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260 THE ADVANCE OF PHOTOGRAPHY
The negatives obtained can be made use of to obtain a set of positives for three-colour work as follows : —
In making one of these positives a panchromatic plate is placed in a special printing-frame with its sensitive film upwards ; a thin sheet of glass or celluloid is then placed in contact with this film, and on this is placed the negative with its film side down. The glass side of the negative is placed facing the light. Exposure must be made to monochromatic light, and so the arc light, if used, must be filtered through a suitable screen. Suppose green light is in use ; then, if the arrangements are so made that the rays of light are parallel and striking the surface of the negative at right angles, since only one-third of the screenplate filter-surface consists of green bands, only one-third of the sensitive film will be affected. Now, when the source of light, or printing-frame, is moved a little to one side, so that the light strikes the face of the negative obliquely, a fresh part of the sensitive film of the positive will be exposed, and by making the exposure with the light, first on one side then on the other, after placing it in the central position, a full-tone positive in any one of the three colour sensations can be produced. The amount of movement made for the above need not be at all accurate. If three sources of parallel light can be used at once, then when two are arranged, one on the right, the other on the left, so that their light strikes the plate at small angles with the vertical, and the third placed directly in front of the plate, only one exposure is necessary.
In this manner all three positives can be produced by using suitable light-filters. The negative may, if desired, be also used for printing a colour positive, in which case a positive screen plate must be used in the place of the panchromatic plate, and the negative must be illuminated by white light. The film of the negative should be in contact with the glass of the positive plate,