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BOOK ILLUSTRATIONS 269
The correctness with which this half-tone dot is formed depends not only on the fineness of the screen used, but also on the size of the diaphragm and the camera extension.
The copy for reproduction with the half-tone process may of course be a photo or even a wash drawing. It is, however, often necessary to touch up the copy by working upon it in colour ; this may either be done by hand brush work, or as is now more often the case by what is technically known as the air brush. This air brush consists of a mechanically worked pump which is used to force tiny jets of colour from a very small hole. The size of the jet can be varied by altering either the air pressure or by valves, and the effect on the plate can be changed by altering the distance of it from the jet.
The glass which is used as a support for the film of the negative must be perfectly clean. The collodion which is required for coating the plate can be purchased already prepared. The sensitizing bath for this work should contain not less than forty grains of silver to every ounce of distilled water, and it is very important that the silver bath should be kept up to its proper strength. After coating the plates with collodion, they are immersed two or three minutes in the silver bath, after which they are ready for use.
This preparation of the sensitive plate must of course be performed in a specialty darkened room. The amount of light admitted may be much in excess of that allowed when dry plates are used, since these wet plates are not so sensitive to light, but great care must be taken to admit only such light as is chemically inactive. This is done by covering up all sources of light with a suitable yellow screen.
The negative is developed with ferrous oxalate developer, and fixed by means of potassium cyanide. The negative must next be intensified, after which it is rinsed and then dried.