Chemically, photographic development is now understood to be a reduction process. Many of the features of its mechan ism, however, are relatively unknown, as, for example, the differentiation by certain reducing agents between the silver halide which has been affected by light and that which has not. To clear up these matters and to determine the nature of the latent image will require extensive experimental work on details which at first seem to have little bearing on the subject. Although it is impossible at present to rate the various phases of possible investigation according to their importance, it is evident that all effects of development must be eliminated one by one. The study of development characteristics and of the reduction potentials of developers is thus of the greatest importance.