4 



STUDIES IN GELS 



119 



highly contrasting images, the aperture of the objective lens must be as 

 small as possible. On the other hand, a small aperture is unfavourable 

 to the resolving power of the microscope, for, according to Abbe's 

 theory of image formation, the 

 resolving power increases with 

 the aperture and reaches a maxi- 

 mum when this becomes ^-^ i. It 

 would be useless, however, to 

 make high aperture electron lens- 

 es because their lack of correc- 

 tion would produce imperfect 

 images, in the same way as un- 

 •corrected lig-ht lenses. Thev 

 •could only be improved by cut- 

 ting down the aperture, but this 

 would reduce the resolution. 

 The present quality of electron 

 lenses can be compared to that 

 -of the optical lenses at the time 



Fig. 80. Electron scattering by a specimen and 



selective effect of the objective Jens aperture 



(from HiLLiER, 1946). 



when Abbe began to eliminate their spherical and chromatic defects. 

 The necessary screening of the scattered light and the defects of 

 the lenses require very narrow bundles of electrons with apertures 

 of only 0.00 1 to 0.005. As a result of the small apertures a applied, the 

 Tesolving power d is not as large as could have been expected from 

 the exceedingly small wavelength. As calculated for the ordinary 

 microscope (probably Abbe's theory cannot be applied without altera- 

 tions to the electron microscope, but, curiously enough, the results 

 are plausible), the resolving power \s l\a = 0.05 A/0.002 = 25 A. 

 This minimum, however, is only seldom reached. Usually the resolving 

 power amounts to about 50 A (Kinsinger and co-workers, 1946). 

 This is near to the smallest gold particles which have been demonstrated 

 in the ultramicroscope (60 A). Instead of luminous points, however, 

 true images are obtained. Thus the electron microscopy covers the 

 whole field of particle sizes in colloid chemistry, completing this 

 science by the new branch of colloid morphology. Considering the 

 hard work needed to increase the resolving power of the ordinary 

 microscope from dry systems with d = o.^ fxto quartz immersion for 

 ultraviolet light with d = o.\ /u, we cannot sufficiently express our 



