The numerical aperture of light microscopes is larger than that of electron microscopes at their present state of development by a factor of 100 to 1000. This is due to the fact that electron lenses have higher aberrations than the highly corrected glass lens systems used in modern light microscopes. In the design of an electron microscope care has to be taken therefore that the proper numerical aperture should be used to give the optimum resolution. The majority of transmission type electron microscopes use at present magnetic lenses. For magnetic lenses of the axial field distribution $H\left(z\right)=\frac{H_0}{[1+{\left(\frac{z}{a}\right)}^2]}$ optimum conditions for size and location of apertures will be stated. The behavior of the condenser lens-objective lens system with respect to the angular aperture of the illuminating electron beam also will be discussed.

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