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/ Evans: Notes on Maxwell's Colour 243

Some Notes on Maxwell's Colour Photograph

R . M. EVANS

Eastman Company, Rochester, U.S.A.

ABSTRACT. An investigation of Maxwell's famous demonstration of three-colour in /86/ reveals that the picture he projected was in fact based on three-colour separation negatives in spite of the fact that the photographic material was not sensitive to either or . The filters used gave colour separation in the and ultra-. The reason he obtained red separation correctly seems to be due to the fact that the red cloth he used hod a secondary reflectance bond in the ultra-violet as do most red cloths available today

JUST over 100 years ago J. Clerk Maxwell enlisted In pursuance of his engagement Sutton1 placed the services of a well-known if somewhat contro­ "a bow made of ribbon, striped with various colours" versial expert on photography to help him with a on a piece of velvet, took it and his lecture he was preparing for the Royal Institution. into the sunlight and proceeded to photograph it by The person thus engaged was Thomas Sutton, blue, green, and red . teacher and lecturer on photography and editor of (It apparently was Maxwell's contention that the Photo News, a lively but short-lived publication yellow would act like a combination of the green concerned with photography and lenses. Sutton and the red.) himself later designed a wide-angle lens remarkable In so doing he produced the colour separation for its time. pictures which subsequently, in Maxwell's lecture, As nearly as one can judge from the few published became the world's first trichromatic colour photo­ statements, the task which Maxwell set for Sutton graph. He also bequeathed to posterity a pretty was to help him prove two points for his lecture, by little technical problem in that the photographic means of photography. The first point was that all materials that he used for the purpose were sensitive colours () could be produced by the use of just only to the extreme blue end of the and had three colours of light and hence if a scene were no sensitivity at all for spectral green, yellow or red. broken into three colours it could be reassembled. As we approach the centennial of their public (Maxwell had suggested the possible use of photo­ display, it seems appropriate to reconsider what graphy for three colour synthesis several years actually occurred. It does not detract from the fact earlier.) The second point was that green and not that the result was the first three-colour photograph, yellow was the best choice to go with red and blue to inquire how it happened to be successful. The for such a task. The burden of his famous lecture fact• is that "When these (projected separation was not so much the trichromatic nature of visual positives) were superposed, a coloured was facts as such- since this was fairly well known at seen, which, if the red and green had been the time- but the fact that green and not yellow was as fully photographed as the blue, would have been the centre member of the trio. a truly coloured image of the ribbon." Sutton says "was produced in the natural colours." The photographic material used by Sutton was wet Paper read at the Maxwell Colo ur Cen(enary organized by the collodion in which the sensitive material is Colour Group and the Inter·Society Colour Council of America on 16-18 May 1961 , in London. iodide. While the actual sensitivity of MS. received 24 May 1961 wet collodion does not appear to have been published 244 The Journal of Photographic Science, Vol. 9, 1961 (probably because of its extremely low sensitivity at found through Sir William Pope that the original any wavelength) the sensitivity to light of silver positives used by Maxwell were still in existence at iodide in other media is well known•. It is sensitive the Cavendish Laboratories. Spencer borrowed from the shortest to about 430mp., these positives and published a colour reproduction where it cuts off sharply; there is no sensitivity at of the projected appearance of the picture in the above all for wavelengths longer than about 430mp. in the cited article. In this reproduction are seen , extreme blue. , and and the background is To get his colour separation Sutton used for the distinctly green. red, green and blue filters glass cells filled with In pursuance of our problem (since Spencer's liquids well known to chemists of the time, and for copies had been destroyed during the war) copy the