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 



