8 Microscopic Histochemistry 



h) Fluorescence— K number of substances show various 

 kinds of fluorescence under ultraviolet light. Some of the 

 most typical are the fading green-blue fluorescence of vita- 

 min A, the golden-yellow fluorescence of enterochromaffin 

 granules, and the brown one of ceroid. Great care should be 

 exercised in the evaluation of the results, and, if possible, the 

 emitted fluorescent light should be analyzed spectroscopi- 



cally. 



Staining with fluorescent dyes is not a histochemical 

 method, any more than staining with other, nonfluorescent, 



dyes. 



c) Ultraviolet spectrography— This was developed by 

 Caspersson^ and his school. A number of compounds, such as 

 nucleic acids and some amino acids, have characteristic ab- 

 sorption spectra in the ultraviolet and can be identified by 

 them accurately and in a quantitative fashion. A fairly com- 

 plicated and expensive equipment is required, consisting of 

 a suitable ultraviolet light source, a quartz optical system for 

 the microscope, and a highly sensitive microphotometer. 

 Areas of the size of 1 fx^ can be used for analysis. 



d) X-ray spectrography (Engstrom) .^— This is an even 

 more complicated procedure, with a difficult theoretical 

 background. However, it permits a highly accurate quanti- 

 tation of almost any element (but not of compounds) in very 



small areas. 



e ) Spark spectrography.^— The emission spectrum of a part 

 of a tissue section vaporized in a spark gap is analyzed. 



/) Tracer techniques^— These techniques utilize the emis- 



1. Caspersson, T.: Arch. f. Physiol., Vol. 73, Suppl. 8, 1936, and J. Roy. 

 Micr. Soc, 60:8, 1940. 



2. Engstrom, A.: Acta radiol., Suppl. 63, 1946. 



3. Gerlach, W., and Gerlach, W.: Die chemische Emissionsspektralana- 

 lyse. II. Anwendung in Medizin, Chemie und Mineralogie (Leipzig: Voss, 

 1933); Policard, A., and Morel, A.: Bull, d'histol. appliq. a la physiol., 9:57, 

 1932; Policard, A.: Protoplasma, 19:602, 1933; and Scott, G. H., and Wil- 

 liams, P. S.: Proc. Soc. Exper. Biol. & Med., 32:505, 1934. 



4. Hamilton, J. G.: J. Appl. Physiol, 12:440, 1941; Simpson, W. L.: 

 Radioactive isotopes, in Cowdry, Laboratory technique in biology and medi- 



