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248 THE ADVANCE OF PHOTOGRAPHY
that the three are in perfect register. Adhesion is secured in this process by a gelatine mountant, and each film must be allowed to dry in its new position on the temporary mount before trying to mount another.
When all the three films are mounted in their proper positions, they are stripped as one from the paper and transferred to a permanent glass support.
All these methods, however, have been more or less overshadowed by the success which has been attained in the last few years with colour photography in which only one exposure is necessary and in which only one plate is used.
Autochrome Process. — In the front rank among these is the Autochrome process of Lumiere. The manufacture of this plate has been so improved, and the method of developing so simplified, that it is now possible with just ordinary precautions and practice, to obtain most satisfactory results of very difficult subjects.
The plate consists essentially of a glass support on which a layer of very minute coloured starch grains is coated. These starch grains are only about ^Vo to 20W inch m diameter, and before they are used for coating the plates are dyed, some violet, some green, and some an orange red. After dyeing, they are mixed together until the mixture assumes a neutral tint. They are then applied to the plate so as to form a very thin but complete coating. When looked at through a microscope such a plate would seem to be covered with a transparent screen composed of innumerable coloured dots. If the plate is held up to the light and viewed as a transparency, the starch grains give it a slightly pink tint.
When the plate has been so coated, a layer of waterproof varnish is next placed on it, and lastly a highly sensitive panchromatic film.
As it is desired that the light should pass through the starch grain screen before impinging on the sensitive film,