For photomicrography with high powers, a sub-stage condenser, a more powerful light, and for critical work a projection eyepiece are neces sary. Colour filters, c, are of ten of great assistance in photomicrography, especially when, as some times happens, the visual and actinic foci of the objective do not coincide. In the older types of objectives this fault was not uncommon. A yellow screen will overcome this difficulty, but except orthochromatic plates are used, the exposure is unduly prolonged. Objects which have been stained with a blue or violet dye should always be photographed with a yellow screen. When opaque objects are to be photo graphed, unless the objective is furnished with a Lieberkiihn, the light must be placed at the side of the object, upon which it is focused by means of a bull's-eye condenser. A strong light is necessary, and unless a Nernst or incan descent gas lamp is available magnesium wire should be used.
A Lieberkiihn is a parabolic mirror, named after the inventor, for illuminating opaque ob jects. It is fitted on to the objective, which projects through the centre of the mirror.
The approximate comparative actinic values of the various illuminants are as follow : — Oil lamp, i-in. wick, r ; incandescent gas (new mantle), 15 ; Nernst, 20 ; acetylene, 3o ; arc lamp, 5 amps., 1,000 ; magnesium ribbon, 1,5oo. The oil lamp is somewhat feeble, and its yellow colour is a serious drawback. Incandescent gas is convenient, but in critical work some difficulty is caused by the pattern of the mantle. Acety lene gas gives perhaps the best all-round light, but the trouble of manufacturing the gas before beginning work militates against its popularity. The arc lamp is excellent ; small arc lamps for use with an ordinary 16 c.-p. plug and switch
are now available. The Nernst lamp, with its small bright flame, gives an excellent light, but the filaments are fragile and are liable to give a troublesome image. The metallic filament electric lamp gives a fairly actinic light, but the long zigzag filament is a serious objection to its use. The use of ground glass or tissue paper to obviate the formation of a pattern is not satis factory, as direct rays from a small illuminating area are essential.
Transparent objects or sections are photo graphed by transmitted light ; opaque objects by reflected light. In the former case the source of light is placed in a direct line with the objective and object, and the light, brought to a focus by some form of condenser, passes through the object into the lens. In the photo micrography of opaque objects the light is con centrated by means of mirrors or condensers upon the object, and is reflected by the object into the lens. Opaque objects require a much more powerful illuminant than transparent ones, and the difficulty of obtaining sufficient light renders the work extremely difficult. A method of illuminating opaque objects which is especially useful for high powers is by means of a cover glass fixed diagonally inside the tube of the microscope. The light reaches the cover-glass through an aperture in the side of the body tube, and is deflected, from the surface of the glass, through the objective to the object, which reflects it through the microscope to the camera. Thus the light has to pass twice through the objective, in this way producing much scattered light, which mars the brilliancy of the image. J. I. P.