image at the other. Whether the image is larger or smaller than the object depends on which focus it occupies. In the case of the object-glass of a telescope the image was at what we may call the short focus.
Now, a compound microscope is practically a telescope with the object at the long focus, very close to a short-focus lens. A greatly enlarged image is thrown (see Fig. 129) at the conjugate focus, and this is caught and still further magnified by the eye-piece. We may add that the object-glass, or objective, of a microscope is usually compounded of several lenses, as is also the eye-piece.
THE MAGIC-LANTERN.
The most essential features of a magic-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.
