
desired to substitute the limelight for the oil lamp.
Owing to the great heat which oil lamps emit, it is
necessary for the lantern body to be jacketted ; that is to
say, for the body to consist of two distinct shells, between
which is an air space. Even with this precaution the outer
casing, which is sometimes of wood, but more often of
Russian iron, is almost sure to get very hotfar hotter, in
fact, than should be the case with the limelight or with an
arc lamp, but this is unavoidable.
CHAPTER III.
the limelight.
THE most popular of all lights for lantern purposes is one
or other form of what is known as the limelight. This
means of illumination was invented by Lieutenant Drummond
in 1826, and although it has been employed more or less
ever since, it has received its greatest impetus by the sale at
a moderate price of oxygen gas compressed into steel
cylinders, which are fairly portable.
The use of Limelight as an illuminant, as its name
implies, is based on the fact that, by directing a very
hot flame against a piece of lime, the latter soon becomes
white hot, and emits a very brilliant light of great penetrative
power. To achieve this, the flame must be of the
most intense character ; ordinary house gas, spirit or
petroleum lamps are quite useless for heating the lime,
and recourse has to be had to flames in which oxygen gas,
freed from the nitrogen, argon, or whatever other diluent
chemists may eventually decide to be present,- is employed.
By the aid of this oxygen the combustion of most inflammable
substances is rendered much more vigorous
and complete, and the heat emitted more concentrated and
intense.
The instrument by which the flame is obtained and
directed against the lime is known as a jet, and some idea
of its appearance may be gathered from Figs. 7, 8, and 9,
which represent the three types of jet in common use, the
oxy-calcium, the blow-through, and the mixed jets. Most