FIXING DAGUERREOTYPES. 109 mical change has been effected, represented by the high black polished face of the silver. (169.) This picture can, of course, be immediately destroyed by a touch; it is, therefore, necessary to protect it at once, by a glass or some other means. Berard, Fizeau, and others, have recommended the use of the chloride of gold, for the purpose of giving adhesion to the mercurial amalgam. It is thus applied, according to the method recommended by M. Fizeau, and which appears to have been the most successful of any of the plans which have been adopted. A double salt of hyposulphite of soda and gold is formed, by mixing very dilute solutions of the respective salts. After the iodine has been removed, as before directed, the plate is placed in an iron frame, and a sufficient quantity of the solution of salt of gold to cover the plate, poured over it. It is then heated by a spirit-lamp for a minute, or until the impression_acquires the greatest strength. The liquid is then poured off, the plate well washed and dried. Gold is deposited over the plate, and thus the picture rendered less liable to injury. Dr. Berres of Vienna, states that considerable success has attended his plan, which consists in exposing the plate for a few minutes to nitric acid vapour, and then placing it in nitric acid, at 611° Fahrenheit, in which a considerable quantity of silver and copper is dissolved. Shortly after being placed therein, a precipitate of metal is formed over the photograph. It is then removed, and the plate polished with chalk or magnesia, until the picture can be clearly seen. (170.) The power of multiplying these very beautiful productions is much to be desired. Several plans have been proposed and used by the inventors, with certain degrees of success, particularly Professor Grove's method of etching by electrical agency. All of them, however, are of great uncertainty; and even with the most careful manipulation, a successful result cannot be depended on. It does not form a part of the plan of the present work to embrace these matters, though they are of the highest interest to every one who is concerned in the success of photographic publication. It must, however, be acknowledged, that the only successful method of multiplying original photographic designs, appears to be the use of processes on paper, or some more transparent material which, giving negative pictures in the first instance, will afford positive copies. (171.) As long back as 1841, it was announced that M. Daguerre had discovered a process by which an instantaneous effect was produced; and in a commuication with which the author was favoured from Daguerre himself, he states, "By means of that new process it shall be possible to fix the images of objects in motion, such as public ceremonies, market-places covered with people, cattle," &c. But some great obstacle appears to have interfered with the successful practical use of this new and important discovery. Daguerre's attempts to improve the process which bears his name, were all of the most complex nature; and his processes singularly unscientific. (172.) The expense of metallic plates and their inconvenience, particularly to travellers, render it very desirable that some material, such as paper, might be employed instead of them. Some very tolerable effects have been produced upon silvered paper; but the pictures thus formed, want the fine black surface, which is to contrast with the mercurial vapour, and which forms the chief charm of a good Daguerreotype. In the Philosophical Transactions, Part II. for 1840, is a paper, by the author, "On the Influence of Iodine in rendering several Argentine Compounds spread on Paper, sensitive to Light." As many of the results are of considerable importance, particularly as they bear upon some of the opinions which we shall have to examine, I shall briefly mention such facts as appear necessary to the complete elucidation of this very interesting subject. (173.) Any of the ordinary photographic papers will DAGUERREOTYPE ON PAPER. 111 darken, by exposure, to a brown or dark olive colour. Exposed to the vapour of iodine, the paper becomes of a steel blue or violet colour. If subjected to solar influence in this state, mercurial vapour attacks all the parts on which the Light has acted, in the same manner as it does the iodized metallic plate, giving a tolerable picture. I also found, that perfectly pure oxide of silver, spread on paper and iodized, was similarly disposed to receive the mercurial vapour, after it had been submitted to the sun's rays. The yellow-brown phosphate of silver was found also to acquire additional sensitiveness under the influence of iodine, and to yield a tolerable picture when exposed to the mercurial fumes. (174.) Papers which were prepared by first saturating them with strong solutions of the nitrate of silver, and then exposure to phosphuretted hydrogen gas, until there was a complete revival of the silver over the surface of the paper, were found to be acted upon by iodine, in a similar way to the silver plates themselves, and for most puposes