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PHOTOGRAPHY IN NATURAL COLOURS 251
purpose of the solution saved from the first development. When the high lights are sufficiently darkened, which should take about three or four minutes, the plate is washed for four minutes and immediately placed on one side to dry. It is not necessary to use a fixing solution unless the plate is intensified, which is sometimes done after the second development, when, owing to over-exposure, the image is too transparent, and the colours are lacking in brilliance.
It is found that varnishing the film not only protects it, but also adds greatly to the transparency. Special varnish is supplied for this purpose, which can be applied cold by flowing it over the plate in the usual manner.
There is one great drawback to the autochrome plate. Every coloured photograph must be obtained by a fresh exposure. This, of course, renders it absolutely impossible in many instances to get two exactly similar photographs of the same object. Then, again, the image of the object only appears in its natural colours when viewed by transmitted light, and for this purpose white light should be used as the means of illumination.
The Thames Colour Plate. — The Thames Colour Plate can also be used for obtaining a coloured photograph in one exposure. These plates have been on the market for just about two years, the first trade circular being issued in October 1908. This circular commenced with the following paragraph : — " The Thames Colour Plate is for use in any camera, to obtain without additional apparatus, at one operation, on one plate, a photograph of the scene or object in all its natural colours. It consists of a screen of the primary colours laid down in minute regular-shaped patches of pure transparent colour with mathematical accuracy ; and a sensitive surface."
The screen mentioned can be best understood by examining one with a pocket lens. It will then be seen that there are a very large number of circles, some of which are coloured red and the others coloured green,