The silver side-reflector is usually made with a ball-and-socket joint, so that it can be turned in any direction. It is secured to the stage of the microscope by the pin, which drops into a hole purposely drilled to receive it, and facility given for turning up and down, or in any position. If daylight is used the microscope should be placed in such a position that the light from a white cloud falls upon the speculum, but the light of the lamp is far more manageable for use with the reflector.
Fig. 157.—Silver Side-reflector.
The Lieberkühn is only intended to be used with low powers—a 2-inch, ½-inch and a 2⁄3-inch. Such objects as the elytra of the diamond and other beetles are well suited for examination.
Fig. 158.—Sorby’s Modification of the Parabolic Reflector.
While experimenting with a parabolic reflector ([Fig. 158]), Mr. Sorby saw the value of examining objects under every kind of illumination. As on viewing specimens of iron and steel with this reflector he found that, from the great obliquity of the illumination obtained, the more brilliantly polished parts of the specimen reflected the light beyond the aperture of the objective, and these could not be distinguished from those parts which absorbed light, he thereupon proceeded to place a small flat mirror in front of the objective, and cover half its aperture, and at the same time stop off by means of a semi-cylindrical tube the light from the parabolic reflector. This arrangement produced the reverse appearance of that first employed, and it proved to be a useful aid in determining structure.
The Bull’s-eye Condensing Lens.
This accessory is brought into constant use for the purpose of converging rays from a lamp or mirror; or, for reducing the diverging rays of the lamp to parallelism with the parabolic illuminator, or silver side-reflector. The form in use is a plano-convex lens of about three or four inches in focal length ([Fig. 159]). It is usually mounted on a brass stand, so that it may be placed and turned in any direction, and at any height. When used by daylight, its plane side should be turned towards the object, and the same position maintained when used for converging the rays of light from the lamp; but when used with the side-reflector the plane side must be towards the lamp. Much attention has been paid to this very necessary accessory, the bull’s-eye lens. A doublet has been brought into use which has increased the value of the bull’s-eye condenser in bacteriological research, and in micro-photography generally.
