PHOTOMICROGRAPHY The photography of minute objects by the aid of the microscope. The essential apparatus in cludes microscope and stand, lenses and camera, and it may be said that many kinds of hand cameras can be utilised. A stage or base board is necessary to receive the microscope and camera (with lens removed), which are placed in position on the stage as shown at A. A small platform is fixed on the stage to bring the lens flange of the camera, A, to the level of the micro scope tube when the latter is brought over to a horizontal position. If a fairly long camera extension is used, the fine adjustment of the microscope will be out of reach of the hand ; consequently a connecting-rod will be necessary for focusing purposes. A brass rod, B, fitted with a grooved wheel at one end and a milled head disc at the other may be fixed on the camera stage ; the grooved wheel, V, is placed immediately beneath the fine adjustment of the microscope and the milled head disc, H, below the focusing screen of the camera. A band or small strap 0 is placed over the groove of the fine adjustment of the microscope, and passed round the groove in the connecting-rod wheel. The image can then be focused by turning the milled head below the focusing screen. If the camera has a short bellows extension, no connecting-rod will be necessary, as the fine adjustment can be readied by the hand while the operator examines the image on the focusing screen. When transparent objects are photographed the light is placed behind the camera stage in a direct line with the objective and focusing screen. For low power work, an oil lamp, E, is a convenient illuminant. With high powers, the Nernst, incandescent gas, or acetylene lamp is preferable. As the mirror is not used, some kind of condenser is necessary to focus the light on the object. If no sub-stage condenser, D, is available, a bull's-eye condenser will answer the purpose.
For experimental purposes some well-mounted object with which the operator is perfectly familiar should be selected. Failing this, the wing
of a house fly can be mounted (dry), and being very transparent, with opaque lines, is easily focused. An objective of low power should be used. The object is placed on the stage and focused ; the eyepiece and draw-tube are removed, and the microscope brought over to a horizontal position. The inside of the tube must be lined with black paper, or the light reflected from the microscope tube will be found to give a flare spot on the plate. The microscope is now placed in position on the baseboard with the body-tube, B, projecting into the lens flange of the camera, the camera lens of course being removed. The photograph can be taken with the eyepiece in the microscope, but the consequent loss of light is considerable, and the difficulty of focusing proportionately increased. The lamp is placed in position behind the microscope, and the light is focused by the condenser on to the object.
When the focusing screen is examined no image of the object may be visible. The coarse adjustment is turned slowly to bring the objective nearer the object, and, when the image appears, sharp focus is obtained by means of the fine adjustment. If the lamp and condenser are properly arranged, the image will appear brightly and evenly lighted. If there is any unevenness of illumination, the position of lamp and con denser must be readjusted.
When the operations of arranging the illumina tion and are completed, a dry plate is inserted in the dark-slide and a trial exposure made. No definite rules can be given for calcu lating exposure, but the following data of a trial exposure will assist the beginner in form ing an estimate :— Object photographed, head of gnat larva, clear specimen, mounted in Canada balsam ; oil-lamp, 2-in. wick ; objective, in. ; bull's-eye condenser, if in. diameter ; distance of object from focus ing screen, 26 in. ; distance of flame from object 7 in. ; rapid plate ; exposure, ro seconds.