ANALYTICAL ELECTRON MICROSCOPY 



149 



The tissue holder assembly is made of a 25 mm. glass tube fitted 

 snugly with a brass sleeve soldered to a heavy brass plate. A 

 shoulder on the plate fits into the microscope tube and the joint is 

 rendered vacuum-tight with Apiezon Q sealing compound. Proper 

 centering alignment is maintained by brass cylinders {A and ^i) 

 which, fit over the glass tube. The flat top of the nickel cathode 

 cylinder (B) serves as support for the tissue sections and the 

 cylinder itself is held by a wire fixed to its wall. The cathode is 

 heated by a coiled filament (H), which consists of 10 mil tungsten 

 wire supported by a ceramic core not indicated in the diagram. The 



Fig. 29. Diagrammatic sketch of the essential features of the electron 

 microscope. A, tissue holder (cathode) support; Ai, inner shell of same; B, 

 nickel cathode; C, fluorescent screen; D, diaphragm; H, heating filament for 

 cathode; Li, objective magnetic lens; L2, ocular magnetic lens; P, pumping 

 port; T, transformer. Other letters and symbols are standard usage in vacuum 

 tube technique. From Scott and Packer (1939a) 



cathode filament is supplied with current from two 6 volt storage 

 batteries in series which are insulated from ground for 15.000 volts. 

 The insulation is required since the tissue holder assembly and 

 batteries are at a potential of 6000 volts with respect to the grounded 

 microscope tube. 



The fluorescent screen (C), which is sealed to the microscope 

 tube with wax, is a commercial oscillograph type. The bare portions 

 of the inner walls are coated with colloidal graphite (Aquadag) to 

 furnish electrical conductivity from the screen to the ground. This 

 prevents the accumulation of charge, which would distort the image 

 on the screen. 



