As a rule, the photographer should remember that blues and violets come out lighter, and yellows and reds darker, in a direct, or printed positive, than they ought to be.
It seems probable that different coloured artificial lights, or sun shine transmitted through coloured media, might be successfully employed in illuminating coloured objects to be copied by photo graphy. A variety of highly interesting experiments might be made in this direction, with the view of removing, if possible, from photography the reproach under which it now somewhat justly labours. As yet very little has been done in the way of careful scientific experiments in the photography of coloured objects, and our information is consequently scanty on this head. The difficulty found in copying old oil paintings arises in a great measure from their exceedingly dingy and low tone of colour, which will be perceived by holding a white card against the highest lights ; and also from the varnished and uneven surface, texture of the canvas, and other defects.
Yellow paper, or calico, or glass, u.sed for cutting off actinic light, should have a tinge of orange, rather than green, as the red are much less actinic than the blue rays.
CoLounnco Merrzus. Organic coloming matters are generally destroyed by the action of light and oxygen, or chlorine. The affmity of the carbon and hydrogen which they contain for oxygen and chlorine is increased by light, and the oxygen which they take up, and which may escape as carbonic acid, alters their composition, and destroys their colour. They have been examined photogra phically by Sir John Herschel : he found the juice of the red poppy and the double purple groundsel to be especially sensitive to the bleaching power of light, and that th.e fading of flowers was due to
the same cause, which increased in force as the vitality of the plant was diminished. The juices formed in spring, in the more active stage of the plant's growth, are also more easily destroyed when they are expressed, than those which are elaborated at a later season. The colouring matters of leaves go by the names of chlorophyl, xanthophyl, and erythophyl, or green leaf, yellow leaf, and red leaf. A solution of the first in ether passes, by the action of light, into the yellow colour of autumn, that is, into the second -variety : the red matter of leaves has already been oxidized, and is much more stable in the aix, but long exposure changes it to brown.
These fugitive animal and vegetable dyes have slightly acid pro perties, like gum and sugar : hence they may be precipitated with the oxides of iron, lead, and aluminium, and, in this state, are much more permanent ; and these oxides having an affinity for cotton and other fibres, fabrics may be dyed with these compounds, though they have no attraction for the colour itself, and many of these compounds are purer in colour when formed in the light than in the dark. Many photographic processes are analogous to these.
These weak organic acids are, like pyrogallic acid, more easily oxidized when mixed with an alkali, and, therefore, fabrics intended to be bleached are rendered alkaline before they are exposed, by being bowked, or boiled, with a weak solution of caustic potassa ; and like pyrogallic acid, though in a less degree, they have the power of decomposing solutions of nitrate of silver, and carrying the oxide down.