A method is proposed and the theory briefly given for focusing to a line all electrons of the same energy reflected or emitted at any given angle from a plane surface.
It was found that the lines of focus lie on the surface of a cylinder that is tangent to the center of the reflecting surface and having a radius equal to the radius of the path of the electrons.
D. A. WELLS, "FOCUSING ELECTRONS REFLECTED OR EMITTED AT EQUAL ANGLES FROM A PLANE SURFACE," J. Opt. Soc. Am. 16, 355-356 (1928)