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November 26, 1880.J THE PHOTOGRAPHIC NEWS. 573 I readily agree that an artist under the circumstances would not always succeed in producing exemplary work ; the number of the customers, as well as ofttimes their class, probably renders that impossible; yet, if photo graphers studied more the requirements of pictorial com position, it would wonderfully reduce the number of incon gruities which are so often at present the prominent features of a photograph. When a great many sitters are to be dealt with it is well to adopt a “set” of accessories for the day—at least, as far as possible—otherwise the photographer expends most of that time on the arrangement of furniture, &c., which should be dedicated to the subject. This plan is carried out by many good photographers, and it is wonder fully practical; nor do I apprehend being considered in consistent by advocating it, after treating on the indivi duality be'onging to pose and model, since the latter is embodied rather in the sitter’s self than in the accessories, and a chair alone is sufficient to produce innumerable positions. Indeed, the pose of the head alone is a host in itself, and it is the due attention to this which, in many cases, gives an inexplicable completeness to some por traits. The finest balance of lines iu a picture may be easily destroyed by the wrong contour of the head, and its elevation or depression tends materially to change the entire expression of the features. Yet there is considerable variation in the position of the head natural to different sitters, and I would warn students of the danger they run, especially when long exposures are necessary, by pressing the head into a position in unity with the body, without first noting this particular; if it is neglected, invariably the sitter will return unconsciously to his or her most natural position during the exposure, and, of course, blur the features. On the other hand, if this seemingly trifle be strictly regarded, it is surprising how few failures will result from long exposures, because a subject who can re main steady for (say) twenty seconds, is generally able to remain free from motion with almost equal ease for at least three times that period—that is, as far as tremor alone is concerned. There is room for the exercise of much discrimination in selecting the amount and size of the figure to be pourtrayed in photographs, as well as the position of it. No particular size should be adhered to, as is sometimes the case, merely because it is fashionable. Busts filling a whole carte-de-visite should be approached with caution, and it should be also well ascertained that the lens to be used is qualified to bear the strain to which it is thus subjected. Picturesque old men, or young persons with really good features, are the best subjects for large heads ; middle-aged persons do not naturally like the prominence thereby given to their wrinkles and other indications of the approaching ‘ ‘ sear and yellow leaf,” and yet their removal by the retoucher, beyond what is exaggerated, is equally undesirable. Plain-looking sitters have obviously a kindred objection; and, moreover, there are generally redeeming graces in their figures which are lost in this sized photo graph. I think that sitters inclined to be stout are, of all others, most sensitive of their appearance, and subjected to the greatest disappointment on beholding their portraits. The most likely size to meet with approbation in their cases is between three-quarter and full-length, as thereby plenty of space is left about the body, and the absence of the feet or bottom of the dress also leaves a pleasing doubt as to how much taller the individual represented may really be. The arms should be judiciously disposed to conceal the actual outlines of the body to a great extent. The next size most likely to please is a small vignetted bust, with as little body shown as is compatible with the balancing of the head. If I write more I fear I shall be disregarding our Editor’s request for short practical articles, so 1 must con clude, feeling how inadequate is a brief paper, such I as this, for treating on so exhaustive a subject as I am attempting. With Mr. II. P. Robinson, I consider that studentswill derive great benefit by becoming acquainted with good painting and engravings of portraiture. These the student should note in every detail, and question closely why cer tain lines, accessories, and other auxiliaries hold their particular positions, endeavouring meanwhile to apply to them what knowledge he possesses of the rules of balance, unity, and sympathy. I would, however, recommend the student to avoid photographs as studies—at least, unless they are the pet productions of our best men—for the reason that painters and sculptors must build up their productions from the foundation ; in their works there is only visible what is conceived by them; hence even the smallest object re ceives their immediate supervision, and, if they are artists, it has a purpose with it; each point symbolizes some idea. But not so the photographer; his duty is to render artistic that principally which already has existence, and, though I doubt not but that this sometimes requires greater skill than the painter or sculptor is called upon to display, yet lack of time, or observation, be knowledge may—nay, does—frequently cause the photographer to overlook many faulty details which nevertheless come to light in the subsequent photographs, and render them far from desirable as examples of pictorial composition. The “ Topic” for next week will be, " On Intensifying Gelatine Plates with Silver,” by Captain Abney, .E., F.R.S. ON THE APPLICATION OF PHOTOGRAPHY TO CHEMICAL RESEARCH.* BY W. N. HARTLEY, F.R.S.E., ETC., Proje sor of Chemistry, Royal College of Science for Ireland, Dublin. (2). On the Photographic Transparency of Substances.—It was found that coloutless bodies which possess equal powers of transmitting the luminous rays vary greatly in permeability to the invisible rays. Thus, water is perfectly transparent, that is to say, thin layers transmit all rays, visible and invisi ble, from one end of the spectrum to the other. In speaking of thin layers I allude to thicknesses of three or four inches, or less, which are small in comparison with the considerable thickness of twenty feet, which is necessary to show the blue colour of water. Carbon di-sulphide, which is as trans parent as water to the visible rays, cuts off the greater part of the visible spectrum. In comparing solid bodies such as quartz and Iceland spar it was found that these are quite transparent; so likewise is fluor-spar, while even thin glass used forcovering’microscopic objects, and thin films of mica, cut off the greater part of the ultra-violet spectrum. Diactinic solids—that is to say, solids wbich are trans parent to the chemical rays—preserve their diactinic power, both when liquefied and when converted into vapour. Colourless solids, wbich exert a considerable absorption upon the chemical rays, preserve their absorptive power with greater or less intensity, both in the liquid and the gaseous state. Dr. Miller laid stress upon the care which it is necessary to bestow upon the preparation of compounds for exami nation as to their photographic transparency. Very slight traces of impurity were believed to be present in some of his most carefully-prepared specimens, wbich could not be detected by ordinary tests, but which were opaque to the chemically active rays. It was found, for instance, that filtering through very pure paper sensibly impaired the diactinic quality of a solution. It will be seen presently that we have since shown that photographic transparency is an exceedingly delicate test of the purity of organic compounds. An immense number of salts in solution of • Continued from page 562.