PHOTOMICROGRAPH. A photomicro graph is a photograph of a microscopically small object. The term microphotograph is sometimes used, but is very inaccurate, and should be used only to designate a very small photograph; e.g., a photograph an eighth of an inch square of the Niagara Falls is a micro photograph. Simple photomicrographs may be made without any expensive special apparatus. A microscope and any camera with a ground glass for focusing will suffice. The lens, or lenses, but not the shutter, are removed from the camera; the eyepiece is removed from the microscope, which is placed in a horizontal posi tion. The eyepiece end of the microscope tube is now inserted a little into the lens barrel of the camera and a black cloth is wound around the place where they come together in order to exclude the light. The tube of the microscope should be exactly perpendicular to the ground glass of the camera. Any strong light may be used, as direct sunlight, a gas mantle lamp, an acetylene lamp, a kerosene lamp, or an elec tric arc; but ordinary incandescent bulbs are not satisfactory. Remove the mirror of the microscope, since it will not be needed in any photomicrographic work, and let the light come directly into the optical axis of the microscope. If the illumination of the ground glass is un even and shows a aflare look at the inside of the tube of the microscope. Probably it was not blackened. Put a black paper inside the tube. Modern microscopes are well blackened inside. Move the light back and forth and sidewise until the best illumination is secured. About six inches from the stage is likely to be a good position.
Suppose we are to photograph a transverse section of a vascular bundle, using a two third-inch (16-millimeter) objective. With such a low power lens and with all lenses of longer focus, the condenser in the microscope is not only unnecessary but it interferes with good work. Remove it. However, the con denser should be used with objective of one third inch (eight millimeters) focus and with all objectives of shorter focus. Carefully focus the object upon the ground glass. The ground side of the glass should be next to the micro scope. After a sharp focus has been obtained, close the shutter of the camera and insert a plate, just as in ordinary photography. The time of exposure will depend upon the intensity of the light, the speed of the plate and the magnification. Small negatives may be made from lantern slide plates, especially when a contrasty negative is desired. With a gas
mantle lamp, a lantern slide plate and a 16 millimeter objective used without an ocular or condenser, try an exposure of 30 seconds. Un til one becomes proficient in estimating expos ures, it is a good plan to expose 10 seconds; then close the shutter and push in the slide of the plate holder just an inch; then expose 10 seconds, close the shutter and push in the slide another inch; then repeat twice more and you will have exposures of 10, 20, 30 and 40 seconds on one plate. It is easy to determine which is the best exposure and only one plate is sacri ficed. A print on printing-out paper from such a negative is very instructive, since it shows the comparative merits of negatives of vari ous densities. A filter, such as is used in out of-door photography, is likely to improve the negative. Experts use filters of various colors, according to objects and the features which are to be emphasized. The developing is the same as in case of lantern slides (q.v.), or as in case of ordinary photography, if a fast or medium plate is used. With higher powers, use the con denser and, if necessary, the ocular. If much work is to be done, the need for a solid sup port for both camera and microscope will be realized. A simple contrivance which any one can make for himself is illustrated in Fig. 1.
Use a clear board an inch thick, a foot wide and five feet long. On top of the board, at the sides, screw two strips, an inch thick, one and one-half inches wide and five feet long, so as to form a guideway for the camera. A long slit, one-quarter inch wide, should be made to accommodate the screw which is ordinarily used to fasten the camera to a tripod. If the same camera is to be used for all work, the space between the two long strips should be just wide enough to admit the camera. A strip of wood one-half inch thick, an inch wide and five inches long, placed over the horse-shoe base of the microscope and held in place by a bolt put through the long slit, will hold the microscope in position.
For the finest class of work, especially where very high magnifications are desired, elaborate and expensive apparatus is very convenient. In these elaborate forms, experts do not always use the same arrangement. The relative posi tions of the various parts, as shown in Fig. 2, is very effective.