The manipulations in using the microscope, other than the selection of lenses, focusing, and placement of slides, relate to the illumination. When the illumination is not handled with judgment, glare and lack of contrast result, and crisp images are not obtained. Effective illuminations depend equally upon three factors, the condenser, the field stop, and the illuminant proper. The adjustments for superior glare-free illumination are discussed and the term “controlled” illumination is suggested as an appropriate designation for such illumination. Aspects of the lighting requirements of the microscope are analyzed, and it is shown that suitable regulation of the apertures of the field and condenser stops are the basic requirements for controlled illumination. Various types of lighting may be used in conjunction with these adjustments. The paper is presented against a background of the pertinent historical developments and the various factors relating to illumination are reviewed in a broader fashion than is common in texts on microscopy. Explanations of the principles and the defects of illumination are presented with the general biological user of the microscope in mind. The shapes and proportions of the various illuminating beams that play on specimens are illustrated. Aberrations and the origin of glare are explained. In general, there are two extremes in the practical use of illumination—illumination for high resolution and illumination for maximum contrast. The microscopist should ordinarily vary his illumination as he works to obtain the advantages of each type of illumination. The conditions of each are fully treated.
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