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THE PHOTOGRAPHIC NEWS. Vol. XXVII. No. 1312.—Octoler 26, 1883. CONTENTS. PAGR The Nature of Green Fog 673 The Preparation of Glass Plates before Coating with Emulsion 674 The Application of Electric Light to Photography 675 The Photographic Exhibition 675 Photographs Transformed into Typographic Blocks 676 How the Chittywee was Taken. By A. J. West 678 French Correspondence. By Leon Vidal 679 Notes 680 Patent Intelligence 681 PAGE Lessons in Optics for Photographers. By Captain W. de W. Abney, 682 Glasgow and West of Scotland Amateur Photographic Associa tion’s Exhibition • 684 Correspondence 684 Proceedings of Societies 686 Talk in the Studio 688 To Correspondents • 688 The Photographic News Registry 688 THE NATURE OF GREEN FOG. This defect in gelatine plates still occupies a prominent place in photographic discussion, and any new light that maybe thrown upon the subject, photographers will no doubt gladly welcome. The causes of its appearance and how to avoid it, and getting rid of it when once it has ap peared and threatens to ruin the printing qualities of a negative, are matters of no small moment to both amateurs and professionals. The actual nature of the deposit itself may interest a smaller number, but it is of no less impor tance. “ Never mind what it is, but tell me how to get rid of it,” is the remark of a rapidly diminishing class of workers when such matters are brought under their notice ; but there are probably few indeed who would say this in reference to the subject of this article. Probably all will be prepared to acknowledge that when it can be decided beyond dispute or cavil as to what green fog really consists of, the difficulty will be more than half overcome. The question can only be decided when the substance itself can be separated in considerable quantities from the gelatine with which we usually find it in contact, or, better still, when it can be prepared chemically in a manner that shall show not only that it is actually green fog, but shall also resemble the way in which photographers are in the habit of preparing it, unintentionally or otherwise. Several experimenters have at times thought they were in a fairway of thus deciding the question, but as in most cases they were rather wide of the mark, there is no neces sity to repeat their observations here. Of the opinions ex pressed by those who had carefully experimented on green fog in gelatine plates, but had not met with it, or thought they had met with it apart from gelatine, that of Captain Abney received most attention. His opinion was that green fog was simply a finely-divided form of silver, and be pointed out several experiments tending to confirm his views, notably the method by which he was able to convert it into a light greyish veil which had less detrimental effect on the printing quality of the negative. His method, which is often put into practice, consists, as our readers are aware, in soaking the negative in a solution of some ferric salt, and blackening it again by the application of fresh iron developer. Since Captain Abney’s opinion was put forward, photographers have been divided into two parties on the subject—those who do, and those who do not, think that green fog is metallic silver or asilvercompound. Which party holds the more correct view we shall speedily be able to demonstrate, for in this article we purpose show ing our readers how they may produce what is practically green fog, in a test-tube, beaker, or bucket, whichever the experimenter may prefer. Before doing so, however, we must ask the reader to follow us into a brief repetition of tho conditions under which green fog appears in working gelatine plates, for not only has that an important bearing on the truth of tho statement we have to make, but it will also explain how we arrived at our result. In the first place, green fog accompanies alkaline deve lopment, and is undoubtedly due in some way or other to the ammonia, for as Mr. Woods once pointed out at a technical meeting of the Parent Society, dense green fog may be produced by the iron developer if only a slight trace of ammonia be present. Again, it is most prominent when the plate has been under-exposed, for on such occa sions the film is in the developer for a longer period, even if more ammonia is not added to force up the image ; the smaller the proportion of ammonia used, the less chance is there of fog. Finally, we must again draw attention to a matter which we noticed last week, that chloride of silver, which is more soluble in ammonia than the bromide, has a greater tendency to produce green fog. With these facts before us, the actual nature of green fog should have been readily solved ; but, in company with photographers generally, we have had a very simple thing staring us in the face for some time without having been able to see it, until certain matters already discussed in connection with alkaline development suddenly thrust it before us. And here we beg to draw the reader’s atten tion to a little experiment. Precipitate some chloride or bromide of silver, using an excess of the haloid salt, and add a large excess of ammonia. Pour off into another vessel a portion of the clear liquid, and add to it a grain or two of pyro. A dense brown precipitate, very similar in colour to that of exaggerated green fog by reflected light, is produced. If the experiment has been conducted in a glass vessel, a film of it will be found clinging to the sides ; but unless it is very thin and fine, it will appear of a brownish green tinge. Use a test tube, and, if a solution of chloride is being used, dilute with water ; then drop in the pyro., place the thumb over the tube, and, giving it just one shake, pour out the contents, and rinse the tube with water. If the film on the side of the tube is suffi ciently fine, a green colour by reflected, and a pink, by transmitted, light will be well marked. This we believe to be green fog. If a large quantity be prepared, the ordinary characteristics of green fog appear less marked, but we must remember that we are producing it in a very rapid manner, whereas its production in a plate is com paratively slow; moreover, we are in this experiment using no gelatine to keep it in the fine state of division requisite. As the production of green fog in a gelatino- bromide plate coincides exactly with this experiment, that fact alone would be almost sufficient to show that the two things are identical. We have, however, a number of experiments which we think will leave no doubt in the minds of our readers that our view is correct, but cannot