. Electron microscopy; proceedings of the Stockholm Conference, September, 1956. Electron microscopy. 344 J. H. L. MCAUSLAN AND K. C. A. SMITH. Fig. 2 a. Needle crystal of silver azide—partly decomposed by heat. Fig. 2 b. End of needle crystal of lead styphnate. Note crystal- lographic break-up due to dehydration. directly in the microscope by means of a hot stage (fig. 1). Bombardment of the specimen in the conventional and scanning,' microscopes.—In order to extract a given amount of information from the image in the electron microscope a certain minimum number of electrons, as determined by
. Electron microscopy; proceedings of the Stockholm Conference, September, 1956. Electron microscopy. 344 J. H. L. MCAUSLAN AND K. C. A. SMITH. Fig. 2 a. Needle crystal of silver azide—partly decomposed by heat. Fig. 2 b. End of needle crystal of lead styphnate. Note crystal- lographic break-up due to dehydration. directly in the microscope by means of a hot stage (fig. 1). Bombardment of the specimen in the conventional and scanning,' microscopes.—In order to extract a given amount of information from the image in the electron microscope a certain minimum number of electrons, as determined by quantum and fluctuation considerations, must interact with the specimen. This minimum number will be determined, among other factors, by the quantum efficiency of the trans- mission process between the specimen and the brain of the observer; the quantum efficiency being defined as the ratio of the number of electrons or quanta associated with the point of minimum quanta trans- fer to the number falling upon the specimen. The bombardment of the specimen is smaller in the scan- ning instrument mainly because of its superior quan- tum efficiency. The conventional instrument operating in trans- mission will have a quantum efficiency, during re- cording of the image, not far short of unity since virtually all of the electrons passing through an element of the specimen will fall on the correspond- ing element of the plate and be recorded. However, when observing the image directly the quantum level falls because of the low efficiency of the fluorescent screen and the small angle subtended at the screen by the eye. It may be estimated that under these conditions the quantum efficiency is about 4 10~^ (4), that is, for every single visual stimulus which the observer receives, about 250 electrons must pass through the specimen. The quantum efficiency of the conventional in- strument operated in reflexion is very much worse because, under the usual conditions of operation, only abou
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