After the acquiring of the necessary skill in manipula tion, an inquiry into the nature of a photographic process is the first step in the direction of the systematic experi ment which brings such pleasure to the genuine enthusiast and has done so much to advance the science and art of photography. Good handbooks of photographic manipu lation are abundant; but as the real business of a hand book is to give directions and not to explain principles, they are apt to be unsatisfactory to the thoughtful worker, interested in reasons as well as results. If he turns to complete treatises, of which there are also not a few, he is likely to find' himself overwhelmed with an avalanche of detail and history, with much of which he is not at all concerned, and of no immediate interest or necessity. Neither do monographs offer great help, as a rule, for they are usually highly technical and confined to such limited portions of the photographic field that the desired informa tion generally lies in the gaps between them.