I 



CHAPTER 13 



THE ULTIMATE LIMIT OF 

 ELECTRON MICROSCOPY 



T will have become sufficiently clear from the last chapter, 

 that future development of the electron microscope is bound 

 to be slow and arduous. This is mainly due to the factor 



(C/)^ which promises only an improvement by a factor of 3.16, 

 even if 99 per cent of the spherical aberration could be success- 

 fully eliminated. There are also other difficulties to be considered, 

 such as the question of mechanical stability, and especially the 

 question of intensity. There is no difficulty in providing suffi- 

 ciently intense illumination. If the resolution is improved say Jx 

 times, this means that in order to get all the available detail the 

 object has to be imaged on a k- times larger area of the photo- 

 graphic plate. But as by the Airy-Abbe relation the aperture 

 has also increased in the ratio k, the solid angle of the illuminat- 

 ing beam increases in the ratio k^, which means that the exposure 

 remains the same. But the electronic bombardment of every 

 part of the object is now k" times stronger, and the danger arises 

 that beyond a certain point progress must stop, as we destroy 

 the details that we want to study more closely. This is particu- 

 larly true in the case of highly complex organic molecules. As 

 regards living matter, as von Ardenne "^ has shown on the basis 

 of experiments by C. P. Haskins, the limit is already reached in 

 present-day electron microscopy. Even the most tenacious spores 

 would be killed if we tried to take high resolution pictures of 

 them. This reminds one of the somewhat pessimistic saying by 

 N. Bohr, that we shall never know life entirely, as we have to 

 destroy it in order to study it. But though we may never know 

 everything, this does not mean that we cannot learn a great deal 



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