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PHOTOGRAPHY IN NATURAL COLOURS 239
following method succeeds well : — A glass plate is heavily silvered and is then flowed with a thick solution of celluloid in amyl acetate. When this varnish is dry the plate is placed under water ; this slowly works under the coating of celluloid, lifting it from the glass and bringing with it the silver. This flexible silver mirror is immediately laid, silver surface down, on a wet Lippmann plate and allowed to dry there. When dry the gelatine film will have the silver surface in optical contact with it. The plate may then be exposed at any time, and only an ordinary dark slide will be required. After exposure the celluloid film is stripped from the gelatine, taking with it most of the silver ; the plate is then developed, and after a thorough washing, any silver which remains can be removed by a mop of wet cotton wool. This works well for all types of colours, but has a disadvantage that sometimes proves troublesome, viz., some of the best sensitizers are apt to lose their power during the slow drying. Erythrosine is found to act perfectly, while pinacyanol and pinaverdol are likely to fail.
The Three-colour Process. — Another process by which many attempts have been made to obtain photographs in natural colours is that known as the Three-colour Process.
In 1867 we find that C. Cros patented a process of threecolour heliochromy, in which three negatives were taken — one with red, another with yellow, and the third with blue light. The positives obtained from these negatives were to be thrown into register so as to form one picture by means of suitable apparatus, the positives having previously been dyed with proper care as to correct tint.
In the next }Tear L. Ducos du Hauron brought forward ■a very similar process, and also one which in principle was very like that now known as the Joly process.
Ducos du Hauron, in his patent of 1868, outlined a process of preparing a three-colour screen consisting of bands of the three primary colours in juxtaposition which was