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192 THE ADVANCE OF PHOTOGRAPHY
service, and there are several well-known methods open to the experimenter.
First, there are mechanical or physical methods in which methylated spirit or some prepared solution is rubbed over the parts it is required to reduce, until the desired effect has been obtained. A very soft piece of washleather is most suitable for this purpose, and the result achieved is entirely due to friction. Then there are various chemical reagents which by their action upon the fixed plate can reduce the opacity which has been produced by the deposit of silver in the film. Some of these will be found useful for one kind of plate, others for plates of a different type. Thus, the Ferricyanide reducer, which is composed of a dilute solution in water of potassium ferricyanide, will be found very useful when it is required to reduce the shadows in greater proportion than the lights, while ammonium persulphate reducer acts more on the lights than the shadows, and ferric chloride will be found to act fairly uniformly.
No matter what care is taken with these supplementary processes, an error in the exposure, or even in the development, can never be absolutely rectified, although, of course, a very great amount of improvement in the printing properties of the negative may be obtained.
Fixing of Negative. — Before either of the above processes are tried upon a negative, it must be properly fixed.
When the developing process is complete, there exists side by side with the dark metallic silver deposit a quantity of unchanged silver bromide. This silver bromide would not only interfere with the value of the plate for printing purposes, but would also darken gradually when exposed to light. It must therefore be removed from the film, and for this purpose a solution must be employed which has the power of dissolving it, while leaving the silver untouched. The substance which is used for this purpose is commonly known as Hypo or hyposulphite of sodium,