152 MICROSCOPIC TECHNIQUES 



the microscope tube and the vacuum pumps are started. When the 

 pressure falls to 10 •'' mm. mercury, or less, the cathode-heating 

 filament is turned on and the temperature is gradually elevated 

 over an hour or more to the operating level. The slow heating is 

 essential to avoid distortions in structure, to minimize or prevent 

 curling on the cathode, and to approximate microincineration condi- 

 tions in order to permit a comparison. The practice is followed of 

 heating the cathode higher than operating temperature for a short 

 time to volatilize the sodium and potassium and to initiate active 

 emission of electrons. When this activation period is over, and the 

 operating temperature obtained, the high-voltage and lens currents 

 are turned on. The electron image formed on the screen is compared 

 with stained or incinerated control sections. When the tissue curls 

 away from the cathode and is improperly oxidized, dark areas appear 

 in the image and carbonization is evident by optical examination. 

 The magnification is altered by changing the position of the mag- 

 netic lenses on the tube and then bringing to focus by adjustment 

 of the lens current. Electron-accelerating voltages of 5000-6000 were 

 employed. Various portions of the section are focused on the center 

 of the screen by virtue of the mobility of the lenses. 



Photographs of the image are made with a high speed camera 

 (/ = 2.9) at a camera magnification of 0.73 on Eastman Kodak 

 Superspeed Ortho Portrait or Panchro-Press film. Exposures of 1-5 

 sec. are usually employed. The films are developed with Eastman 

 Kodak D-72 developer. Some photographs obtained in this fashion 

 are shown in Figures 31 and 32. 



G. RADIOAUTOGRAPHY 



The novel and thus far very limited technique of radioautography 

 has been employed in a few instances for the localization of radio- 

 active elements in tissue sections. The use of these isotoi)es as tracers 

 in biochemical investigations, particularly as the result of the 

 pioneering work of Hevesy, is a well-established device; however 

 histological distribution cannot be determined quantitatively, as yet, 

 on the basis of radioactivity, by any very satisfactory means, 

 since the order of the intensity of the radiation produced in the 

 quantities of tissue commonly employed for histological examina- 

 tions is far too small to permit suitable measurements M'ith the 



