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MICRO-PHOTOGRAPHY AND PROJECTION APPARATUS 323
microscope, a camera, properly supported, can be placed directly on the eye-piece. This camera does not require a lens like the ordinary photographic camera. The eye-piece of the microscope serves this purpose, and this is inserted through a light-tight sleeve into the front of the camera. It is necessary, in these operations, that all light not emitted from the object should be excluded. If the reflector at the bottom of the microscope is used by itself, many rays pass by the sides of the object, and fall upon the lenses, bringing about reflections that materially disturb the clearness of the image. Hence it becomes advantageous to use in such work the substage condenser, by means of which the rays of light are concentrated on the object ; in addition to this, an iris diaphragm will often be found extremely useful.
The lenses used for microscopic objectives are either achromatic or what are known as apochromatic. Such lenses differ from one another in that the apochromatic lenses are so constructed that a very much higher order of colour correction is obtained in them than in the case of the achromatic lenses.
On the other hand, the superiority of apochromatic lenses in micro-photographic work is only very apparent when the preparation to be photographed is unstained, and extremely minute details are required, such as can only be resolved with light of short wave-length. Hence it is that the cheaper achromatic lenses are much more frequently used in conjunction with stained preparations, autochrome plates, or orthochromatic plates and colour screens.
The method by which the image is formed in the microscope will be understood by referring to fig. 135 ; r s is the small object of which it is desired to obtain an enlarged image ; S R represents the enlarged image projected by means of the objective. This is when