THE COLOR SALT If we trace back the origin of almost any great discovery or invention, we find that it was heralded by a whole series of "dreamers of dreams." With the dream came the irre sistible impulse to set to work and make it a reality. Each would-be discoverer is like to a man groping across a trackless desert, guided only by the bones of those who have fallen by the way. His dream may prove but a will o'-the-wisp, a " Belle dame sans merci," leading only to failure and disappointment. Or he may eventually stumble on to a great success, and then the world will call his vision " foresight and enterprise." The story of photography itself begins with its generations of dreamers, who convinced themselves that there must be some means of recording permanently the pictures shown in the camera obscura. Thirty years after Thomas Wedg wood had commenced his vain experiments, Daguerre was threatened with imprisonment in an asylum for the ridiculous conceit ; then came the lucky accident of an upset mercury bottle, and the escaping vapours developed the first Daguerro type. It would be rash to prophesy that any such happy chance is destined to reward the seekers after the colour salt, or method of sensitising plates that will chemically reproduce the colours of nature. It is irrational to regard the goal as being hopeless of attainment, as was the Philosopher's Stone or the Elixir of Life, elusive though it has hitherto shown itself. The arguments in its favour are, at least, plausible, and the facts already known point to something more than mere coincidences.
Analogies in animals and insects have the property of assuming, very rapidly, the colours reflected by their immediate surroundings. The case of the chameleon is well known ; also the caterpillars of sensitive skin, that are bright green when feeding on cabbage leaves, but become bright yellow when transferred to a sunflower in bloom, or dull brown on a dead leaf. All the fauna of the Arctic regions take on a pure white coat during the months when the ground is covered with snow. The flatfish, white in colour below, has an upper surface exactly imitating the hues of the sea bottom which he frequents. Some of these phenomena will admit of ordinary physical explanations ; others seem to point to the existence of a colour-sensitive secretion.
Allotropic Forms of a century ago Seebeck discovered that chloride of silver, when exposed to the action of light, partook of slight variations of colour, accord ing to the particular rays of light to which it was exposed. Thus violet rays produced a brown substance, and blue rays a species of blue, while, during its transformation under white light, it frequently passed through the stages of yellow and red. Later on, after the advent of the photographic process, M. Bequerel succeeded in securing definite but fleeting bands of colour on a plate of silver, prepared by immersion in dilute hydrochloric acid in connection with an electric current.
Xiepce de St. 1851 and 1867 this famous experimenter—a nephew of the great Niepce, the colleague of Daguerre—devoted considerable attention to the phenomena of colour photography, and finally came to the conclusion that chloride of silver was actually the colour salt. At the Paris Exhibition he showed numerous specimens of photo graphs in colour. According to eye-witnesses the colours were exact and life-like. Flowers, stained-glass windows, and brightly dressed dolls were portrayed in a manner to satisfy the most exacting critics. Unfortunately the colours disappeared as soon as any ordinary fixing agent was applied ; and even as shown in a dark box, somewhat similar to the old-fashioned stereoscope, they faded away within a week. Niepce prepared them by treating silver plates with a solution of ferrous and cupric chlorides, afterwards coating them with dextrine containing lead chloride. It is not possible for us, from the data given, to decide how far the colour was produced by chemical means, or whether these plates were a foretaste of the interference process, since perfected by Prof. Lippmann. A quasi-colour process was afterwards devised by Niepce & Kopp, developing the colours one by one with the aid of various chemicals ; the finished portion being protected by varnish, while the remaining colours were subjected to fresh treatment in order to develop them.