Supcniiicroscopes 33 



of 1934. The whole coil is enclosed in the vacuum envelope. As 

 outgasing of the insulation and of the long and narrow interstices 



Fig. 11. E. Ruska's magnetic electron microscope of 1934 



between the windings would be a very slow process, the whole 

 coil is encased in metal. The gap in the iron casing is covered 

 with non-magnetic material, such as brass. The pole pieces are 

 interchangeable. In addition to these features, which recur in 

 practically all modern microscopes, Ruska's design provides 

 water cooling, a complication which was later found unnecessary. 



Comparing the optical systems of an electron microscope and 

 an optical microscope of the projection type, which are shown 

 side by side in figure 10, it can be seen that they are qualitatively 

 identical. There is, however, a striking quantitative difference 

 in the illuminating beams. Optical microscopes w^hich aim at 

 high resolution are fitted with wide-angle illuminator systems. 

 The advantage of wide-angle illumination for a good resolution 

 w-as a result of Abbe's theory of the microscope, and though 

 later investigations proved that this conclusion was really a 

 rather daring extrapolation from the original theory, the result 

 remained true, though it had to be buttressed by another theory. 

 The best result is obtained when about two thirds of the aperture 

 is filled by the illuminating beam, but this condition is not 

 critical. 



In electron microscopy, however, it appears from figure 10 

 as well as from practically all the literature of the subject, as if 

 a radically dififerent method had been chosen : parallel illumina- 



