Few speculations are more replete with interest than that of the probability of our succeeding in the production of photographic images in their local colours. M. Biot, a great authority, says,— " Substances of the same tint may present, in the quantity, or the nature of the radiations which they reflect, as many diversities, or diversities of the same order, as substances of a different tint; inversely, they may be similar in their property of reflecting chemical radiations when they are dissimilar to the eye ; so that the difference of tint which they present to the eye may entirely disappear in the chemical picture. These are the difficulties inherent in the formation of photographic pictures, and they show, I think, evidently, the illusion of the experimenters who hope to reconcile, not only the intensity, but the tints of the chemical impressions produced by radiation, with the colours of the objects from which these rays emanate." It may be remembered that two years since, Sir John Herschel succeeded in procuring upon photographic paper a coloured image of the solar spectrum ; and that eminent inquirer has communicated to me a recent discovery of great interest, which I have his permission to publish. " I have got specimens of paper," says Sir John Herschel, " long kept, which give a considerably better representation of the spectrum in its natural colours than I had obtained at the date of my paper (February 1840), and that light on a dark ground ; but at present I am not prepared to say that this will prove an available process for coloured photographs, though it brings the hope nearer." Here we have the speculations of one philosopher representing the production of such pictures as hopeless, while the experiments of another prove these to be within the range of probabilities.
My own experiments have, in many instances, given me coloured pictures of the prismatic spectrum, dark upon a light ground, but the most beautiful I have yet obtained has been upon the daguerreotype iodidated tablets, on which the colours have, at the same time, had a peculiar softness and brilliancy. Daguerre himself has remarked, that when he has been copying any red brick or painted building, the photograph has assumed a tint of that character. I have often observed the same thing in each variety of photographic material, i.e., where a salt of silver has been used. In the Philosophical Magazine for April 1840, will be found a paper,—" Experiments and Observations on Light which has permeated Coloured Media,"—in which I describe some curious results on some of those photographs which are prepared with the hydriodic salts exposed to luminous influence with coloured fluids superimposed; permitting, as distinctly isolated as possible, the permeation of the violet and blue, the green, the yellow, and the red rays, under each of which a complementary colour was induced. During January of the present year, I prepared some papers with the bichromate of potash and a very weak solution of nitrate of silver : a piece of this paper was exposed behind four coloured glasses, which admitted the passage respectively of, 1st, the violet, indigo, and blue rays; 2d, the blue, the green, and a portion of the yellow rays ; 3d, the green, yellow, and orange rays; and, 4th, the orange and red rays. The weather being extremely foggy, the arrangement was unattended for two days, being allowed to lie upon a table opposite a window having a southern aspect. On examining it, it had, under the respective colours, become tinted of a blue, a green, and a red : beneath the yellow glass the change was uncertain, from the peculiar colour of the paper, and this without a single gleam of sunshine. My numerous engage-ments have prevented my repeating the observations I desire on this salt, which has hitherto been considered absolutely insensible to light.
The barytic salts have nearly all of them a peculiar calorific effect; the muriate, in particular, gives rise to some most rich and beautiful crimsons, particularly under the influence of light which has permeated the more delicate green leaves ; and also in copying the more highly-coloured flowers, a variety of tintings having been observed. We may always depend on producing a photographic copy of a leaf of a green colour by the following arrangement: — Having silvered a copper plate, place it in a shallow vessel, and lay thereon the leaf of which a copy is desired, maintaining it in its position by means of a piece of glass ; pour upon it, so that the plate beneath the glass may be covered, a solution of the hydriodate of potash, containing a little free iodine : then expose the whole to sunshine. In about half an hour, one of the most beautiful photographic designs which can be conceived is produced, of a fine green yellow. The fluid is yellow, and cuts off nearly all the "chemical" rays, allowing only of the free passage of the less refrangible rays; the most abundant being the yellow. This retards the process of solariza-tion, but it produces its complementary colour on the plate.
These facts will, I think, prove that the possibility of our being enabled to produce coloured photographs is decided, and that the probability of it is brought infinitely nearer, particularly by Sir John Herschel's very important discovery, than it was supposed to be.
M. Edmond Becquerel has recently succeeded in obtaining bright impressions of the spectrum in colours, and copying highly coloured drawings on metallic plates prepared with chlorine. The results of M. Niepce de St.-Victor have been of a satisfactory character : the main particulars thereof we select from a memoir entitled " Upon the Relation existing between the Colours of certain coloured Flames with the Heliographic Images coloured by Light".
