The Dictionary Of Photography

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

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Photogravure
obtained very fine results^ and this method of producing photo-graphs in colour appears one of the most promising from the artistic point of view. An old method (Ducos du Hauron, 1868) has recently been revived, in which the original negative is taken under a screen covered with transparent lines in three colours, and a monochrome print from the negative is viewed in connection with a similar coloured screen. M. Louis Ducos du Hauron has recently devised a two-colour system of heliochromy, the novelty consisting of leaving out the yellow image and only superimposing the red and blue. The full colour effect does not show in bright white light, a yellowish light being best, although M. Ducos du Hauron contends that the yellowish illumination is not essential. Many interesting modifications of the three-colour process have been suggested and experimentally carried out during the past few years, among which one may be mentioned in which Professor R. W. Wood reproduces the hetrochrome in diffraction colours. Three diffraction gratings are so spaced that the deviation of the red of one corresponds to that of the green of the second and the blue of the third. A viewing lens can now be so placed in relation to these gratings, that to the eye all three colours overlap and produce the effect of white. Cutting out any one grating is now like cutting out the corresponding colour in the photochromoscope. A grating re-production of each element of the heliochromic triplet is printed on a glass plate covered with bichromated gelatine and developed in hot water, and the three grating prints being superimposed and suitably viewed the effect of a colour print becomes visible.
Photogravure. A general term equivalent to photo-engraving, but most usually applied to the intaglio methods. There are several methods of working, but the most popular, and the easiest for an amateur, is that known as Klic's. The apparatus necessary for this is, first, a dusting box, which should be at least three times the area on all sides of the largest plate to be coated. This is provided with a narrow door at one side. The box should be mounted on trunnions or side pivots, so as to swing freely. Raw Syrian asphalt, finely powdered, or a mixture of asphalt and resin, is also required. Finely polished copper plates and perchloride of iron ; this latter should be solid, and not the acid solution of the chemist. The perchloride is dissolved in water by adding one-
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