Abstract

Experiments in which the numerical apertures of the objective and condenser of an ordinary microscope are reduced progressively and alternately in a systematic manner are described as regards contrast in the images of nonabsorbing particles that differ slightly from their surrounding medium in optical path. The observed contrast remains poor as the numerical apertures of the objective and condenser are reduced by means of variable diaphragms such that the ratio s of the numerical aperture of the condenser to that of the objective is held at unity. The observed contrast in the images of platelike particles tends toward similarity for a given value of s as the size of particle is increased. The various observations are interpreted theoretically and found to be in qualitative agreement with theory. Conditions under which distributions of irradiance in the images of different platelike particles become similar are formulated for in-focus and out-of-focus image planes. Knowledge of these conditions leads to improved understanding of the effects of numerical aperture on contrast in the image.

© 1961 Optical Society of America

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