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.
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.