Immersion refractometry has been used to measure the changes in 

 refractive index and concentration of soHds in cell structures during 

 such activities as cell division and fungal spore development. The method 

 is also useful for the study of X-ray and drug effects on living cells. The 

 variations in cytoplasmic refractive index and solid concentration during 

 different stages of meiosis in the male grasshopper is shown in Figure 

 11-15. 



Figure 11-15. Histogram Showing the Changes in Solid Concentration 

 (Per Cent) and Refractive Index in the Ground Cytoplasm of Primary and 

 Secondary Spermatocytes of the Grasshopper, Chortippus, During Spermato- 

 genesis. (From Barer, R. and Joseph, S., 1957. "Phase-Contrast and Inter- 

 ference Microscopy in the Study of Cell Structure," Symp. Soc. E.xptl. Biol., 

 10, Fig. 3, p. 172.) 



While use of the phase-contrast microscope makes it possible to detect 

 phase changes by converting them to visible intensity changes, the instru- 

 ment is not designed to permit precise measurement of these phase 

 changes. As pointed out, a diffraction plate is used in the phase-contrast 

 microscope to separate the direct and diffracted light so that they can 

 be altered in phase relative to each other to give increased or decreased 

 brightness of the object. Because this separation of direct and diffracted 

 light is never complete, it is possible to detect only relatively large 

 changes in phase or optical path by phase microscopy. The phase 



234 / CHAPTER 11 