When a plate of silver is plunged into a solution of sulphate of copper and chloride of sodium, at the same time that it is rendered electro-positive by means of the voltaic battery, the chloride formed becomes susceptible of colouration, when, having been withdrawn from the bath, it receives the influence of light.
M. Niepce de St.-Victor, from observing that when chloride of sodium (common salt) was employed, the plate became more susceptible of receiving a yellow colour than any other, and knowing that it imparted a yellow colour to flame, was led to believe that a relation existed between the colour communicated by a body to flame, and the colour developed upon a plate of silver, which should have been chloridated with the particular body.
To avoid complexity, it may be briefly stated that the bath in which the sensitive surface is obtained is prepared with water, holding free chlorine in solution, to which has been added the salt which is essential to give a predominance to any particular colour.
It is well known that strontian gives a purple colour to flames in general, and to that of alcohol in particular. If we prepare a plate of silver and pass it into water saturated with chlorine to which is added some chloride of strontian, and when thus prepared we place upon it a coloured design of red and other colours, and then expose it to the sunshine, after six or seven minutes we shall perceive that the colours of the image are reproduced upon the plate, but the reds much more decidedly than the others. When we would produce successfully the other rays of the solar spectrum, we operate in the same manner as we have indicated for the red ray—employing for the orange the chloride of calcium, or that of uranium for the yellow, or the hypochlorite of soda, or the chlorides of sodium or potassium. Very fine yellows have been obtained with a bath composed of water slightly acidulated with muriatic acid and a salt of copper.
The green rays are obtained with boracic acid or the chloride of nickel; also with all the salts of copper: The blue and indigo rays are obtained with the double chloride of copper and ammonia. The violet rays are obtained with the chloride of strontian and sulphate of copper. Those substances which give white flames, as the chloride of antimony, the chloride of lead, and the chloride of zinc, yield no colour by luminous action. All the colours of a picture have been produced by preparing a bath composed of the deuto-chloride of copper; and M. Niepce states that this salt thrown into burning alcohol produces a variegated flame according to the intensity of the fire, and it is nearly the same with all the salts of copper mixed with chlorine. Niepce says— " If we put a salt of copper in liquid chlorine we obtain a very sensitive surface by a single immersion in the bath; but the colorific result of this mixture is seldom good. I prefer a mixture of equal parts of chloride of copper and of chloride of iron, with three or four parts of water; the chloride of iron has, as those of copper, the property of being impressed on the plates of silver and of producing many colours, but they are infinitely more feeble, and the yellow always predominates; and this agrees with the yellow colour produced in flame by this salt".
It should be understood that when the plate of silver, being previously connected with a voltaic battery, is plunged into the bath and the circuit completed, it becomes covered with a dark coating, probably of a sub-chloride of silver mixed with the salt, on which the colour to be produced by solar radiation depends.
If we form a bath composed of all the substances which separately give a dominant colour, we obtain very lively colours; but the great difficulty is the mixing of the salts in such proportions as to give an equality to the tints, as it commonly happens that some colours are excluded by others. We cannot always depend upon obtaining the same results with the same materials, owing principally to the difficulty of preserving the solution at a uniform strength. Liquid chlorine is necessary—the application of dry chlorine will not produce the same results, and the volatile chlorine is continually escaping from the water.
Niepce de St.-Victor has made many experiments to produce the colours upon salts of silver and copper spread on paper, but without success; the metallic plate appears absolutely necessary, and the purer the silver the more perfect and intense is the impression. The following is recommended as the most effectual mode of manipulating. The silver plate is highly polished with the best tripoli powder and ammonia; being perfectly cleaned it is connected with the battery, and then plunged into the bath prepared in any of the ways stated. It is allowed to remain in the bath for some minutes, taken from it, washed in a large quantity of water, and dried over a spirit-lamp. The surface thus produced is of a dull neutral tint, often almost black ; the sensibility of the plate appears to be increased by the action of heat, and when brought by the spirit-lamp to the cerise red colour it is in its most sensitive state. The sensibility, how-ever, of the plates is low, two or three hours being required to produce a decided effect in the camera obscura. The name of Heliochromes has been given to these naturally coloured photographs, some of which, the personal gift of the inventor to Mr. Malone, I have inspected. These, when I first saw them, were perfectly coloured in correspondence with the drawings of which they were copies ; but the colours soon faded, and it does not appear as yet that any successful mode of fixing the colours has been discovered.