are capable of being substituted for them. The pictures, when the papers are well prepared, are formed as readily as upon the iodized plates, and are not at all wanting in the beauty of their general effect, or in the delicacy of their minute detail. It unfortunately happens that a considerable degree of risk attends the preparation of the paper by this spontaneously inflammable gas. be pre (175.) Papers prepared in a similar way, substituting the sulphuretted for the phosphuretted hydrogen, are in nearly all respects equal to them. Some difficulties attend the preparation; but by observing the following directions, papers of a very uniform dark gray surface may pared: The paper is first soaked in a solution of the hydrochloride of ammonia, carefully wiped with cotton cloths, and then dried. It is next dipped in a solution of nitrate of silver, dried in the dark, and then carried into a vessel in which sulphuretted hydrogen is slowly forming. When it has darkened to an iron brown, the paper must be passed through water slightly impregnated with chlorine or muriatic acid, and again dried. It is once more dipped into an argentine solution; and when dry, subjected a second time to sulphuration. These papers are best iodized by drawing them slowly over a saturated solution of any salt of iodine in which is dissolved a considerable quantity of iodine; care must be taken that one side only of the paper is wetted. is wetted. It is then dried near the fire, and subjected in the camera to the solar agency. After mercurialisation, the picture is fixed most effectually by a strong solution of common salt used moderately warm. (176.) If when these drawings are finished, they are placed in a solution of corrosive sublimate, the images entirely disappear; but after a few minutes they are seen, as if by magic, unfolding themselves, and gradually becoming far more beautiful than before; delicate lines, at first invisible or barely seen, are now distinctly marked, and a rare and singular perfection of detail is given to the photograph. A singular obliteration is described before (94.); but in this case the results are very different, the picture being again restored by the agent which caused it to disappear. It would appear that the mercury on the paper is slowly converted into a protochloride; the modus operandi is not, however, quite evident. (177.) From the great interest which attaches to the discovery of Daguerre, we must now endeavour to examine some of the most remarkable points in the phenomena which are afforded in pursuing his photographic practice. If we expose a prepared plate, for a considerable time, to the action of the chemical radiations in the camera, it is darkness and a negative picture results. The discovery of Daguerre is that, before the negative image is formed so as to be visible, the iodide of silver has undergone a change, which has given it the property of condensing over well defined spaces the vapours of mercury. It has been shown by M. E. Becquérel, that if the plate be removed from the camera, before it has been exposed suf BECQUÉREL'S RESEARCHES. 113 ficiently long to afford a visible picture, or at least any. thing beyond the faintest outline of an image, the process may be continued by exposing the plate to sunshine under a red glass. E. Becquérel assumes, from these results, that there exists two orders of rays, which have not been previously described, and which he calls Rayons excitateurs, and (178.) It will be found, when we come to an examination of the subject in all its details, that the process, supposed to be carried on by the rayons continuator, considering them as a new class, is probably due to the calorific rays, and that very nearly the same effect is produced by the application of artificial heat to the under surface of the plate. But it is not entirely to the calorific rays that the effect is due. We know, that the least refrangible rays of the prismatic spectrum are not without some chemical power, and that this is a power quite independent of their heating effect; and it will be found, with nearly all the sensitive preparations, that the action, under any circumstances of sunshine, is carried on much more rapidly upon those parts over which a chemical disturbance has been already begun, than over the other parts. Moser, indeed, admits that some blue and violet rays penetrated the red glasses he used in his experiments, therefore the only effect which, as I conceive it, the red medium produced, was to retard the change, over the undisturbed parts of the plate, whilst a sufficient quantity of the most refrangible, and extra spectral chemical rays, had passed the glass, to continue the change which had been already begun. Yellow glasses were found by Moser to produce, first negative and then positive images; but "these positive images have always a blackish covering." Now this is as easily explained as the action of the red glass. The change is carried on "by the large quantity of white light" which these yellow glasses allowed to pass, with much greater rapidity over the parts on which the chemical rays I |