Why do stereoscopic pairs show striking increases in resolution over the individual micrographs of which they are composed? Of course, stereoscopy increases information by permitting a three-dimensional reconstruction of the specimen to be made. Stereoscopic views also tell whether observed variations in image density are due to Bragg reflections in crystalline materials or to differences in electron scattering power in amorphous materials. Also, with two separate exposures, imperfections in photographic materials tend to cancel out; and there is a two-fold gain in the numbers of electrons or photographic grains that contribute to the net image seen. An additional feature is perhaps more subtle but no less important : No matter how many exposures are made from a single point of view, the images of some objects in a threedimensional specimen are inevitably confused with those of other objects such as the supporting film. If, however, two exposures are made from different points of view, i. e., a stereoscopic pair, the previously superimposed images separate from each other. What would have caused confusion in a single exposure may now be ignored by the viewer of the stereo pair. By eliminating confusions like these, stereoscopy increases the yield of information and the effective resolution at all levels in the specimen.
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