148 MICROSCOPIC TECHNIQUES 



conception of this possibility and the ingenuity to carry through to 

 fruition, credit is due to Gordon H. Scott and his associates, J. H. 

 McMillen and D. JVI. Parker, wlio carried out their investigations 

 at Washington University. 



Scott and co-workers utiHzed the well-known fact that when 

 metals or certain of their compounds are heated in vacuo they emit 

 a number of electrons depending in part upon both the nature of 

 the metal and the temperature. Hence identification of the metals 

 might be possible on the basis of their differential emission of 

 thermally excited electrons. The localization of these metals in tissue 

 sections would be possible since the electrons emitted could be 

 focused by the magnetic lenses of the electron microscope to yield an 

 image, on a fluorescent screen, of the topographical disposition of 

 the emitting substances. 



The first apparatus developed for this purpose was that of 

 McMillen and Scott ( 1937) but, since a number of changes have 

 been made, only the later apparatus of Scott and Packer ( 1939a) 

 will be described. 



The Scotl-Packer Analytical Electron Microscope 



A diagram of the instrument is given in Figure 29. Basically it is 

 an electron microscope of relatively low magnifying power fitted 

 at one end with a chamber (C), lined with a material capable of 

 fluorescence by electron streams for the visualization of the electron 

 image, and at the other end with a special cathode (B), carrying 

 a tissue support (.4) to hold the paraffin sections that are employed. 

 The microscope tube is made of brass, and is 1 m. long and 63 mm. 

 in diameter. Two magnetic lenses (Lj and Lo), swung on gimbals, 

 surround the tube and are free to move along it as well as rotate 

 around it to some degree. Each lens is composed of 1550 turns of 

 No. 22 enameled, single-cotton-covered copper wire wound on a 

 copper inner ring 75 mm. in diameter. The coils are enclosed in 

 sheaths of soft iron having a wall thickness of 3 mm. The lenses have 

 an axial width of about 43 mm. It is apparent that the object-image 

 distance is fixed; therefore focusing for any magnification must be 

 accomplished by altering the power of the lenses. This is done by 

 varying the current passing through the lens coils. The power for 

 these coils is supplied from two 30 volt banks of storage cells in 

 parallel. 



