The electromagnetic field in the focal plane of a finite axially-symmetrical paraboloidal reflector illuminated by plane wave of arbitary polarisation incident nearly normally at vertex, has been investigated. This paper describe the investigation, explains the method, and summarizes some of its results. The reflector is assumed to be of a focal length which is large compared with the wave length of the incident wave, and the plane of aperture is assumed to be about halfway between the focus and vertex. The purpose is to view of the diffraction pattern in the focal plane by treating the current induced on the concave surface of the reflector, so that the electromagnetic effect due to them on one part on the surface is taken into account at another by means of waves of current and charges on the paraboloid.