If any illustration were needed of the great popularity now attained by the practice of photography, reference might be made to the large number of periodicals devoted to the subject, and appearing weekly, fortnightly, quarterly or annually, in every civilised country, and also to the multitudes of societies that have been formed for the promotion of the art. In Great Britain alone there are now at least 150 such societies in active operation, and they are correspondingly numerous elsewhere. If, when we consider all that has been accomplished up to the present time, with the jubilee year of photography scarcely passed, and observe the increasing numbers of its cultivators guided by the explanations of its phenomena that science is beginning to furnish, we can expect a corresponding progress in the next fifty years, then the centenary may be reached with a roll of achievements that could we know them now we should think marvellous.
As already remarked elsewhere, the practical side of photography has outstripped the theoretical one, for so far its progress has been much less indebted for processes and technic to the direct guidance of science than almost any other of our Nineteenth Century acquisitions, such as telegraphy, electric lighting, etc. The materials employed, and the mode of manipulation, have certainly not been deduced from previous knowledge of the nature of light or from the laws of chemistry, although when, by repeated trials and happy guesses, the right direction had been found, the field into which it led could be more easily explored under the direction of chemistry and physics. But even yet the fundamental principle, or the precise nature of the action of light on certain compounds, has not been definitely made out, and although some theories on the subject have been proposed, no one has been generally accepted as an adequate explanation of the known facts, and still less have any quantitative relations been established for these actions. The photographer cannot compose a formula for the composition of his emulsions and developers from assured data like those that enable the chemist to weigh out with accuracy the constituents that go to produce a required compound.
The attainment of permanency in its products, which, by several processes, photography can now boast of, is one of its triumphs, and will tend greatly to enlarge the sphere of its utility. For example, we have a public institution, known as the National Portrait Gallery, in which it is sought to gather together and preserve the likenesses of the most eminent Englishmen, and presentments of such of far less fidelity than photographic portraits are eagerly sought after. It has been suggested that something like a National Gallery of permanent photographic portraits of the chief men of their time would be a fitting and acceptable legacy to the public of the future. This idea has much to recommend it, particularly as authentic likenesses would thus be secured for the nation beyond the chance of loss.
Photography has been applied in preparing blocks in relief for printing along with letterpress in the same way as woodcut blocks. The process has the great advantage of producing in a wonderfully short time a perfect facsimile of the artist’s drawing without the intervention of any engraver. A plate of zinc, brass, or copper, coated with a dried film of bichromated albumen, is exposed to light under the transparent negative of a drawing in pure line, that is, one having in it only lines of uniform colour throughout. The parts of the film reached by the light, which correspond with the lines of the original design, are rendered insoluble, while the rest can readily be removed by water. These unprotected parts have then to be removed by the action of acids, but these are used alternately with the application to the plate of certain compositions, the purpose of which is to prevent lateral erosion of the lines in relief before the requisite depth of the metal has been removed. Fig. [147f] is the reproduction of a pen-and-ink sketch by this or some similar process. But nature and the ordinary photograph show us graduated tones which ordinary printers’ ink cannot really reproduce, inasmuch as it is incapable of gradation, and can give the effect of gradation only by such devices as are mentioned on page [642] (last sentence). Now, the photograph cannot yield a printing-block until its continuous tones are broken up into lines or dots. Not a few methods of doing this have been contrived, but that which is by far the most commonly used, and is most successfully practised on the commercial scale, is simple in principle, although in actual working it calls for much experience and skill. The negative is taken upon a wet collodion plate, in front of which, within the camera, and at a very short distance (say 1
30th inch) from the film, is a transparent screen, bearing two sets of parallel opaque lines at right angles to each other. These lines are mechanically ruled with the utmost regularity, and are separated by only very small intervals. There may be from 80 to 200 of them in the space of one inch, according to the class of work required. The effect of this is that the light reaches the photographic film through a series of minute transparent squares, the sides of which may be only from the 1
140th to the 1
400th of an inch in length. Now it is found that the brighter lights from the original positive, after passing these small apertures, spread so as to more or less cover the opposite parts of the negative, while the feebler lights, from the shades of the original, impress the plate to a less degree, the developed image in these showing, perhaps, merely a small dot or, in the very darkest parts, a blank. In this way, then, may the photographic negative be obtained with a granulated texture following in graduation the tones of the original. After this, the rest is easy, for the process of exposing a metal plate, coated with a sensitive film under the negative, and of etching it with acids, etc., is essentially the same as in the foregoing. Such is the half-tone process, which is now so largely superseding wood and other engraving. It is unnecessary to describe technical details here, such as the employment of bitumen of Judæa as the coating for the metal plate, or how the image must be reflected into the lens from a mirror to avoid a reversal in the final print, etc. There are endless modifications of the processes briefly mentioned above, and some of these are guarded as valuable trade secrets. Several of the illustrations in this work are prepared by the half-tone process, of which plates I., IV., V., etc., are examples, and they should be examined with a strong lens, in order that the different rendering of the light and the dark parts may be compared.
