FLUORESCENCE MICROSCOPY 105 



They reported that in the dry state the amide has a weaker fluores- 

 cence than the acid, but that in a 1% aqueous solution the reverse 

 is found. EHinger (1940) was not able to observe fluorescence with 

 purified solutions of these compounds, and he also disagrees with 

 Hirt and Wimmer (1939b) that ascorbic acid can be detected 

 microscopically by fluorescence. The histochemical opportunities of 

 studying vitamin K by means of its well-known fluorescence are 

 obvious. One may expect that studies of this nature will be made in 

 the future. 



Lipids. During the course of his work on vitamin A, Popper 

 (1941) also studied the histological detection of lipids by means 

 of the fluorchromes: methylene blue, thioflavin S, rose bengal 

 magdala red, and phosphine 3R. The last stain appeared to be the 

 best. Further examination revealed that fatty acids, soaps, and 

 cholesterol are not made apparent bj^ phosphine 3R which, however, 

 does visualize neutral fat as a silver-white fluorescence on a brown 

 background (Volk and Popper, 1944b; Popper, 1944). The advan- 

 tage claimed for this method is that, because of the water solubility 

 of the dye, more and finer droplets of lipid can be detected than 

 would be possible by the usual stains. Popper (1944) recommends 

 the use of a 0.1% aqueous solution of phosphine 3R {Pfaltz and 

 Bauer) for 3 min. on frozen sections of tissue. 



Pigments. The fluorescence technique has been applied to studies 

 of certain biological pigments. Thus the red fluorescence of porphy- 

 rins has been employed in histological studies of these compounds 

 (Lison, 1936, page 256; Ellinger, 1940; Dobriner and Rhoads, 1940; 

 Grafflin, 1942) . Chlorophyll has been localized microscopically in 

 plant tissues by Tswett (1911) and Wilschke (1914) by means of 

 its red fluorescence. The fluorescent properties of bile pigments in 

 the presence of zinc acetate, and of uropterin, might be adapted to 

 microscopic studies of these pigments. 



Carcinogenic Hydrocarbons. Investigations of carcinogenic 

 hydrocarbons in tissues have made use of the fluorescence of certain 

 of these compounds. Graffi ( 1939, 1940) investigated the distribu- 

 tions in normal and tumor cells of pyrene, benzpyrene, anthracene, 

 dibenzanthracene, and methylcholanthrene, while Gunther (1941) 

 and Doniach et. al. ( 1943) confined their studies to benzpyrene, and 

 Simpson and Cramer (1943-1945) explored the histological localiza- 

 tion of 20-methylcholanthrene in skin. 



