The following is M. Niepce's last address to the French Academy of Sciences, on the abovenamed subject, which we have translated from the " Lumiere " : " In this new memoir I shall chiefly treat upon the optical phenomena that I have ob. served in trying to fix the colors m the cament. After having obtained, by contact, that IS to say by applying the plane of a colored engraving on a sensitive plate, and coveTing It with a glass to expose it alterwards to the light, all that it was possible to obtam H1 the present state of things, I have sought to arrive at the same results in the camera. The attempt was difficult and I made up my mind to encounter great difficulties, which I h.ave succeeded to a certain extent, in surmountmg. I have discovered the possibility of copying every colorall that is required for this purpose being a suitable preparation ot the plate. I began by copying, in the camera, colored en gravings, and afterwards artificial and natural flowers, after this inanimate nature,a figure which 'I clothed with garments of different colors, and always with gold and silver lace I obtained every color, and, what IS most extraordinary and singular, the gold and silver were depicted with their metallic lustre, .as were likewise glass, alaha8ter, and porcelain, with their peculiar brilliancy. I hR:e produced pictures of precious stones and stained WIU dows and these attempts have brought under my notice a curious circumstance which I think ib proper to mention here. A dark green glass placed before my objective, g.ave a yellow instead of a green picture, whilst a light green glass placed beside the dark green was copied exactly with its color. The great difficulty that has hitherto stopped me, has been that of obtainingseveral colors together ; it is however possible, for I have often done it. All li"ht colors are produced much quicker and better than dark colorsthat is to say, the nearer colors approach to white the more easily are they copied, and the nearer they approach to black the more difficult it is to copy them. This is to be expected, for their photogenic action is greater according as colors are more luminous. Bodies that reflect most white light are likewise those that are best copied, so that white light, far from .being in jurious to copying colors, .renders It, on the contrary, more easy, as will be seen Hav. ing observed that light and brilliant colors are copied much better than dull colors, provided, however, that the former are not exposed to the direct rays of the sun, because, in that case, they would reflect the light like. a looking-glass, and burn the picture in certain parts, I conceived the idea of operating m a room, the interior ot which should be as much illuminated as possihle ; for that purpose I em ployed at first a room papered white. The results have been at least equal to those that the camera gave, as far as regarded the copying ot colors, which it was important to prove. After this I covered the inside of a camera with tinned looking. glass, and again obtained the same results, such : camera is, however, contrary to all photogemc las.. I cannot, nevertheless, certify m a posi.ti.ve manner, that there is really an advantage to use, in preference, an apparatus of these two kinds, either for the force of the effect or for the rapidity, because the means at my dlspo. sal have not, as yet, permitted me to make comparative trials sufficiently conclusive. On account of the light colors belUg copIed more easily, and above all more rapidly than the dark colors, it is of great importance that the shades of tho object copied should have shades of a similar tone, if it is required to co- I py them all at the same time; unless thiS 1$ the case, the light shades would be obhtera. ted before the next were copied., Colors of different tones can, however, be fixed by tao king care to select light dead colors ar d dark colors that are bright or glassy, .whICh I have done successfuly. The most difficult color to obtain with all the others, IS the dark green of foliage, because green rays have little pho togenic action; and are almost as lUactlve aS black; light green, however, is very well co pied, particularly if it is shining, as 1U green paper glazed. T, obtain dark reens, the plate must be scarcely warmed before expo. sure t0 the light ; whilst, to obtain most oth,er colors. and particularly line whites, it is necessary, as I have said elsewhere, that the sen. sitive coating be brought by heat to a cherry red tint. This red tint has great disadvantagesthe dark parts and the shaJes remain almost red ; some times, however, it happens that the dark pa'ts are well expressed, parti. cularly when it is done by contact. I have endeavored by all the means at present in my power, to do away with this preparation by lise of temperature, but I !have not yet succeeded. The following experiments have di. rected me on a road which will conduct me, I hope, to a complete solution of the problem ot Heliocheome. If, when taken out of the bath, the plate is only dried, without raising the temperature so as to change its color, and then exposed to the light with a colored engraving before it, there is actually obtained, after a very short time of exposure, a copy of the engraving with all the colors. But the colors, most commonly, are not visible, some only appear when the exposure tQ the light has been long enough, name. ly the greens, reds, and sometimes the hlues ; the other c(.lors, and frequently all the colors, although for certain produced, remain latent. The tollowing is a proof of this: if a small hall of cOttOIl, impregnated with ammonia, 81Jd that has already been used for cleaning a plate, is taken, and the plate gently rubbed with it, the picture will gradually appear With all its colors. For this purpose it is necessa. ry to take off the surface coating of chloride of silver, to get at the lower coating below, namely, the one that adheres directly. to the silver plate, and on which the picture IS formed. It is clear from this that the only ques. tion is to find a substance that can develope the picture and that will, perhaps, at the same time fix the colors. The problem would then be completely solved. In the numerous ex. periments made for this purpose, I have observed, it the vapor ot mercury IS employed, the picture is very well developed, but it is of a uniform gray tint without any trace of eolor ' its appearance differs from that of the Daguerrea picture, although like the latter it is presented under two different aspects, that is to say, a positive picture, in one sense, and a negative in the other. If a weak solutIOn 01 gallic acid, with a few drops of ammonia is used, the picture is similarly made to appear, especially when the plate is heated and afterwards dried without washing. The picture which then appears, resembles, in some degree thab produced by mercury, and if to the gallic acid there are added a few drops of aceto-nitrate of silver, it becomes almost bJack. The time of exposure necessary to obtain the colors varies considerably, according to the manner of preparing the plate; I have already shortened it, for I have taken pictures in the snn with a German objective for a half plate, in less than a quarter of an | hour and with diffused light in less than an \ hour. The colors fade more quickly, accord- j ing as the plate is more sensitive, and hitherto I have only succeeded in fixing the colors I momentarily, the question of permanent fix- i ing has yet to be resolved; it is connected, ! perhaps, as I have pointed out above, With the ; discovery of a substance that will transfer the ' picture from its latent to its visible state. Notwithstanding what remains to be done, I believe that I have already obtained extraordi. nary results, which have surprised every one to whom I have showed pictures of the figure ' copied by me, in which the gold an.d silver lace is depicted with its metalhc bnlliancy, and in which the contour of the figure and all the colors of the clothes are copied with much clearness. My best pictures already realize, in part, the enthusiastic expectations of my uncle, who said to one of his friends, the Mar. quia de Jouffry, that one day he .would take his picture the same as seen lU a lookmg-glass. This vast advancement has unfortu. nately, not yet been attained, but we may hope to arrive at it some day or other, and although the difficulties to be overcome are still numerous and great, ! have placed, It ap- I pears to me, out of doubt the possibility of complete success." The stock of hemp in 'odtSt., wLouis is stated to be only one hundred and twenty .five bales, and held at SU5.
This article was originally published with the title "Helichrome or Sun Coloring" in Scientific American 8, 23, 179 (February 1853)