Supermicroscopes 31 



electron-optical laboratory for E. Ruska and his numerous col- 

 laborators. f The R.C.A. took an interest in the supermicroscope, 

 in 1938, when L. Marton (until then in E. Picard's laboratory in 

 Brussels) continued his researches in the Camden Laboratories 

 of the R.C.A. under V. K. Zworykin. 



J. Hillier and A. Prebus built, in 1938, a highly successful 

 supermicroscope at Toronto University, under the supervision 

 of E. F. Burton and W. H. Kohl. In 1940, J. Hillier joined the 

 staff of the R.C.x\., and with A. W. Vance developed the first 

 commercial supermicroscope, details of which were first pub- 

 lished in 1941.19-29 



The principle of the magnetic microscope is explained in figure 

 10 which shows the electron-optical arrangement side by side 

 with its optical counterpart, the projection microscope. 



The electron source in modern magnetic microscopes is a 

 thermionic cathode, almost always a tungsten wire, bent to a V, 

 pointing toward the object. This produces, in combination 

 with an electron gun arrangement which w^ill be discussed later, 

 a divergent electron beam which is made approximately parallel 

 by a first magnetic lens, the condenser lens. The beam next 

 strikes the object which in some cases can be suspended in the 

 meshes of a fine gauze, but in most cases is resting on an ex- 

 tremely thin supporting membrane. This is followed by a second 

 magnetic lens, the objective, which forms a fifty to one hundred 

 times magnified intermediate image. This in turn is magnified 

 in about the same ratio by a second lens, the projector lens, and 

 forms the final image, with a magnification of the order of sev- 

 eral thousand, on the photographic plate. 



In the early instruments, of which Ruska's magnetic micro- 

 scope of 1934, shown in figure 11, is an example, the electrons 

 wxre produced by a gas discharge from a cold aluminum cathode, 

 in the same way as in high speed cathode ray oscillographs. In 



t Reinhold Riidenberg, at that time Chief Electrician of the Siemens- 

 Schuckert A.G., patented, already in 1931, several fundamental principles 

 of the electron microscope, but experimental work in the Siemens con- 

 cern did not start until 1935.-^5 



