VISIBLE AND U.V. ABSORPTION HISTOSPECTROSCOPY 113 



SO that, if iron is to be investigated, another electrode metal should 

 be used. For an adequate exposure (15-30 sec), 2-4 /xl. of tissue 

 are usually sufficient. When tissue is subjected to fixation, a control 

 experiment is necessary to test the fixing fluid spectrographically; 

 the filter pulp method may be employed for this purpose. 



The Williams and Scott ( 1935) photoelectric apparatus for dark- 

 field photometry and densitometry has been used to determine the 

 intensity of the spectral lines in order to obtain a more quantitative 

 estimation of the elements. A description of this apparatus is given in 

 the section dealing with microincineration (page 146) . The blackness 

 of the photographed spectral lines is measured by placing the photo- 

 graphic plate on the mechanical stage, adjusting the reflecting prism 

 so that the light emerging from the ocular is reflected directly down- 

 ward on the slit of the photocell box, and observing the galvanometer 

 deflection after the proper focusing has been made. The deflection for 

 an unexposed portion of the photographic plate is then taken. 



C. VISIBLE AND ULTRAVIOLET ABSORPTION 

 HISTOSPECTROSCOPY* 



The application of the quartz microscope to measurements of the 

 absorption spectra of cellular components in situ, particularly as 

 developed and applied by Caspersson and co-workers at Karolinska 

 Institutet, Stockholm, offers a new and promising approach to the 

 solution of many histo- and cytochemical problems. The ingenious 

 apparatus of Caspersson (1940), subsequently modified by Gersh 

 and Baker (1943), has already yielded valuable information con- 

 cerning the nucleic acids of chromosomes (Mirsky, 1943), the nature 

 of thyroid colloid (Gersh and Baker, 1943), and chemical character- 

 istics of the Nissl bodies in nerve material (Gersh and Bodian, 1943- 

 a,b). (It should be pointed out that the absorption method cannot 

 differentiate between ribonucleic acids and desoxyribonucleic acids 

 since their absorption spectra are almost identical, having maxima 

 at about 260 m/i. However, the differentiation can be made qualita- 

 tively by staining reactions, (pages 65, 66). 



The great advantage of studying cell structures m situ by this 

 technique is made particularly impressive by the fact that the 

 spectral measurements can be carried out on quantities of material 



*See Bibliography Appendix, Ref. 31. 



