It is well known that to determine the true shape of an object two eyes are better than one. In the examination of such things as firing pin impressions (and many others) it is very important to determine their true, shapes and this cannot be done with the ordinary monocular microscope because one sees the object from one viewpoint only. With a binocular microscope (the Greenough type, having two separate optical systems) one uses both eyes and consequently secures two images of the object, from slightly different angles, which blend together to give a true impression of the shape of the object. For this reason all such examinations should be made with a binocular microscope. Two firing pin impressions, which with the ordinary microscope may appear to have the same shape, may be shown to have quite different shapes when viewed under a binocular microscope. One may have been made with a blunt firing pin while the other was made with a more pointed (or tapered) firing pin. Viewed with the ordinary microscope the impressions might appear to be identical, but not with the binocular microscope.
It may be very important to demonstrate such differences in court. The unsupported word of the expert that two firing pin impressions have different shapes (or the same shape, as the case may be) when viewed under a binocular microscope, an instrument about which the members of the jury may not have any knowledge, will certainly not be as convincing as a visual demonstration which each of them can see for himself. Since the average jury member is quite unfamiliar with even an ordinary compound microscope it would serve no good purpose to try to have each jury member look at an object through a binocular microscope, where the pupillary distance has to be adjusted for each individual, and by himself. Stereo photographs, however, are something about which practically everyone has knowledge and experience, and these can be shown to the members of the jury in a short time.
Stereo photomicrographs are not particularly difficult to make if one has suitable equipment. There are cameras on the market which are especially designed for the purpose. Some of these are used in conjunction with the binocular microscope and some are simply stereo cameras with very short-focus lenses which produce the necessary magnification. Some have even used a single camera to secure the two photos from a slightly different angle. This is difficult and is not recommended.
The author uses a camera and binocular microscope, as shown in Fig. 98. The microscope is an older type of Bausch and Lomb instrument having three pairs of objectives and is well adapted for the purpose. The camera consists of a box with a partition down the middle, and a removable back in which are mounted two ground glasses in such position that each will be at right angles to the pathways of light through the optical systems of the microscope. A plate holder, for two 31/2x31/2-inch plates, is provided. When this replaces the ground glasses upon which the images are focused, the plates are in precisely the same positions the ground glasses were in. At the bottom of the camera there are „muffs" to make light-tight connection between the microscope and camera. The camera, as made by Bausch and Lomb, was modified so as to permit lateral movement of each of the „muffs." As received, these were in fixed position on the bottom plate of the camera. This plate was cut in two and the pieces mounted in a slide which permits the lateral motion of each which is necessary to bring the images into proper position on the ground glasses. The images must not only be in focus but must be exactly centered in order that the two pictures will register properly when viewed in a stereoscope. Panchromatic plates rather than cut films are used because the two photographs, when viewed in the stereoscope, must be in exact register. Since films of the size used do not always lie perfectly flat they are unsuitable. The slightest „buckling" would destroy the register. In printing, the two plates are put into a printing frame in such a manner as to bring them into the same position in relation to each other that they were in when the exposure was made. Obviously a good stereoscope is also necessary to secure proper register of the two images.
The accompanying stereo photomicrographs, reproduced full size, (Figs. 99, 100) will show, when viewed through a stereoscope, the advantage of this type of photography.
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