Xll. ELECTRON MICROSCOPY 385 



built by E. Ruska, achieved a resolving power somewhat greater than 

 that possible with the light microscope. The development was con- 

 tinued until by 1940 resolving powers of 20 A. had been achieved 

 by a number of workers and electron microscopes were made avail- 

 able commercially. Further improvements in the resolving power 

 were not forthcoming until, in the latter part of 1945, the present 

 writer designed a special objective able to resolve approximately 

 10 A. (Im/x). 



As would be expected from the preceding, the electron microscope 

 is quite analogous to the light microscope as far as the arrangement 

 of its lens system is concerned (see Fig. 1). However, in its physical 

 structure, it is an entirely different instrument. Figure 2 is a photo- 

 graph of a modern electron microscope, while Figure 3 is a somewhat 

 simplified schematic cross section of this instrument. The most ob- 

 vious physical characteristic of an electron microscope is that the 

 entire lens system including the specimen and the photographic plate 

 or film is enclosed in a high vacuum chamber. In view of the fact 

 that it is necessary to align the lenses, to position the specimen, to 

 manipulate the shutter and photographic plate in the vacuum cham- 

 ber, and at the same time to be able to gain easy access to the inside 

 of that chamber for replacing the specimen or replacing a photo- 

 graphic plate, a very special type of structure is necessary. Fur- 

 thermore, it is necessar}^ to provide a fast pumping system, which 

 will exliaust the air rapidly from the instrument after changing the 

 specimen or plate and keep it exhausted during its operation. As 

 little as ten years ago, many of the vacuum techniques used in the 

 electron microscope at the present time were not possible, even in the 

 laboratory, but fortunately — due to the simultaneous development 

 of such techniques for many other purposes — they have now become 

 routine and seldom provide any great difficulty. 



A complete description of the electron microscope and the details 

 of the construction of its various components cannot be given here. 

 These can be found in any of the general references. There are, how- 

 ever, a number of points in connection with the operation of the 

 electron microscope that should be given in order to clarify the 

 understanding of its use. For instance, the lenses used do not have 

 fixed focal lengths, particularly in the case of those instruments using 

 magnetic lenses. Since the focal length of the magnetic lens is 

 controlled by the strength of the field produced in the lens 

 and that in turn is controlled by the current passing through 



