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THE PHOTOGRAPHIC NEWS. Vol. XXVII. No. 1271.—January 12, 1883. CONTENTS. PAGE Coating Plates with Gelatine Emulsion Investigations of Cellulose, and on an Oxidation Product Derived from it A New Organic Substance Sensitive to Light BY-the-Bye.—Judging Pictures at Exhibitions On the Rowland Diffraction Grating. By Captain W. de w. Abney, R.E., Notes Patent Intelligence French Correspondence. Dy Leon Vidal.......... 17 18 19 20 21 24 25 2G 29 29 30 30 31 32 PAGE 26 27 28 Review «... Notes on Photography. By E. Howard Farmer Photographic Exhibition in Brussels Stiffening or Animalising Vegetable Tissues by Chromatised Gelatine. By J. Wolff Improvements in Letter-Copying Correspondence Proceedings of Societies Talk in the Studio,.;,.,.....,,. To Correspondents COATING PLATES WITH GELATINE EMULSION. First Article. WHILST the number of amateur photographers is rapidly on the increase, it is probable that the number of those who manufacture their own plates becomes steadily less. It is certain that even if the aggregate number does not decrease, the percentage does. The same probably holds good of professional photographers. What is the reason of this? Are photographers less of enthusiasts than they were of yore? Not so, we believe. Is it that any enor mous difficulty is to be found in the manufacture of emul sions ? No, surely not; for even if some do find difficulty in producing emulsions of the most extreme sensitiveness with uniformity, there is not the veriest dabbler but can with a reasonable amount of care, and without excessive expenditure of time, produce emulsions of excellent quality and of a sensitiveness sufficient for nine out of ten of the purposes to which dry plates are applied. He can make the emulsion, we say ; but how about the plates ? It is here the difficulty comes in. But half the battle is over when the emulsion has gone through the final process of filtering. Plates have to be coated and dried. For this, special appa ratus is necessary, not to mention much room to place it in, and no inconsiderable dexterity. It is in this latter that there is the falling off. During the days of collodion a certain amount of manual dexterity was necessary before even the poorest results could be got. Now there is no such necessity. To place a plate in a dish, and pour the developer over it, requires little manual skill. Judgment may be necessary now, as before, even possibly to a greater degree, but this is a quite different affair, consequently the manipulative power of the average photographer has fallen off greatly. Add to this that the coating and drying of emulsion plates, at all times troublesome, is proportionately more difficult the smaller the scale on which operations are carried on, and we will no longer wonder that the rising generation of photographers, seeing plates of excellent quality offered.at exceedingly low prices, should not care to wrestle with the difficulties which they would encounter did they determine to manufacture their own plates. Yet there is a body of workers—by no means inconsider able—who, from love of experimental investigation, and for the sake of the pleasure they gain from total self dependence, do continue themselves to make all the plates they use. Long may they continue to do so, for it is to these that we have to look for advancement now, and at all times. To such it may be useful, to all it may be interesting, to know in what manner plate-coating is con ducted on a large scale in commercial plate factories. There are two methods in use—namely, hand-coating, and machine-coating. The former is that generally adopted. In fact, we know of only two manufacturers who coat largely by machinery, although, of course, there may be more. The first object in all coating is, to get the plate properly cleaned, and to have an emulsion which will flow well. Glass may be cleaned in any of the usual methods to get off adhesive dirt, and may be polished afterwards with moist whitening, or any of the similar pastes used for cleaning glass. Of course it is desirable to use nothing which will have an action on emulsions, although the paste ought to be so thoroughly cleaned off that there would be small danger, even were such introduced. As all who have experimented even to a small extent in emulsion work must know, there is a vast difference be tween various emulsions, as to the ease with which they flow when poured on glass. We do not know what are all the factors which influence this quality, nor do we suppose that they have been investigated by anyone. A good emulsion in this respect may, however, always be relied upon if the following conditions be fulfilled. A suitable gelatine must be used. There are many such in the market now specially prepared for emulsion work. Those which combine to the greatest extent the two properties of stiff setting and absorbing power are the best. The proportion which the silver haliod bears to the gelatine must not be too small. An emulsion made with a suitable gelatine, and in which 360 grains of gelatine have been used to 400 grains of silver nitrate, will generally produce a well flowing emulsion. The emulsion must not be too much diluted with water. With a hard gelatine, an ultimate proportion of 18 grains of gelatine to each ounce of emulsion is good. Five to ten per cent, of alcohol must be added to the emulsion. Not more than the latter quantity should on any account be used, as it is liable to produce a peculiarly non-absorbent state of the film, which retards development. With an emulsion containing 30 per cent, of alcohol the average time of development may be increased to thirty minutes. This is a curious and somewhat inexplicable fact, seeing that plates dried with the assistance of alcohol do not exhibit the same peculiarity. With an emulsion containing the proportions of gelatine, silver salts, water, and alcohol which we have described, it is necessary to coat at a pretty high temperature; 120° Fahr, is by no means too high. Nor should the plates themselves be too cold ; 60° to 65° is a good temperature for the coating room, levelling slab, and plates. Hand-coating, it is common to say, is performed exactly in the same manner as is the coating of plates with col lodion. It is the same, inasmuch as emulsion is poured on the plate, flowed to the corners, and that afterwards a certain amount of the excess is poured off ; but in reality