A difference equation method for the calculation of the probability that two molecules will collide

The probability that one molecule will collide with another is calculated for the case in which the molecule is confined to equally spaced rings about the second larger molecule except when it jumps from one ring to the next. The result is practically the same as that obtained for an infinitesimal mean free path, not having first-order terms in the ratio of the mean free path to the particle radius as does the result of the calculations of R. Wijsman (1952).