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328 THE ADVANCE OF PHOTOGRAPHY
required when the beam of light is small. Besides, a very narrow source of light is apt to produce coloured fringes due to diffraction. The source of light is movable, and its distance from the illuminating lens will depend upon the magnification which is required. For low magnifications it is not necessary to use a substage condenser, and in this case the lamp may be placed at the extreme end of the apparatus, the lens being situated about 15 cms. in front of it. When the higher powers are used an Abbe condenser becomes an essential addition to the apparatus, and now the lamp and lens are gradually moved nearer this condenser until its back lens is uniformly and brightly illuminated. It may sometimes be found advisable to interpose a groundglass screen between the lamp and camera, especially so if any structure of the source of light, such as filaments of an electric lamp, or meshes of an incandescent mantle, are visible on the screen.
In such cameras as those just described, it must be remembered that there is a large amount of metal-work, and that the correct adjustment depends upon this metalwork support. It can therefore be easily understood that if the best results are desired, it is imperative that the temperature of the room should be kept as nearly constant as possible after focussing and during the exposure.
In the photographs of some microscopic preparations it is desired to make the contrasts as strongly marked as possible ; for this purpose it is necessary to use coloured filters with the apparatus, unless the preparation itself is of such a nature as to transmit only those colours which are best adapted for the plate used. A number of such coloured filters are supplied with the apparatus.
The accompanying figure (138), which represents the transverse section of the leaf of the tea-plant, has been taken with a ground green glass disc in the apparatus.