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PHOTOGRAPHY IN NATURAL COLOURS 261
and three exposures made, or three parallel beams of white light used. No cross-line pattern appears on the positive, owing to the exposure through the two screens.
There are some interesting applications of the use of a bichromated gelatine and suitable dyes, in order to obtain a coloured print on paper.
Pinatype Process. — One of these, the Pinatype Process, depends upon the fact that certain dyes are not so easily washed out from bichromated gelatine when it is soft, as when it has been hardened by the action of light.
In this process three negatives are obtained, as is usual, by means of orthochromatic plates and the proper filters. Next, since the dyes used have a selective action upon the gelatine, only adhering to the unexposed parts, positives must be made from these negatives. Good lantern-slide plates will do for this purpose, but of course enlarged positives could be made if desired.
Under these positives, plates coated with gelatine which has been sensitized by means of a two per cent, solution of a bichromate salt are exposed. The plates so obtained are washed so as to free them from the bichromate, and then dyed as follows : —
The plate obtained by using the transparency made with negative produced with red filter is stained blue, that with green filter is dyed red, and that with blue filter, yellow.
When the plates are thoroughly stained, gelatinized paper is well soaked in water, and then squeegeed to one of the dyed plates and left ten or fifteen minutes. The blue-dyed plate is used first, the yellow one next, and lastly the red. If it is found that any one colour is too faint, it can be strengthened by again squeegeeing the print on the corresponding dyed plate, or if it is found to be too strong, then some of the dye may be removed by squeegeeing the paper down to a gelatine-coated plate.