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PHOTOGRAPHY IN NATURAL COLOURS 247
grating appear, and so on ; where yellows, spacings of both green and red, and where white, all three spacings are superimposed.
One great advantage of this method is that it is possible by a single exposure to obtain a new positive when one positive has been made.
The apparatus for viewing the pictures consists of a cheap double convex lens mounted on a simple frame, at the other end of which is a small screen which is so perforated as to bring the eye into the correct position for obtaining the desired colour effect. Wood found that when a lens of proper focal length was used, it was possible to view the picture by both eyes, since similar overlapping spectra are then formed on each side of the central image.
Before leaving this three-colour method of obtaining photographs in their natural colours, we will briefly mention a process brought out by Lumikre in which threecolour niters — viz., blue-violet, green, and orange — are required. Three negatives are obtained using the three different screens, and from these negatives three prints are made on special paper which has been coated with bichromated gelatine and glue. These prints are developed in the ordinary manner with water, and when dry are dyed.
The print obtained from the green negative is dyed red, that from the blueviolet is dyed yellow, and the orange print is dyed blue. The dyes are so prepared that the prints should remain in the dyes about twelve hours, the excess of colour being afterwards removed by washing before drying the prints. It is possible so to correct any colours, since if not sufficiently transparent, washing may be continued until the proper stage is reached, while any deficiency in colour can be rectified by a longer immersion in the dye. These three prints are then stripped from their paper supports and mounted in superposition on a temporary paper support, great care being taken to ensure