The electron microscope, its development, The electron microscope, its development, present performance and future possibilities electronmicrosco00gabo Year: 1948 122 The Electron Microscope keep present-day objectives constant within - . But it must be remembered that the short focal length of the suggested objective has been obtained not by a reduction of geometrical dimensions, but by a sort of compensation between two large systems. Fluctuations in the driving voltage will produce in it a new phenomenon which deserves special attention. Space charge region Fig. 44. Achromatic condition
The electron microscope, its development, The electron microscope, its development, present performance and future possibilities electronmicrosco00gabo Year: 1948 122 The Electron Microscope keep present-day objectives constant within - . But it must be remembered that the short focal length of the suggested objective has been obtained not by a reduction of geometrical dimensions, but by a sort of compensation between two large systems. Fluctuations in the driving voltage will produce in it a new phenomenon which deserves special attention. Space charge region Fig. 44. Achromatic condition It can be easily shown that fluctuations of the driving voltage will produce relative variations of the focal length of the com- posite lens of the same order as the relative fluctuation of the voltage. Because of the very short focal length, its absolute variation will be entirely negligible. But the new phenomenon arises that unless special measures are taken, the focal point will shijt along the axis by about the same amount as the varia- tion of the focal length of either of the two component lenses, taken by itself. Assuming that the constancy of the driving volt- age is not better than in present-day microscopes, the shijt in the above example would be about ten times the focal depth, and would frustrate the superior resolution. Instead of calculating the shift, we can at once indicate the measures to compensate it. The intercept of the tangent to the trajectory where it enters the space charge lens is b^(-\ =LeCothL (50) and if -^ is large, this will be very nearly
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