114 MICROSCOPIC TECHNIQUES 



down to 10"^ /tg. Investigations may be made on selected structures 

 in microtome sections or on mechanically separated cellular com- 

 ponents. When sections are to be employed, they are best prepared 

 from tissue embedded in paraffin or celloidin after freezing-drying 

 treatment. The technical requirements in absorption spectra meas- 

 urements by means of the ultraviolet microscope have been examined 

 critically by Cole and Brackett (1940). Laviri' (1943) simplified 

 the focusing of the ultraviolet microscope by using a willemite 

 screen which produces a visible image with ultraviolet illumination. 



The absorption technique applied in situ has certain drawbacks 

 that should be considered, elegant though the technique is. Thus, 

 Danielli ( 1946a) has sounded a warning that hazards exist in as- 

 cribing to particular substances the absorptions found in different 

 parts of a cell. Effects of molecular interactions and interferences 

 by other substances are possibilities that are not to be ignored. Hence 

 the method will be of greatest value when the results are interpreted 

 with appropriate regard to these limitations. 



A most ingenious microscope arrangement for the colorimetry of 

 0.5-1.0 ix\. drops of liquid was developed by Norberg (1942), also 

 at Karolinska Institutet, and applied by him to the measurement of 

 phosphorus in quantities down to 0.5 n\[xg. This technique requires 

 the removal of the specimen from the rest of the tissue and its 

 chemical treatment to yield a solution which can be subjected to 

 absorption analysis. 



Stowell (1942) designed an apparatus for the measurement of the 

 amounts of stain or pigment in tissue sections. For the measurement 

 of stained constituents the quantitative significance of the method 

 depends on the degree of correlation between the amounts of the 

 stain and the substance for which the stain is specific, a correlation 

 often poorly defined. Stowell applied his technique to the estimation 

 of desoxyribonucleic acid by means of the Feulgen stain. 



1. Casper sson in Situ Technique 



A diagrammatic representation of the apparatus is given in Figure 

 9. The source of radiation may be either the Philips water-cooled 

 super-high-pressure mercury lamp (A), a tungsten lamp (B) sup- 

 plied with current from storage batteries, or Kohler's rotating elec- 

 trode spark gap (P) . The mercury lamp may be used for radiation in 

 the visible portion of the spectrum and in the ultraviolet range down 



