Page:Popular Science Monthly Volume 61.djvu/154

148 slides that may be readily photographed with the microscope in a horizontal position. A mechanical stage is of great convenience in this work as it saves much time in accurately placing a slide, especially where high power objectives are used and when focusing must be done at arm's length. In the making of photographs of rock sections, it is frequently necessary to use Nicol prisms in order to properly differentiate particular portions. The one is placed over the ocular, or between it and the objective, and the other beneath the sub-stage condenser. As the latter is free to be rotated, it is an easy matter to bring out clearly the special features desired. Fig. 2 illustrates the apparatus in a vertical position. It must be operated in this position when freshly mounted slides are used and where photo-micrographs are to be made of living organisms. A special shutter devised by the writer for the photographing of living specimens, a detailed description of which will be found in the Scientific American of March 24, 1900, is shown in place on the draw tube of the microscope. In place of the mirror a hole is drilled through the base of the camera stand in order to make transmitted light available. With this arrangement, as illustrated, photographs have been made in 1/50 of a second, (see Figs. 3 and 4).

The character of the plates used in connection with the work and a word as to their manipulation is worthy of notice, because ultimate results are not dependent upon first quality optical apparatus alone. With ordinary light and low power objectives, a slow plate is to be preferred, and with such conditions the Carbutt's B brand has given excellent results. Where polarized light is employed a color sensitive plate is preferable. Such light often gives the most beautiful colors with rock sections and crystal specimens, and the Cramer isochromatic plate renders the color values admirably.

The question of exposure must be decided principally by experience. The candle power of the illuminant; the character of the