Discoveries and Inventions of the Nineteenth Century

canvas, or after it has been cut into thin strips. It is then drained, dissolved at a gentle heat, and filtered warm. The clean glass plates are coated over with it, at the temperature of 120° F., and are set aside in a perfectly horizontal position until the gelatine has set, when they are placed for twenty-four hours in a drying cupboard, maintained at 80° F. It will be understood that these operations are conducted in a room where no light enters, except through a frame of ruby-coloured glass, and the plates, when dry, are carefully packed and stored in light-tight boxes. They are marvellously sensitive, and receive the photographic impression in about one-sixtieth (160th) of the time required for wet collodion plates. Half a second exposure in the camera may be sufficient to impress the image of a well lighted landscape, even when a very small stop is used, and it is not unusual to employ for extra sensitive plates, a so-called “instantaneous shutter,” when the exposure may be no more than 180th to 1100th of a second, and yet obtain a perfectly strong image. Dry plates are manufactured in vast numbers in many large establishments, and the operations are carried on to a great extent by the aid of machinery, by which the plates are uniformly coated and automatically carried into drying chambers, etc. If photography were popular before the introduction of the dry gelatino-bromide 621plates, it has since become a hundred-fold more so. Indeed, the camera is now seen everywhere, and few are the family circles in which at least one amateur practitioner of the art is not to be found; indeed, the technical terms of the art have become “Familiar in their mouths as household words.” The daguerrotype, notwithstanding its cost, had no sooner become a practicable process for taking likenesses, than it began to supersede miniature painting, and how rapidly it rose into general favour may be inferred from the fact that, in 1850, ten years after its introduction, it was estimated that in the United States of America, at least ten thousand persons had made it their profession, and, probably half as many more were occupied in making and selling chemicals, plates, cameras, lenses, mounting cases, and other apparatus connected with its practice. Such being the demand for photographic portraits, at the period when the sitter had, as we have already seen, to remain motionless for two whole minutes in sunlight, we can hardly be surprised at the increased popularity the art has acquired in the last decade, when a picture can be produced with one-hundredth the length of sitting, and at about the same reduction of cost. It may here be mentioned, that Daguerre’s process is still occasionally used for special purposes; it was, for instance, the method selected for obtaining the photographic records in the expedition sent out by the French Government, in 1874, to observe the transit of Venus. Fig. 311a.—The Roll-Slide. The dry plate processes have given an immense impulse to landscape photography, and travellers have been able to bring back authentic representations of the scenery and inhabitants from every part of the globe. This advantage arises from the fact that having the camera, and its appurtenances, the tourist or traveller is not obliged to carry anything about with him except his plates, and when these have once been exposed in the camera, and stowed away in light-tight boxes, the latent images may be developed months, or even years, afterwards. But glass plates are heavy, and are liable to accidental breakage. Inventive ingenuity has been actively at work for the past few years, to find a means of obviating these remaining inconveniences. The first method adopted was to employ paper instead of glass, as a support for the sensitive gelatine film. The paper, having been cut to the proper size, is placed on a film-carrier, which is usually a thin plate of ebonite, by which the paper is kept flat. These carriers take the place of the glass plates in the ordinary dark slide, and after exposure in the usual way, the papers are removed in the dark room and made up into light-tight packages, where, of course, a large number will occupy but a small space, and the weight of them be wholly negligible. Many persons make use of this arrangement, which has the advantages of simplicity and of requiring no special apparatus. But an improvement was soon brought out, which consists in substituting for the carriers and pieces of sensitive paper a continuous roll of the material. For this purpose a special piece of apparatus, called the roll-holder, is made to take the place of the dark slide at the back of the camera. The arrangement will be readily understood from Fig. 311a. The figure shows the apparatus in section, but only the disposition of the principal parts, most of the mechanical details being omitted. R R´ are two metallic or wooden rollers, which admit of being readily put in their places and taken out. Upon one of these, R, the full length of the material is previously wound, and the free end is passed over another roller, r´, and across the opening at E O, where the exposure is 622made. There is in front of this a dark slide (not here shown) to be drawn up when everything is ready for uncovering the lens. Immediately behind the paper is a flat plate of ebonite, E, or a smooth black board, the object of which is to keep the material quite flat as it passes over the opening to the roller, r´, which guides it to the roll, R´, on which it is wound as required. S S´ are two small rollers always pressed by springs against the rolls to prevent the turns working loose. There is a registering apparatus outside in connection with one of the rollers, r, or r´, to show when the proper length of material has been wound across the opening for a new exposure; and at the same time a mark is automatically made on the paper