yellow a piece of "-coloured glass." The positives were obtained by the writer through the yellow glass we cannot identify but its exact nature courtesies of Dr. Spencer, the Cavendish Laboratories is rather incidental to the present problem. The and Kodak Limited. other colours we have from Sutton's own description. Since the chemicals of the filter solutions and the "The experiments were made out-of-doors, in a times of were known, all that was needed good light, and the results were as follow:- to repeat the "curious experiments which our readers lst. A plate-glass bath, containing the ammoniacal will like to hear about" (Sutton) was a photographic sulphate of which chemists use for the blue material having the same sensitivity distribution as solution in the bottles in their windows, was first wet collodion. This material was kindly supplied placed immediately in front of the lens. With an to the writer by Dr. Burt Carroll of the Kodak exposure of six seconds a perfect was Research Laboratories, Rochester. obtained. This exposure was about double that This new material, of course, was of a different required when the coloured solution was removed. "speed" than that used by Sutton, but Sutton had 2nd. A similar bath was used, containing a green carefully noted that with the blue filter the exposure solution of chloride of copper. With an exposure was twice that without any filter. This is the necessary of twelve minutes not the slightest trace of a negative clue to the determination of all concentrations, none was obtained, although the image was clearly visible of which were stated. (Sutton evidently was no upon the ground glass. It was therefore found chemist.) advisable to dilute the solution considerably; and Accordingly, successive trial exposures at different by doing this, and making the green tinge of the concentrations were made for each of the three water very much paler, a tolerable negative was solutions until the ratios of all exposures were the eventually obtained in twelve minutes. same as those used by Sutton. The blue concentration 3rd. A sheet of lemon-coloured glass was next was altered until it took twice the exposure time placed in front of the lens, and a good negative required without a filter to get a "perfect negative." obtained with an exposure of two minutes. The concentration of the green copper chloride was 4th. A plate-glass bath, similar to the others, and decreased (to "very much paler") until a "tolerable containing a strong red solution of sulphocyanide of negative was eventually obtained," at 120 times that iron was next used, and a good negative obtained of the blue filter exposure. Similarly the red solution with an exposure of eight minutes." was modified until a "good negative" was obtained "The thickness of fluid through which the light with an exposure 80 times that of the blue. had to pass was about three-quarters of an inch." It is interesting that copper chloride has since "The negatives taken in the manner described been used as the classical example of a solution were printed by the Tannin process upon glass, and that changes colour (from green toward blue) as exhibited as transparencies. The picture taken through the solution is diluted. the red medium was at the lecture illuminated by The fact that images were obtained at all, of course, red light,- that through the blue medium hy blue indicates that all the filters transmit light of wavelength light,- that through the yellow medium by yellow shorter than 430mp.. light,-and that through the green medium by green Spectrophotometric curves were then run on all light; and when these different coloured images the solutions, as used, with the results shown in were superposed upon the screen a sort of photograph Fig. I. At the left is shown the cut-off caused by glass of the striped ribbon was produced in the natural of the approximate thickness of that in Sutton's lens. colours." At 430ml--' is shown the cut-off due to the film (In spite of Sutton's statement above, it is quite sensitiVIty. It is at once apparent that these filters clear from other sources that the positive from the rather neatly divide the blue and regions yellow filter was not used by Maxwell in his demonstra­ of the spectrum into three quite distinct bands, tion at the lecture.) although the green is contained within the blue. They In 1940 Dr. D. A. Spencer' wrote that he had are separation filters but for the short wave rather / Evans: Notes on Maxwell's Colour Photograph 245

