no 



MICROSCOPIC TECHNIQUES 



(Fig. 5). A platinum or gold needle embedded in a cylinder of 

 insulating material, which is fixed in one of the objective holes of 

 the microscope's revolving nosepiece, serves as a movable electrode. 

 The circuit is completed through the frame of the microscope which 

 is grounded. The cellular region to be examined is chosen with an 

 objective in position, and then the objective is swung out and the 

 electrode needle brought into place. The needle should be in line with 

 the optical axis of the microscope so that it is centered over the 



Fig. 5. Arrangement of Policard's 

 apparatus for emission histospectros- 

 copy: 1, Microscope stage under re- 

 volving nosepiece bearing tlie upper 

 electrode; 2, high-frequency appara- 

 tus — dermatological type; 3, Hilger 

 quartz spectrograph; 4, cylindrical 

 quartz condensing lens; 5, optical 

 bench; 6, ground lead; 7, current 

 source; 8, upper electrode lead. From 

 Policard (1931-1932) 



selected spot. The distance between the point of the needle and the 

 section is fixed between 1-2 mm. When the circuit is closed, the 

 radiation from the spark produced is condensed by the quartz lens 

 and directed through a 0.1-0.2 mm. slit into the quartz spectro- 

 graph. For a strong spark, a period of not more than 1 sec. seems 

 to be sufficient for the exposure of the photographic plate on the 

 spectrograph, but for weak sparks 20-30 sec. may be required 

 according to Policard. The best spectral lines were observed in the 

 ultraviolet range, 100-300 m/x. 



2. Scott and Williams Technique 



Scott and Williams (1935) modified the apparatus chiefly by 

 screening the electrodes, placed 1.4 cm. apart, in order to practically 

 eliminate electrode lines from the spectra and to obviate the need 

 of cleaning the electrodes often. Since these workers did not employ 

 needle electrodes mounted on a microscope, they could not make 

 as fine a histological selection of the sample as the European investi- 

 gators. Scott and Williams (1934) minimized this point and claimed 

 that removal by fine dissection of the bit of material to be studied 

 is sufficient. In most instances this would be true, but there are 



