Stationary optical paths

Referring to Dr. Karl Darrow's article on quantum mechanics, Mr. T. Smith has pointed out that optical paths are not (as Darrow repeats in his article) paths sometimes of maximum and sometimes of minimum time; but that the time is a minimum if the path does not include an image of an end point, and if the path includes an image then the time is neither a minimum nor a maximum. Smith has further elucidated this point in connexion with Darbyshire's recent letter