ic-lantern are:--(1) The _source of
light_; (2) the _condenser_ for concentrating the light rays on to the
slide; (3) the _lens_ for projecting a magnified image on to a screen.

Fig. 130 shows these diagrammatically. The _illuminant_ is most commonly
an oil-lamp, or an acetylene gas jet, or a cylinder of lime heated to
intense luminosity by an oxy-hydrogen flame. The natural combustion of
hydrogen is attended by a great heat, and when the supply of oxygen is
artificially increased the temperature of the flame rises enormously.
The nozzle of an oxy-hydrogen jet has an interior pipe connected with
the cylinder holding one gas, and an exterior, and somewhat larger, pipe
leading from that containing the other, the two being arranged
concentrically at the nozzle. By means of valves the proportions of the
gases can be regulated to give the best results.

[Illustration: FIG. 130.--Sketch of the elements of a magic-lantern.]

The _condenser_ is set somewhat further from the illuminant than the
principal focal length of the lenses, so that the rays falling on them
are bent inwards, or to the slide.

The _objective_, or object lens, stands in front of the slide. Its
position is adjustable by means of a rack and a draw-tube. The nearer it
is brought to the slide the further away is the conjugate focus (see p.
239), and consequently the image. The exhibitor first sets up his screen
and lantern, and then finds the conjugate foci of slide and image by
racking the lens in or out.

If a very short focus objective be used, subjects of microscopic
proportions can be projected on the screen enormously magnified. During
the siege of Paris in 1870-71 the Parisians established a balloon and
pigeon post to carry letters which had been copied in a minute size by
photography. These copies could be enclosed in a quill and attached to a
pigeon's wing. On receipt, the copies were placed in a special lantern
and thrown as large writing on the screen. Micro-photography has since
then made great strides, and is now widely used for scientific purposes,
one of the most important being the study of the crystalline formations
of metals under different conditions.

THE BIOSCOPE.

"Living pictures" are the most recent improvement in magic-lantern
entertainments. The negatives from which the lantern films are printed
are made by passing a ribbon of sensitized celluloid through a special
form of camera, which feeds the ribbon past the lens in a ser