The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Photography in Natural Colours
kromogram. For the production of prints a slightly different procedure is required. Three negatives are'obtained in some-what similar fashion, but in this case we use red, yellow, and blue as the three printing colours. As in all photomechanical printing processes, like collotype, which we will take as an example, it is the shadows which print. If a collotype plate is exposed under a negative, it will be found that, after wash-ing, only the shadows or those portions of the film acted upon by light will take the ink, the high lights where the film was protected refusing to take the greasy ink. The result of this is that we use that negative for making the printing plate, in which the particular colour corresponding to the ink has not acted: for instance, the collotype plate made from the red negative, or the negative taken through the red screen, is inked with blue ink; the plate from the negative taken with the green screen is inked with red ink; and the plate from the negative taken with the blue-violet screen is inked with yellow. The inks used vary slightly according to the recommendations of authorities, but cadmium or chromium yellow, carmine or madder lake, and prussian or ultramarine blue may be taken. The yellow is printed first, then the red, and. finally the blue. In the production of prints by the aid of process blocks, one line screens only are used for each block, the said lines crossing each other at angles of 300. Whilst from the difficulties of obtaining perfect printing inks corresponding exactly to the spectroscopic absorption curves required, it is at present impos-sible to obtain an absolutely faithful reproduction of every tint. There is no difficulty in making lantern slides of still more faithful accordance with the original, nor is the work beyond the ordinary scope of an amateur. There are many interesting developments of the three-colour method, among which may be mentioned the following: - Cros, Lumiere Brothers, and others have suggested the bleaching out of the pigments or colours of a compound film, each colour light tending especially to bleach out tints complementary to its own, and on this Wiener has based a theory that coloured radiations naturally tend to produce their own colours, and he has applied this doctrine to an explanation of the protective colour adaptation of animals. By successively printing in the three colours on the same sheet by the gum aquatint process (see Gum-bichromate), Watzek has
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