///. PHYSICAL METHODS 



A. FLUORESCENCE MICROSCOPY 



The detection and localization in tissues and cells of certain 

 substances by virtue of their flourescent properties when subjected 

 to ultraviolet irradiation is finding increasing application. Sub- 

 stances investigated in this manner include naturally occurring 

 compounds, such as vitamin A, and others introduced into organisms 

 for experimental purposes, such as 20-methylcholanthrene. The 

 fluorescence exhibited in tissues or cells may be "primary," i.e., 

 produced directly by certain compounds, or "secondary," i.e., result- 

 ing from treatment with so-called "fluorochromes," fluorescent sub- 

 stances taken up selectively by particular cellular structures having 

 no fluorescence of their own. For the most part, the use of fluoro- 

 chromes is limited to purely morphological studies without regard 

 to chemical nature and hence need not be discussed here, except as 

 applied to lipids (page 105). However, for those who may be in- 

 terested, lists of fluorochromes and their properties may be found in 

 Haitinger (1938), Jenkins (1937), and Metcalf and Patton (1944). 

 General discussions of fluorescence microscopy are to be found in 

 Sutro ( 1936) , Jenkins ( 1937) , Ellinger ( 1940) , Simpson (in Cowdry, 

 1943, pages 76-78) , and Metcalf and Patton ( 1944) . The books of 

 Haitinger (1938), Radley and Grant (1939), and Pringsheim and 

 Vogel (1946) are useful for reference. 



1. Apparatus 



The set-up of the fluorescence microscope is shown in Figure 3. 

 Ultraviolet radiation having a high intensity in the range 300-400 

 m/j. is produced by means of a carbon arc or one of the mercury vapor 



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