1. The term “pencil” is used to designate those rays which take part in the imaging of a single point.
2. See V. K. Zworykin, G. A. Morton, E. G. Ramberg, J. Hillier, and A. W. Vance,Electron Optics and the Electron Microscope(John Wiley and Sons, Inc., New York, 1945).
3. The combined effects of chromatic change in magnification and chromatic change in rotation (in the case of a magnetic lens) may be considered as an exception to the above statement. However, these affect only the outer zones of the image and are less, in general, than the defocusing of the entire image caused by the simultaneously appearing chromatic aberrations of the objective.
4. J. Hillier and R. F. Baker, “Miscellaneous research on electron microscope parameters,” presented before Electron Microscope Society of America, January 14, 1944.
5. For simplicity this point will be considered at infinity in the present paper. In critical work, however, it is necessary to take into account the fact that the objective is normally at a finite distance from the projector in which case u1 is substituted for f1 in the following formulas.