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DRY PLATES, FILMS AND PAPERS 185
the development for ten minutes. The actual density of the strips will be found to have somewhat increased, but it will be an increase in the same proportion — i.e. the second will still be 1*6 times as dense as the first.
On the other hand, the opacity of the strips will be greatly changed by the extra development, and the increase in this will not be proportionate ; the opacity of the strip which had the longer exposure will increase much more rapidly than that of the other strip.
It must not be understood from the foregoing that no control is possible with the developer, as that is absolutely incorrect. Control can be effected when developing, but it should be done by altering the length of time the developer acts, and not by altering the ingredients — that is to say, no alteration in the composition of the developer can alter the gradations in the negative ; these are due to the exposure to light.
All the various types of plates that are required for different purposes — such as a fairly dense negative for platinotype work, a much denser one for P.O. P., a thinner one for carbon, and a thin and somewhat flat negative for enlarging — may be readily produced by an alteration in the time the developer is allowed to act, if the correct exposure has been made.
Mees and Sheppard have carried out further researches in this direction, and have come to the conclusion that there are two distinct classes of photographic plate so far as speed of development is concerned. One of these gives the same speed no matter what developer is used, while the other class is much faster with such developers as pyro-soda, hydroquinone, etc.
Factorial Development. — The next great step made in scientific development was due to Watkins, who introduced the system of " Factorial development."
This may be looked upon as an outcome of the work of Hurter and Driffield, for it is based upon the fact that the