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MICRO-PHOTOGRAPHY AND PROJECTION APPARATUS 327
since this will depend upon the plates used, the source of light, the magnification, and the nature of the preparation. It is a good plan before commencing actual work to expose a plate for two seconds, then gradually push in the slide a half inch at a time, allowing one second to elapse between each operation and the next. In this way, strips of the plate will have been exposed two, three, four seconds, and so on, and if the plate has been previously marked, the correct time of exposure can be accurately gauged by noting which strip has just sufficient density when developed.
The carrier of the camera front is fitted with a rack-andpinion movement for conveniently focussing photomicrographs and photographic objectives attached directly to the camera front. These lenses are attached by a threaded flange to a small wooden lens board which slides into a fork forming part of the camera front. This slide ma}' be replaced by another fitted with a double tube sleeve, which serves to establish the optical connection between camera and microscope by means of a short single socket which slips over the end of the draw-tube of the microscope.
When the camera is in the horizontal position, focussing is done by means of the side attachment (see fig. 136). By this means it is possible to observe the image on the camera-screen during the focussing operations.
During the preliminary arrangements very great care must be taken to ensure that the screen of the camera is evenly illuminated, otherwise some parts of the image will receive correct exposure, and others incorrect.
If the outlines of the image are not absolutely clear, and the details not well marked, proceed as in ordinary photographic work, i.e. stop out some of the light. This should be done gradually until the required degree of sharpness is obtained, as much illumination as possible being retained, on account of the length of exposure