• THE PROCESSES OF 'PURE PHOTOGRAPHY.
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"reduction." We start with a compound of (say) silvir and something else. Actinic light either "reduces," or prepares for "reduction," our compound, and the "reduction" consists in removing the something else, and leaving the silver alone to form the visible p4otographic image. The optics of photography are directed chiefly to regulating the size of our image, light unaided is quite able to effect all our purposes, but without the aid of optical appliances light would be for us an unmanageable and unprofitable servant. We do not expect, and still less wish, this summary to be taken as touching more than the extreme outskirts of the subject of photographic theory. We believe that a thorough mastery of the whole theory is almost essential to a thorough mastery over the practice, but our limits absolutely preclude other explanation. Certain processes are not even touched by the above remarks; on encountermg these processes in their turn, we shall say a word or two on the special theories regulating them.