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188 THE ADVANCE OF PHOTOGRAPHY
The complete Thermo-time system of Wat-kins can therefore be stated as follows : " Allow for the variation of the plate by altering the dilution of the developer, and for the temperature by altering the time of development."
The tank development above referred to, is based directly upon the fact that the only control we have over the quality of the negative produced, is the length of time the developer is allowed to act upon the exposed plate, and it is found by experience that if in twenty-four minutes a certain developer at a certain temperature will produce a perfect negative in a plate or film for which correct exposure has been allowed, then it will also produce the most satisfactory results if allowed to act upon under-exposed or over-exposed plates or films for the same length of time.
For developing roll films the Kodak Developing Tank is perhaps one of the best known. By its means one can avoid finger marks, scratches, stains and patches upon the negatives obtained, and, of course, uneven development is impossible.
Again, when using this apparatus no dark room is necessary, as the operations can be carried out in daylight or gaslight as desired.
All that has to be done when using this tank is to enclose the exposed film, with the aid of a covered box supplied with the outfit, in a light-tight celluloid band, and immerse it in the tank of developer, where it should be left for the necessary time, then removed, fixed, washed and dried in the usual manner.
A similar tank developing apparatus can also be obtained for use with plates, but, of course, a dark room of some kind is necessary for removing the plates from their holders and placing them in the tank.
Daylight development without the aid of any particular form of tank has been practised for some time to a limited extent. The method adopted is either to stain the sensitive films to a non-actinic colour, or else to stain