126 MICROSCOPIC TECHNIQUES 



The lamp is housed in a Spencer (No. 367) lamp case which has a 

 filter holder. A microscope fitted with a mechanical stage, a 44 X 

 achromatic objective, and a 15X compensating ocular are employed. 

 The field of observation is limited to an area 50 X 35 /a by inserting 

 a rectangular diaphragm in the ocular. The light from the microscope 

 is thrown on a vacuum photocell (RCA 929) enclosed in a light-tight 

 box having a side tube to extend over the microscope tube. This 

 side tube contains a movable mirror so mounted that it can be 

 interposed in the light path to enable inspection of the field, and 

 then tm'ned out of the path to permit photoelectric measurement. 

 The photocell current is amplified by a General Electric FP54 tube 

 in a Barth circuit with a 10^^ ohm grid resistor (Penick, 1935). The 

 amplified current is measured with a Leeds and Northrup student 

 type potentiometer and a Leeds and Northrup type R galvanometer. 



Measurements are made of the light transmittance through both 

 stained and unstained sections in order to obtain the absorption 

 due to the stain itself. In both cases, measurements are also made 

 of the transmittance through blank portions of the glass slides 

 adjacent to the sections to correct' for variations in the intensity 

 of the light source, changes in amplification or potentiometer bat- 

 teries, and alterations in thickness of cover glasses, slides, or mount- 

 ing media. When it is possible, fifty adjacent areas on each section 

 are measured and the mean percent absorption calculated. 



Stowell and Albers ( 1943) employed a Coleman Model 10 S double 

 monochromator spectrophotometer by means of which they meas- 

 ured absorptions of light bands, having a 5 rap. spectral width, by 

 stained sections of tissue. Since no microscope is employed in this 

 apparatus, the absorption of the section as a whole, rather than a 

 chosen cellular region, is measured. 



The photometric procedure was employed by Stowell ( 1942) for 

 the estimation of desoxyribonucleic acid in tissue sections by means 

 of the Feulgen stain (page 65). Light from the source was passed 

 through a heat-absorbing filter (Corning Aklo No. 396) and a green 

 gelatin filter (Wratten No. 58) before entering the microscope. The 

 extension of the method to absorption studies with a variety of 

 stains commonly used in histological examination was included in 

 the reports of Stowell and Albers (1943) and Stowell (1945b). 

 Subsequently Stowell (1945a) subjected the Feulgen reaction to 

 detailed study and described each step in his method of using it. 



