. Cytology. Cytology. index (11-2) than through region A of lower refractive index (ih). As a result, the light transmitted by B is retarded in velocity with respect to that transmitted by A and emerges out of phase relative to that emerging from region A of lower refractive index. When the differ- ence in refractive index is small, the magnitude of the phase change induced is also small and is measured in wavelengths (^). In Figure 11-10, the light ray transmitted by region B is shown retarded V4 wave- length (- ) behind that transmitted by region A. The phase-contrast microscope transforms s
. Cytology. Cytology. index (11-2) than through region A of lower refractive index (ih). As a result, the light transmitted by B is retarded in velocity with respect to that transmitted by A and emerges out of phase relative to that emerging from region A of lower refractive index. When the differ- ence in refractive index is small, the magnitude of the phase change induced is also small and is measured in wavelengths (^). In Figure 11-10, the light ray transmitted by region B is shown retarded V4 wave- length (- ) behind that transmitted by region A. The phase-contrast microscope transforms such phase changes into corresponding variations of brightness or intensity. This serves to enhance the contrast between. Figure 11-10. Schematic Representation of the Retardation in Velocity of Light on Passing Through Two Adjacent Cell Parts (A and B) which differ from each other in thickness (/) and refractive index (n). the cell, its contents, and surroundings, thus permitting its study in the living state. The principle of the phase-contrast microscope is outlined below. Because most cell structures exhibit irregularities in detail or outline, they are probably best treated as optically inhomogeneoiis objects. Paral- lel light striking such an object is deviated from its original path on passing through and past the edges of an object. This deviated light is retarded or altered in phase (about 14 wavelength) with respect to light transmitted directly by the object and its surroundings (imdeviated light) and is spread over the entire surface of the objective lens (Figure 11-11). The light transmitted undeviated by the object and its surroundings passes, for the most part, through the more central part of the objective. In the ordinary light microscope the undeviated light is brought to focus at the rear focal plane of the objective where it diverges and spreads 228 / CHAPTER 11. Please note that these images are extracted from scanned page images that may have been digitally
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