PHOTOGRAPHY IN COLOURS.
It is the statement as to the futility of assigning limits to scientific discovery that has been justified by facts. The preceding edition of this work was not long in the hands of its readers before the solution of the problem of photography in colours was announced from Paris, where, at the close of 1890, the physicist M. Lippmann had succeeded in photographing the solar spectrum in its natural colours, and at the beginning of 1891, he was able to exhibit at the Academy of Science untouched photographs of a stained glass window in three colours, of a dish of oranges and red flowers, and of a gorgeously coloured parrot, all in their natural tints. The method employed had no apparent relation to that of Becquerel, but was of the simplest, and, moreover, one which any reader who has followed the first few pages of our section on the “Causes of Light and Colours” will have little difficulty in completely understanding, if he has devoted a little attention to Fresnel’s interference experiment. M. Lippmann took a photographic plate, coated to a greater depth than usual with a gelatine film containing the sensitive salts of silver, and in the camera this plate was exposed with the glass towards the lens, while at the other side of the film was a metallic reflecting surface, namely, quicksilver. Supposing a ray of red light to enter the glass and traverse the film, it would be reflected from the metallic surface, and would meet the direct ray within the substance of the film, with a difference of length of path that would produce the interferences already described, and so give rise to alternate lines or bands of darkness and brightness. It would, of course, be in the lines of maximum brightness that the silver would be first deposited by the photographic action, and these microscopically fine lines or striæ of silver would give back, from ordinary light, a colour corresponding to the waves of red light that produced them. Similarly with the other colours. Anyone may observe the production of colour from ordinary white light in the iridescent tints of mother-of-pearl, where the effects are due to the varying distances of fine edges of the layers of the substance. If an impression is taken from a piece of mother-of-pearl by solid paraffin, or by white wax, or even by common red sealing-wax, the colours will seem to be adhering to the impression, but the operation may be repeated times without number. It is the distance apart of the lines or striæ that determinates the colour, and this is always some definite multiple of the wave lengths, given on p. [411], for the various colours. M. Lippmann’s products are true colour photographs, and they form a new and elegant experimental demonstration of the doctrine of luminiferous undulations.
The colour effects of nature have also been reproduced by taking photographs of the same scene through coloured glass. Thus a screen of yellow glass will intercept the blue and the red rays, and the sensitive film will be impressed with images of objects containing yellow rays only, and that in proportion to the quantity of these rays that enter into any given tint. Similarly with images taken through red and blue glasses. The positives from these partial images being projected by three optical lanterns on the same space on a screen, and each being coloured by passing through tinted glasses like the original, the superposed images thus combined give a very lively impression of the natural colours in all their gradations.
Among the many processes for reproducing photographs by non-photographic processes, some have been more or less successfully combined with colour printing. Some of these productions are very effective, and are more attractive to many persons than the monochromatic tints of ordinary photographs.
