236 



NA TURE 



[January 9, 1896 



long exposure, and could even then be satisfactorily viewed at 

 only a single definite angle, it is now claimed that an exposure 

 of only a few seconds is needed, and that the colours are visible 

 at all angles of incidence so long as the plate is moist {Journal 

 de Physiqtte, p. 84, 1894). But, like the daguerreotypes of fifty 

 years ago, they are incapable of multiplication, and great as is the 

 scientific interest connected with them, it seems scarcely prob- 

 able that they can long continue to hold an important place 

 practically. The problem of ascertaining definitely the cause of 

 the return of a colour the same as that which falls upon a given 

 surface may seem to be solved mathematically, but the mastery 

 of the physical conditions required to produce a single coloured 

 negative, from which may be had any desired number of positives 

 with varied hues accurately reproduced, is still in the future. 

 From the very nature of stationary light waves it does not appear 

 probable that the Becquerel method as improved by Lippmann 

 will give the means of multiplying copies of a single picture. 

 Wiener has lately published an elaborate research upon this 

 subject (O. Wiener, Wiedeinanii's Annalen, pp. 225-281, June 

 1895), i" which he recognises the necessity for the employ- 

 ment not of interference colours but rather of what he calls 

 body colours (Korperfarben) due to chemical modification of 

 the reflecting surface. M. Carey Lea {Aiiierican /ournal of 

 Science, p. 349, May 1887), in 1887 obtained a rose-coloured 

 form of silver photochloride which " in the violet of the 

 spectrum assumed a pure violet colour, in the blue it acquired a 

 slate blue, in green and yellow a bleaching influence was shown, 

 in the red it remained unchanged." But in the absence of any 

 means of fixing these colours, a promising prospect brings 

 disappointment. 



While it is abundantly possible that coloured illumination upon 

 suitable colour-receptive materials can give rise to similar body 

 colours, we are still far from having these materials under con- 

 trol. There seems at present to be greater promise in another 

 and quite diff"erent application of optical principles. The 

 suggestion appears to have been first named by Maxwell (Royal 

 Institution Lecture, May 17, 1861) in 1861 that photography 

 in colours would be possible if sensitising substances were dis- 

 covered, each sensitive to only a single primary colour. Three 

 negatives might be obtained, one in each colour ; and three 

 complementary positives from these, when superposed and care- 

 fully adjusted, would present a combination that includes all the 

 colours of nature. Li 1873 H. W. Vogel in Berlin discovered 

 that silver bromide, by treatment with certain aniline dyes, not- 

 ably eosine and cyanine blue, can be made sensitive to waves of 

 much longer period than those hitherto effective in photography. 

 In 1885 he proposed to sensitise plates for each of a number of 

 successive regions in the spectrum, and to make as many com- 

 plementary pigment prints as negatives, which should then be 

 superimposed. This somewhat complicated plan proved difficult 

 in practice. In 1888 F. E. Ives {Journal of the Franklin 

 Institute, January 1889), of Philadelphia, adopting the more 

 simple Helmholtz-Maxwell modification of Young's theory of 

 colour, applied it to the preparation of suitable compound 

 colour screens which were carefully adjusted to secure corre- 

 spondence with Maxwell's intensity curves for the primary 

 colours. The result was a good reproduction of the solar 

 spectrum. But to reproduce the compound hues of nature it is 

 necessary specially to recognise the fact that although the spectrum 

 is made up of an infinite number of successive hues, the three 

 colour sensations in the eye are most powerfully excited by 

 combinations rather than by simple spectral hues. Thus accord- 

 ing to Maxwell's curves, the sensation of red is excited more 

 strongly by the orange rays than by the brightest red rays, but 

 the green sensation is excited at the same time. This fact has 

 to be applied in the preparation of the negatives, while images 

 or prints from these must be made with colours that represent 

 only the primary colour sensations. Properly selected colour 

 creens must therefore be used for transmission of light to plates 

 sensitised with suitable aniline dyes ; and the adjustment of 

 ratios with this end in view is not easy. But it has been success- 

 fully accomplished. From three negatives thus made, each in 

 its proper tint, positives are secured ; and these are projected, 

 each through its appropriate colour screen, to the same area 

 upon a white screen. The addition of lights thus sent from the 

 triple lantern gives the original tints with great fidelity. 



