concentration (n = n,, + a C), it follows that measurement of the phase 

 change, 0, which is the product of refractive index and thickness, can 

 be used to determine the dry mass of cell structures. The concentration 

 of solids is related to phase change by the equation 



= aCt 



Phase change is determined by interference microscopy, C by im- 

 mersion refractometry, and a is taken as equal to protein, or 0.00180. 

 The value of t is determined by the equation 



a C 



The combined use of interference microscopy and immersion refrac- 

 tometry makes it possible to determine the refractive index, solid con- 

 centration, dry mass, and thickness of cell structures. The changes in 

 dry mass, solid concentration, and volume during growth and cell divi- 

 sion have been studied by this technique (Figure 1 1-17). The variations 

 in dry mass of sperm nuclei of various species of mammals have also 

 been investigated in a similar way. 



UI+raviole+ Microscopy 



The use of ultraviolet radiation as a light source has several advan- 

 tages over that of the visible light ordinarily employed in conventional 

 microscopy. These major advantages are: (1) ability to view unstained, 

 living cells because of the strong absorption of ultraviolet by nucleo- 

 proteins of the cell, and (2) to provide a means of measuring quantita- 

 tively the amounts and distribution of nucleoprotein in cell structures 

 by their characteristic absorption in the 270 mu region of the ultraviolet 

 spectrum. 



The ultraviolet microscope differs from the ordinary light microscope 

 only in the use of lenses constructed of transparent fused quartz instead 

 of optical glass, v/hich is opaque to the shorter wavelength ultraviolet. 

 The slides and cover glasses used in mounting the specimen for study 

 are also of quartz. Because the eye does not respond to ultraviolet light, 

 focusing of the specimen must be done indirectly by visible light or by 

 means of a fluorescent screen. The image of the specimen is recorded by 

 a photographic plate, photoelectric cell, or television pickup tube. 



The ultraviolet microscope has its greatest value in studies dealing 

 with the identification and quantitative measurement of chemical com- 

 ponents in cell structures which absorb strongly in the ultraviolet (Figure 

 11-18). This absorption by cell structures is due to the presence of 



236 / CHAPTER 11 



