A general method of magnetic direction refocussing, i.e., the refocussing of slightly divergent electron paths in a uniform magnetic field, has been found, of which the familiar 180° refocussing is a particular case. If we use a wedge-shaped magnetic field, whose lines of force are perpendicular to the plane of motion of an electron beam, the field being produced by a solenoid or magnet pole pieces of such a shape that the effective boundaries of the field projected on the plane of motion of the electrons is V shaped; and if the electron beam enters the magnetic field perpendicular to one edge, with the strength of the field set so that the electron beam will leave perpendicular to the other edge; then the refocussing of slightly divergent electron paths will occur. The position of best refocussing is on a line through the point of divergence of the electron beam and the apex of the wedge field. A magnetic field approximating the desired field has been obtained and the theoretical results have been checked for a particular case in which the angle at the apex of the wedge field was 90°. The particular value of this general refocussing property is that, with it, velocities of electrons can be analyzed without the deflecting magnetic field straying over into the region from which the electrons originate.

• Received 31 January 1934

DOI:https://doi.org/10.1103/PhysRev.45.513