to indicate where the negatives are to be separated for development by cutting the paper. Some forms of the apparatus also call the operator’s attention to the sufficient winding of the roll by an audible signal, a stroke on a little bell tells that everything is ready for a new exposure. In some cases the number of exposures already made is registered by figures that appear on the outside. The paper in these processes is used only as a temporary support; for after the negative has been developed in the ordinary way, the sensitive gelatine film is removed from it and made to adhere firmly on a plate of clear glass, from which prints are taken as usual. The operations required for the transferring require considerable dexterity of manipulation, and to both the paper and the glass special preparations have to be applied, before and after the transference of the film. This plan, therefore, of “stripping films” involves so great a number of delicate and somewhat troublesome operations that very many photographers have preferred to encounter the labour and risks of carrying about with them the more easily manageable glass plates. But what if some grainless, transparent substance could replace the paper in these rolls so that the negatives might be ready for printing from when merely developed and fixed? Many trials have been made to find this desideratum. A material sufficiently translucent, even, and of tenacity enough to bear the stretching strain between the rollers has, it is believed, been discovered in a very singular substance previously used for other purposes. The reader is no doubt familiar with it as the substitute for ivory in combs, knife handles, and other small articles. It is called celluloid, and is a composition the principal ingredients of which would never be guessed from its appearance—namely gun-cotton and camphor! This material is prepared in a plastic condition that enables it to be shaped into any required form. It can be drawn into threads or rolled out into very thin films. Thin plates of it have been used in photography as a substitute for glass, for the sake of lightness, before its employment as a transparent film in the roll-holders. We have now at length the equipment of the travelling photographer reduced to the utmost conceivable limits of lightness and compactness. Thus the complete apparatus 623required for taking hundreds of pictures of a good size need not be more than a few pounds in weight, and can easily be carried in the hand. But even quite small negatives can now be very readily printed in a few seconds on paper, with an enlargement of many times the original dimensions. The resources of the photographic art appear indeed to be endless; but a mere statement of even the more interesting of these would lead us beyond our limits, and descriptions of the details of manipulation are out of our province altogether. But a few of the more recent applications and developments of the art scarcely or not at all alluded to in the foregoing pages should receive some attention. The extraordinary sensitiveness of the gelatine-bromide film which makes it possible to impress on it a photographic image in the merest fraction of a second of time, enables us to take pictures of objects in rapid motion. Express trains at their highest speed have been successfully photographed, and so has almost every moving object in nature. The photographs that have been taken of men, of birds, horses, and other animals in every phase of their most rapid actions, have solved many disputed and perplexing problems as to the nature of their movements, and sometimes the solutions have been of a very unexpected kind. Taking a photographic “shot” at a bird has become almost more than a figure of speech; for there are contrivances by which a bird on the wing may be aimed at with the lens, and hit off on the sensitive plate with a certainty surpassing that of the fowling-piece. There are also photographic repeaters by which six or more successive photographs of the bird, etc., can be taken in a single second. Mr. Muybridge has published a number of such photographs of the horse, and by projection of the different images on a screen from a magic lantern, in rapid succession, he has been able to reproduce the visual appearance of horses trotting, leaping, galloping, etc., on the principle of the zoetrope (page 399). Photography has afforded wonderfully delicate observations in many departments of science, by recording phenomena too rapid for the eye to seize, or too recondite for direct perception. A few examples may be mentioned. First, the advantage of photographing the lines of spectra, such as those described in our article on the spectroscope, will at once suggest themselves, and accordingly this method of recording spectra has been largely used, and in the hands of Mr. Lockyer, Dr. Draper, and others has been successfully applied to the study of the solar and stellar spectra. But more than this, it is the sensitive photographic plate that has enabled us to explore the region of the solar spectrum lying far beyond its visible limits in the red and in the violet rays. The ultra-violet portion of the spectrum is shown photographically to be occupied by multitudes of the thin insensitive spaces—breaks in the continuity of the active rays—which are impressed on the photographic print as black lines, similar in every respect to the lines mapped out in the visible spectrum by Fraunhofer. It is known by these that the ultra-violet spectrum, produced by glass prisms, extends to a distance beyond the last visible rays of nearly double the space occupied by the colour spectrum. The principal lines, or rather the greater groups of lines in the invisible spectrum, are distinguished by the capital letters of the alphabet, in continuation of Fraunhofer’s method, beginning from H and nearly exhausting the letters of the alphabet to designate them. These are