Mr. Ives has devised a special form of camera by which the 

 three elementary negatives are taken simultaneously, and also 

 an instrument, the i^hotochromoscope, in which a system of 

 mirrors and lenses brings to the eye a combination similar to that 



NO. 1367, VOL. 53] 



projected with the triple lantern. A double instrument of this- 

 kind forms the most perfect type of stereoscope, bringing out 

 with great vividness from the prepared stereographs the com- 

 bined effect of colour, form and binocular perspective. It is- 

 only within the past year that these improvements have beea 

 perfected. By further application of the same principles, Mr. 

 Ives has produced permanent coloured prints on glass, which da 

 not require to be examined by the aid of any instrument. Each 

 of these negatives is made with a coloured streen which trans- 

 mits tints complementary to those which it is desired to 

 reproduce. The three gelatine films are soaked in aniline dyes- 

 of suitable tint, and superimposed between plates of glass. 

 When viewed as a transparency such a print gives a faithful 

 reproduction of the natural colours. 



The problem of colour reproduction is thus solved, not indeed 

 so simply, but more effectively, than by the method of inter- 

 ference of light, or by those laody-colour methods that have 

 thus far been applied. To the imaginative enthusiasts who are 

 fond of repeating the once novel information that " electricity is- 

 still in its infancy," it may be a source of equal delight to 

 believe that photography in colours, a yet more delicate infant^ 

 is soon to take the place of that photography in light and shade 

 with which most of us have had to content ourselves thus far ;; 

 but so long as an instrument is needed to help in viewing 

 chromograms, the popular appreciation of these will be limited. 

 We may take a lesson from the history of the stereoscope. Yet 

 it is gratifying to recognise the great impetus that this beautiful 

 art has received during the last few years. We may quite 

 reasonably expect that the best is yet to come, and that it will 

 have an important place among the future applications of optical 

 science. 



The Infra-red Spectrum. 



Among the splendid optical discoveries of this century, 

 probably the most prominent are photography and spectrum 

 analysis, each belonging jointly to optics and chemistr)-. 

 Photography was at first supposed to be concerned only with the 

 most refrangible rays of the spectrum, but Abney and Rowland 

 have photographed considerably below the visible red. Beyond 

 the range thus attained qualitative knowledge was secured by 

 Herschel, Becquerel, Draper, Melloni, Miiller, Tyndall, 

 Lamansky and Mouton. But our quantitative knowledge ot 

 this region began with the invention and use of the bolometer 

 by Langley {" Selective Absorption of Solar Energy," Am. 

 Journal of Science, March 1883, p. 169), whose solar energy curve 

 has been familiar to all physicists during the last dozen years. 

 During this interval the bolometer has been used with signal 

 success by Angstrom, Rubens, Snow and Paschen, who have 

 made improvements not only in the instrument itself but in the 

 delicacy of its necessary accompaniment, the galvanometer. 

 The work of Snow {Physical Review, vol i. pp. 28 and 95),, 

 particularly, on the infra-red spectra of the voltaic arc and of 

 the alkalies, and that done by him in conjunction with Rubens 

 {Astronomy and Astrophysics, March 1893, P- 231), on refraction 

 through rock-salt, sylvite, and fluorite, exhibited the capacities 

 of the bolometer even better perhaps than Langley's previous 

 work on the sun. But more recently with the collaboration of 

 several able assistants, and more particularly the great ingenuity 

 and mechanical skill of Wadsworth, the sensitiveness of 

 Langley's galvanometer has been so exalted, and the bolometer 

 connected in such manner with photographic apparatus as to 

 make it an automatically controlled system, by which an hour's 

 work now brings results superior in both quantity and quality to 

 what formerly required many weeks or even months (Langley, 

 "On Recent Researches in the Infra-red Spectrum"; Report 

 of Oxford Meeting of British Association, 1894). Not only 

 is an entire solar energy curve now easily obtained in a single 

 day, but even a succession of them. . It becomes thus possible 

 by comparison to eliminate the effect of temporary disturbing 

 conditions, and to combine results in such a way as to represent 

 the infra-red cold bands almost as accurately as the absorption 

 lines of the visible spectrum are indicated by use of the diffrac- 

 tion grating. It will undoubtedly become possible to determine 

 in large measure to what extent these bands are due to atmo- 

 spheric absorption, and which of them are produced by absorption 

 outside of the earth's atmosphere. 



With the diffraction grating, supplemented by the radio- 

 micrometer, Percival Lewis {Astrophysical Journal, June 1895, 

 p. I, and August 1895, P- i°6), has recently investigated the 

 infra-red spectra of sodium, lithium, thallium, strontium. 



