Interpretation of Fermat's Principle

Abstract

IN order to demonstrate the possibility that an optical path between two fixed points may be merely stationary, that is, neither maximum nor minimum, Mr. T. Smith¹ includes in the infinite number of paths with which an actual path is to be compared those which in one homogeneous medium are non-rectilinear. Thus in Fig. 1, if A is the image of A due to the lens PQ, the actual path APAB is compared with the longer and shorter imaginary paths APCB and AQDB.