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SYNOPSIS. In order to observe a specimen at high magnifications in the electron‐microscope whilst keeping the instrumental magnification and intensity low, an optical magnifier is used to observe the image on the fluorescent screen. The brightness of the fluorescent screen at various instrumental magnifications and in combination with several optical magnifications is given in a table. A second table shows how the latitude encountered in the electron‐microscope when determining “sharp focus” is compressed by using the optical binocular magnifier at increasing magnifications. When observing finest details at the highest magnifications and taking photographs with the electron‐microscope the findings of the “sharpest focus” with the aid of the binocular magnifier results in the highest resolution becoming both visible and recordable. So far a resolution better than 25° has been obtained. The combination of the highly efficient zinc‐cadmium‐sulphide screen with the binocular magnifier is applied to the study of bacteriophage of Bacterium coli. A number of photographs show the sharpness obtained with the help of this combination and it is possible to follow the mechanism of bacteriophage action and behaviour in finest detail: the moment of entry of one ‘phage body into the bacterial cell, beginning and end stages in the multiplication of bacteriophage, colony formation, and the reaction of bacterial protoplasm during and after lysis.