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DECEMTBER 3, 1880.J THE PHOTOGRAPHIC NEWS. 577 Ue ppotograpic YYets, 3, 1880. polou the merits of Albert’s method. He has, in fact, by a combination of Ducos' method with lichtdruck, been the Asisknown, Herschel was the first who prepared pictures with iron salts, as the cyanotype, the chrysotype, &c. These methods of his were at first ignored. Cyanotype is now again groups. In many of the pictures (which were landscapes) blue predominated, in others red, and in others yellow, and many had a brownish colour—in short, they were clearly not the natural colour. We will not, however, criticise too closely first to make the former process capable of being practically used—even though results are not perfection. AUgeyer, the editor of a new book on lichtdruck, and manager for several years in Albert’s printing establishment, says, in his book :—“ Granted that the right kind of print is pro duced with all three negatives, then the fixing of the three pigments to be used opens a wide field for investigation. But even if all these factors are securely established, the changeableness of the surface for printing, which is depen dent on the nature of the lichtdruck process, will always remain an intrinsic hindrance to the production of a pic ture agreeing well with the original, in a process in which the least change of any of the shades of colour is similar, but Eder has proved that ferric citrate has only a slight sensitiveness to light (ammonium ferric citrate ie only about one-sixth as sensitive as ferric oxalate). These results are only shown well in solutions. If the salts are contained in paper, the differences in sensitiveness, according to Dr. Eder, are not so great. However, the mixture of ferric chloride and oxalic acid is the most sensitive in thiscase also. re. We have seen a collection of 1,000 such pictures Albert’s atelier in Munich. These were so different, that it was possible to divide them into seven different while the gelatine plates were exposed for diffe rent lengths of time, with the result that from two to eight seconds were necessary to obtain a picture as well exposed as that produced by a wet plate in twenty-four seconds, thus proving that under these conditions the gelatine plates were about three times as sensitive as the wet plates. We confess that in bright weather, if the whole atelier is open, the circumstances are more favourable for the gela tine plates as regards sensitiveness. In the above experi ments, more than half the atelier was completely veiled with curtains, this being necessary to obtain an artistic picture. According to Dr. Vogel, another circumstance influences the sensitiveness, and that is the colour of the light. The wet plates arc most sensitive for the indigo rays of the spectrum, the gelatine plates more so for the less refrangible blue rays. Now the quantity of blue and indigo rays in daylight is variable, and therefore, if it contains more of the blue rays and less of the indigo, the gelatine dry plate will be more sensitive in comparison with a wet plate than in the contrary case. Pbotor/raphy in Natural Colours.—Some years ago, the news of the invention of photography in natural colours was first published in the newspapers. The fact is, how ever, that various investigators occupy themselves now, as before, with this problem, and pictures, said to be in natural colours, have been published by Albert, in Munich. Albert uses the process of Ducos du Hauron. He takes three negatives of the same object with plates made sensitive for different colours. Dr. Vogel’s re searches show that it is possible to sensitize bromide of silver for the red rays by mixing it with any dye which absorbs red rays; in the same way it can be sensitized for yellow and green rays by the use of suitable dyes. If ,a picture is taken with a plate which is sensitive for all colours except red, a negative is obtained which, printed off in red, gives the red tints of the original. In like manner, a plate sensitive for all colours except blue would give a printing plate for the blue tints, and a plate sensitive for all colours except yellow, a printing plate for the yellow tints. The three prints combined should give one in its natural colours. At first sight it seems absurd to speak of a plate sensitive for all colours except blue, for even the bromide of silver mixed with dyes preserves its sensitiveness for blue. Ducos du Hauron must, on this account, shut out the blue rays in the cases in which the so Very apparent. The production of a really good copy by the lichtdruck process in one colour being difficult, it must be much more difficult with three printing plates in three colours.” The proper time of exposure of the original negative must also be brought into consideration. We have no means of hitting the latter ab solutely accurately. It is only seen on the finished print, e.g., whether the negative for blue was exposed long enough. Finally, there is a drawback in the method of colour printing. Chromo-lithographers are by no means satisfied with three or four lithographic plates to produce all shades of colour. They often take ten or twelve stones for quite simple pictures ; they do not print the colours immediately in full strength, but print one faint tint over another— alternate thick and thin colours—and give the whole a final coating, for harmony. Sometimes as many as twenty or thirty lithographic stones are used for a picture, and in the renowned establishment of Prang, in Boston, occasion ally forty printing plates are used for one picture. Only by this means, delicate harmony of shades and soft transitions are possible, and this can never be attained by using three printing plates, according to Albert’s method. Such a method can only give crude, inartistic results. New liesearches of Dr. Eder.—Dr. Eder, the unwearying and ingenious investigator in photo-chemistry, seems deter mined to examine all branches of this subject. We are indebted to him for much work on the chemistry of chromates, and equally important work on gun-cotton, and the principles of photography with gelatine plates ; now a no less interesting work on the photo-chemistry of iron salts, published in these columns, has emanated from his pen. The chemical composition of the salts in question, and the ex haustive study of their photo-chemical properties, are equally to be admired. Compared with this rapid progress in photo chemistry, it seems as if photographic optics and physics were somewhat neglected now-a-days. With regard to the photo-chemistry of iron salts, the investigation of Dr. Eder, proving that the compound most sensitive to light is a mixture of ferric-chloride and oxalic acid, is interesting. After that comes ferric oxalate in sensitiveness, and then ammonium ferric oxalate. Ferric tartrate is somewhat PHOTOGRAPHY IN AND OUT OF THE STUDIO. Collodion versus GELATINE — PHOTOGEAPHY in Natural Colours—New RESEAROHES of Dr. Eder. Collodion versus Gelatine.—The question opened by us as to the relative sensitiveness of wet collodion and gelatine dry plates, has been taken up by many, and we see that we do not stand alone in our opinion. We will here shortly describe our experiments, so that the reader may understand how we arrived at our opinions. One of Dallmeyer's stereoscopic cameras, with two lenses abso lutely equal in respect to light, was directed at a sitter in black clothes, while a wet collodion plate, and the gela tine dry plate to be tested, were put side by side in a slide arranged for this purpose. The light was so regu lated that artistic pictures were produced. The wet plates required twenty-four seconds to give fully ex exposed negatives with the necessary details in the shadows; blue light has no action ; that can be done by means of orange-coloured glass, which is put before the lens. The choice of the proper printing colour gives more difficulties. It is easy to say that the colour is red, but the question is, which—crimson, alizarin, or Vandyke red? There is also the question as to how clear or how dark the red must be I printed? Of course, all this depends on the pleasure of the I o , printer, and, therefore, itis not really photography innatural I brought into favour. Engineers and designers use it for the