Annexed are four engravings of microscopic objects, the true character of which it is, however, impossible to give in wood, and is difficult indeed to accomplish by any description of engraving.
Fig. 25 shows a scale of the small insect called Podura Plumbea, the common Skiptail, magnified about five hundred times. To define the markings on this scale clearly is the highest test of a deep achromatic object-glass; and this drawing is given rather to explain what the observer should look for, than as a very correct representation. Fig. 26 is a scale or feather of the Menelaus Butterfly; Fig. 27 is the hair of a singular insect, the Dermestes; and Fig. 28 is a longitudinal cutting of fir, showing the circular glands on the vessels which distinguish coniferous woods. These latter objects may be seen by half-inch or quarter-inch achromatic glasses. Opaque objects are generally better exhibited by inch and two-inch glasses, when a general view of them is required, and by higher powers when we wish to examine their minute structure. In the latter case the light must be obtained by condensing lenses instead of the metallic specula.
Although the reflecting microscope is now very little used, it may be expected that we should mention it. In this instrument, at Fig. 29, the object O is reflected by the inclined face of the mirror M, and the rays are again reflected and converged by the ellipsoidal reflector R R, which effects the same purpose as the object-glass of the compound microscope. It forms an image which is not susceptible of the over-correction as to color before described, and which therefore becomes colored in passing through the eye-piece. This fact, and the loss of light by reflection, will probably always render the reflecting microscope inferior to the achromatic refracting.
The solar microscope has been so nearly superseded by the oxy-hydrogen, that a brief description of the latter must suffice, particularly as their optical principles are similar.
The primary object in both is to throw an intense light upon the object, which is sometimes done by mirrors, and sometimes by lenses. In Fig. 30, L represents the cylinder of burning lime, R R the reflector, which concentrates the light upon the object O O; the rays from which, passing through the two plano-convex lenses, are brought to foci upon a screen placed at a great distance, and upon which is formed the magnified image.