Experiments With a Variable Amplitude and Phase Microscope

SYNOPSIS. A special Cooke microscope was used to examine the principles of phase‐contrast applied to a series of simple objects. With transparent objects the image contrast with different methods depends on the phase change introduced by the object. Simple absorption of the direct light without phase change increases contrast of both phase and absorbing objects at first, but eventually produces central dark‐ground illumination. With objects of widely differing phase change optimum phase‐contrast is obtained with a quarter‐Nave phase‐plate. The exact degree of phase change required is not critical. Absorption of the direct light increases the sensitivity and it is suggested that 100 P.c., 20 p.c., and 6 p.c. transmission objectives would suit most requirements. Absorption of diffracted light is useful with objects of high phase change, but as these are already easy to see the scope of this method is limited. Phase‐contrast is maximal at the border of the image of a large object, but it extends further into the image than is generally realized. As a means of immersion refractometry phasecontrast is more sensitive than the microscopical methods commonly employed. Precautions to be observed in the use of the star test and Abbe test plate are mentioned.