Possibilities of Future Development 103 



ton and R. G. E. Hutter.^^ But it appears that it has been put 

 into practice for the first time in the new Phihps commercial 

 electron microscope which was developed on the basis of research 

 work extending for several years by Dr. le Poole at Delft Uni- 

 versity. In the new Philips microscope the bore of the pole 



pieces is 11 mm. The advantage in the product (C/)* as com- 

 pared with typical lenses of narrow bore is about 20 per cent, 

 a very valuable gain. But even greater advantages have been 

 derived from the comfortable manipulating space provided by 

 the 11 mm bore, which makes it possible to accommodate a 

 movable objective aperture. It is also to be noted that the wide 

 bore makes it much easier to achieve axial symmetry, and thus 

 to approach the theoretical resolving power, which for this 

 instrument is estimated at 3.5 A. 



Suggestions for Corrected Microscope Objectives 



As the scope for improvements in electron microscopes with 

 uncorrected lenses is very limited, it appears of interest to ex- 

 plore the possibilities of corrected objectives. Though the way 

 to them bristles with difficulties, the prize may be worth the 

 effort. 



1. High frequency operated objectives 



Two lens errors cannot be corrected by methods as used in 

 present-day electron optics : the spherical aberration, and the 

 chromatic aberration. R. Kompfner ^^ made the very attractive 

 suggestion to use these two enemies of resolution in such a way 

 as to eliminate one another. We have seen in chapter 2 that 

 spherical aberration makes the outer zones of a lens stronger 

 than the inner ones, whereas the lens strength in every zone 

 decreases with increasing velocity by chromatic aberration. Let 

 us, therefore, make the outer electrons by E electron-volts faster 

 than the axial ones, which have an energy V ev. From equa- 

 tions (20) and (21), we obtain for the apparent diameter of a 



