384 



JAMES HILLIEK 



complete and, in fact, stemmed from the basic analogy between opti- 

 cal and mechanical systems first published by W. R. Hamilton many 

 years earlier. The fundamental and practical significance of this 

 theory is that electron-optical counterparts of all optical instruments 

 are physically possible. Applied to the microscope it leads imme- 

 diately to the possibility of building an electron microscope. Sim- 

 ilarly the theoretical resolving power of such an instrument would be 



LIGHT SOURCE 



CONDENSER LENS 



OBJECTIVE LENS 



L 



PROJECTOR LENS 

 (EYEPIECE) 



OBSERVATION SCREEN 

 (PHOTOGRAPHIC PLATE) 



ELECTRON SOURCE 



MAGNETIC CONDENSER 



MAGNETIC OBJECTIVE 



INTERMEDIATE IMAGE 

 PROJECTOR 



SECOND STAGE 

 MAGNIFIED IMAGE 



Fig. 1. Simplified diagram showing analogy between 

 optical systems of light and electron microscopes. 



determined by Abbe's formula and hence, because of the short wave- 

 length of electrons, would be very high. There were, however, in- 

 numerable technical difficulties to be overcome before such resolving 

 powder could be realized and even today, when the resolving power of 

 the electron microscope is as much as 200 times greater than that of a 

 light microscope, the theoretical possibilities of an electron beam are 

 by no means fully utilized. 



The first attempt to build a high resolving power electron micro- 

 scope for the examination of materials that were not themselves 

 electron emitters was made by M. Knoll and E. Ruska in 1932. In 

 1935, E. Driest and H. 0. Miiller, working with another instrument 



