changes introduced by cell structures are, for the most part, extremely 

 small and usually of the order of fractions of a wavelength. To measure 

 phase changes of this order of magnitude accurately it is necessary to 

 completely separate the directly transmitted light and that deviated by 

 the object such as in the interference microscope. The way in which this 

 is accomplished in the interference microscope is shown in Figure 1 1-16. 



Figure 11-16. Schematic Representation of an "Ideal" Interference Mi- 

 croscope System: Sj and So, semireflecting mirror surfaces; M^ and Mo, 

 fully-reflecting mirror surfaces; Lo and L3, microscope lenses; O, object slide; 

 C, comparison or "blank" slide. (From Barer, R., 1959. "Phase, Inter- 

 ference, and Polarizing Microscopy," in Mellors, R. C. (Ed.), "Analytical 

 Cytology," 2nd ed., McGraw-Hill Book Co., Inc., New York, N. Y. Fig. 

 3.22, p. 221.) 



The light originating from a common source is split into two beams, 

 one of which is transmitted through the object while the other bypasses 

 the object entirely. In Figure 11-16 a semireflecting mirror (Si) is used 

 as the beam-splitter. One beam is directed onto a mirror (Mo) where it 

 is reflected through the microscope and transmitted by the object. The 

 other beam is directed onto another mirror (Mi) and is reflected 

 through a lens system. The two beams produce interference when re- 

 combined at the semireflecting mirror (So). 



Because the refractive index of a cell structure is related to its solid 



SURVEY OF CYTOLOGICAL TECHNIQUES / 235 



