The Dictionary Of Photography

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

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Isochromatic Photography
Colour, therefore, is due to the absorption or extinction of certain of the coloured rays of white light within the object, and the remaining rays are reflected to the eye, imparting to that object its characteristic colour. It must, however, be borne in mind, that all objects, irrespective of colour, reflect white light when illuminated by white light. An engraver, when translating into monochrome any coloured objects, gives not only a correct form, but also a correct idea of colour, by giving varying depths of deposit of the pigment used, so as to give to the eye, were a gamut of colours engraved, a steadily increasing depth of tint from absolute white to deepest black, so that each tint or colour receives its quantum of deposit that will accord to some extent at least with the colours as they affect the human eye. In the diagram, fig. 67, are shown what are called the primary colours of the spectrum, traversed by numerous dark lines, which are called Fraunhofer's lines, after their discoverer. To give an idea of the relative luminosity of colours to the human eye, the following diagram has been prepared, from which it will be seen that the greatest luminosity is between D and E, fig. 68, or in the yellow, shading off rapidly through orange on one side to the red, and through yellowish-green and green to the violet on the other. In fig. 69 is given the curve showing the luminosity of colours to the ordinary photographic dry plate. Thus it will be seen that the most luminous part to the dry plate or photographic retina is between F and H, and practically no luminosity between C and F, where the greatest visual luminosity resides. It is obvious, therefore, that to reproduce colours in correct gradation as seen by the human eye, we must in some way exalt the sensitiveness of the ordinary plate to green, yellow, orange, and orange red, and at the same time reduce the sensitiveness to blue. It has been found that the particles of silver haloid are most sensitive to those colours which they absorb, and numerous experiments have been undertaken to find a substance which would enable the silver salt to absorb the whole of the rays in same ratio as we see them ; but this has been found so far to be impossible, and although means have been discovered to render the silver salts more sensitive to the less refrahgible rays between C and F, which are most luminous to the eye, yet they still remain most sensitive to the blue rays about G. These colours are toned down or robbed of some of their actinic value by being
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