Diffraction coronæ due to non-spherical particles

Abstract

It is pointed out that the Fraunhofer diffraction due to an obstacle of arbitrary shape can be replaced by that given by a linear distribution of sources along its boundary. Most of the intensity at any point in the pattern can again be supposed to originate from a finite number (usually two) of point sources, called "opposed points" or "poles", situated on the boundary. In this way, if there are a large number of non-spherical particles distributed at random, then a ring system will be formed whose size will correspond to the distance between the opposed points. If this distance is a constant over an appreciable region of the boundary, then the rings will stand out from the background intensity. This explanation of the formation of the rings, has been verified using spores of Pinus longifolia and of Lycopodium. It is also shown that variations in the size of the particles affect the clarity of the rings detrimentally, the rings becoming less and less clear as the range of particle sizes increases. This is also verified by using two types of Lycopodium.