WHEN the pioneers of photography began to turn their attention to landscape and decorative work they discovered a very serious defect in their productions as either records of facts or as interpretations of nature as they saw it. The rendering of colour was entirely misleading. They did not, of course, anticipate that the bright hues of nature would arrange themselves automatically upon the finished print. But they had expected to find some such relative shading of objects seen by the eye as is expressed by the varying depths of tone in an etching or steel engraving. This is just what the photographic image failed to do. The colour values appeared as a confused muddle. A dark-blue ribbon in a maiden's hair might be represented as nearly white; the golden tresses lost all distinction; and the pink rose pinned on the front of her dress was nearly black. The bright gorse blossom on the hillside came out darker than its foliage, and the thousand varying shades of the trees were lost in meaningless dull tints.
The cause of this difficulty has been already discussed in standard works on the chemistry of light, and we need therefore only refer to it very briefly here. The diagram shows the rays of the spectrum bands in their order divided at proper intervals by the Fraunhofer lines. The black line describes roughly the relative luminosity or brilliancy of the various rays as they appear to the human eye; the dotted line, on the contrary, shows the proportion of these rays acting on the particles of silver halides, or in other words, the relative chemical intensity of the spectrum upon the sensitive dry plate. Thus it will be seen that that part of the spectrum which appeals most strongly to the human eye - the orange and yellow - is just the part that exerts no perceptible chemical influence on the silver salts. On the contrary towards the end of the blue, shading off into violet and ultra violet, the chemical action becomes vigorous - just the part of the spectrum which attracts little notice or is even invisible to the human eye. As a rule, therefore, the perception by the eye with regard to the luminosity of the rays reflected from coloured objects is almost diametrically opposite to the record of luminosity shown on the ordinary dry plate. The fact is demonstrated in a most striking way if we place side by side a yellow viola and one of the very dark-blue variety. We have no doubt which of the two reflects the brightest and most luminous rays, judged by the test of our own eyesight. If an artist were called upon to sketch them in pencil or any other monochrome method he would convey the impression by shading the yellow viola as nearly white and the blue one as nearly black. But if we photograph them on an ordinary plate, in which no attempt at colour compensation has been made, a reverse result will be attained in the print. The dark-blue flower will be white, the bright yellow one black.
The science of orthochromatics has properly, then, nothing to do with colour pet se. It attempts merely to render in more truthful gradations the illumination of objects to be photographed. We can only reconcile the optical nerves with the haloid salts by reducing the action of the short length waves of light upon the plate, the blue, violet, and ultra violet; and on the other hand increasing the sensitiveness of the silver salts to the rays at the yellow and red end of the spectrum.
Professor J. W. Draper, of New York, who, it will be. remembered, in 1840 photographed the moon for the first time on a Daguerrotype plate, is frequently claimed as the father of orthochromatic photography. He discovered that the chemical action produced by rays of light depended upon their absorption by sensitive bodies. But it was not until 1873 that Dr. H. W. Vogel, of Berlin, applied this, practically by staining collodio-bromide plates with a yellow dye. He was able in this same year to prove that silver bromide stained with proper yellow or red dyes had its sensitivity to the yellow and green rays considerably increased. Colonel Waterhouse shortly afterwards suggested the application of the coal dyes, and especially eosin for the purpose; and in 1879 Mr. F. E. Ives obtained some valuable results with an alcoholic solution of chlorophyll, the green colouring matter of leaves. Later on M. Tailfer, a French chemist, applied orthochromatism to the dry plates, by staining the previously prepared plates with eosin or erythrosin in conjunction with ammonia. The dyes still most in favour are erythrosin, pinachrome, pinacyanol, and cyanine, the two latter being chiefly used when considerable sensitiveness to red rays is demanded.
But although by the use of suitable dyes the silver salts are rendered much more sensitive to green, yellow, and red rays, there are still a vast number of rays impinging on the plate and being absorbed by it at the opposite end of the spectrum. Dr. Kenneth Mees and Dr. Sheppard are agreed that, if no glass is used, practically two-thirds of the whole effect on the ordinary plate is due to the ultra-violet rays, and if a sheet of glass is used, about one-third. Messrs. Newton and Bull have shown that the ultra violet rays are even more effective with enclosed arc rays and wet collodion. Such rays are most disastrous for colour rendering. This light is quite invisible to the eye; its effect on colour cannot be gauged, and it entirely falsifies the tones. In a recent lecture before the Royal Photographic Society, Mr. R. W. Wood exhibited a number of photographs taken by these invisible rays, when Chinese white appeared as black. Incidentally they have their value in detecting certain chemical and other phenomena unseen by the eye, but in pictorial photography we would rather be without them. Hence the yellow filter, which may be introduced either as a lens cap in front or behind the lens, or in the diaphragm slot.