100 100

90 90

80 8C /' I I \ I I I I I 70 RED CLOTH 70 I I I I I I I 0 .J. I I I ~ ~Green/ 60 60 I Red/ ~ I ~ .§ I I u: I I I ·= "'0 I c: I I "'0 50 50 c: ~ I I .E · ~ 'I I 0 I I I ~ c: 4i ; I I I I 40 e 40 a:: 1- II,, I I I I I Blue I ~ 30 30 11 I Biue I I\-' I I I I I I II 20 20 I I I I l I I I I I I I / I 10 10 I I \ I I \ / " 0 0 " 300 400 500 300 400 500 600 Wavelength A (millimicrons) Wavelength A (millimicrons)

Fig . /. Transmittance curves for the red, green and blue filters Fig. 2. Reflectance curve far a sample of red doth. and the /ens glass and a curve showing the long wavelength cut-off of the film (film sensitive to all shorter wavelengths).

than the visible spectrwn. It must be remembered in discovery of the first synthetic dye, there could have looking at these curves that the green exposure was been only three or four red dyes available, but pure 120 times and the red 80 times the blue exposure. samples of these were not located. However, a The curves shown have not been multiplied by these systematic study of a considerable number of red­ factors. dyed materials has revealed that nearly all of them The blue is seen to cover the near ultraviolet and have a secondary reflectance region in the ultraviolet. blue, and the green just reaches the longest wavelength A curve for such a material is shown in Fig. 2. blue to which the film is sensitive, but the red centres It is not too much to assume, therefore, that the beyond the blue in the ultraviolet. red used by Sutton had a relatively high reflectance Now one can imagine the blues to be nicely separated in the ultraviolet and, accordingly, photographed from other colours and that a good green might be through his "red" filter much as it would have if separated from blues or reds even though just barely his film were actually sensitive to red light. for some blues, but how about the reds? In fact, using our versions of his filters, fairly good The identity of the tartan displayed by the ribbon colour with reasonably good reds were has not been possible to established. Since the date actually produced. This result was also checked of the work was only two or three years after the using interference filters with narrow pass bands in 246 The Journal of Photographic Science, Vol. 9, 1961 the region 420mfL, 405m,u. and 382mfL, for green, blue It would seem, therefore, that the historical occasion and red respectively. had been reconstructed correctly and that Maxwell Incidentally, it is interesting that had Sutton had and his aide Sutton had in fact produced the first true panchromatic materials his choice of filters three-colour separation negatives and the first would have been excellent! The slight ultraviolet projected three-colour photograph. transmission of the red would have done no harm, A lingering doubt remained, however, that in some and the green form of copper chloride is an excellent way it was possible for Sutton's collodion plates to green separation filter. have had some trace of red and green sensitivity. In addition to this actual separation into three Indeed, it is now known that under certain unusual colour regions it was apparent from Maxwell's circumstances such sensitivities may occur even positives that a number of other forces were at work without using sensitizing dyes, which were not to add colour to his picture. In the first place the discovered until 1874. "tolerable" green negative was obviously rather These doubts were happily dispelled by a discovery badly underexposed. In the second place it was made one day when studying the reproduced Maxwell apparent that the contrasts of the three negatives transparencies. were quite different- a not unexpected effect con­ In making the photograph Sutton had used "a siderin the materials i portrait lens of full ." This could only have have been substituted for it with little change in the been a Petzval lens and this lens did not cover the result. plate used. That is, the image formed was restricted When projected on the screen with the projectors so to a circle of somewhat smaller area than the plate. balanced (through the same three filters) that the best The discovery was that the diameters of these circles colours were produced, these defects would add colours were not all equal. The blue positive had the smallest not otherwise present. For example, in Spencer's diameter, the green next, and the red the largest. It reproduction the "black velvet" comes out green. was apparent that Sutton had refocused for each Dr. Hunt of Kodak Limited reports that the colour of light and that for red light the lens had Maxwell positives are very yellow. If this was true been farthest away from the plate. at the time the picture was projected, a still further At the same time it was apparent, as had been variation of would have been introduced. noticed earlier, that the red image was the most out We cannot be certain of the light sources in his offocus of the three images. He had focused by visual "magic lanterns" because an "electric lamp" was red but photographed by the invisible ultraviolet! used as part of one of the other lecture demonstrations, The pieces of the puzzle thus all fit together nicely, but the typical "magic lantern" of the day was the and we can say that Maxwell's ingenuity in devising famous -light in which a block of calcium car­ a proof by the new technique of photography for an bonate was heated to by an oxy­ old theory of Young's on the three-colour nature hydrogen flame. (Imagine three of these with a bag of vision, coupled with Sutton's knowledge of photo­ of hydrogen and one of oxygen for each of them!) graphy and lenses, led to the invention and demonstra­ The colour of this source was around tion of colour photography some 20 years before it 3000 K . and would have seemed quite blue. It is was " possible." possible that electric carbon arcs powered by galvanic [t is to be regretted that Maxwell did not feel the cells were used but the result would be much the experiment was very successful, apparently because same. In either case the yellow colour of the positives he could not then, as we can now, also demonstrate would have given a far higher contrast picture through that yellow is not the correct primary for this kind the blue filter than through the red and the green of colour photography. would have been intermediate. So Sutton concluded that green foliage could not The effects obtained were not all due to contrast be reproduced by photography; Maxwell was and density mismatches, however. The existence disappointed that photography did not demonstrate of true colour separation among the red, green and that green was the correct primary; and we conclude blue pictures was demonstrated by superimposing that Maxwell invented three-colour photography. negatives of various contrasts made from say, the blue positive over each of the other two. This was done for all combinations. In no case was it possible References to "blank out" the image with a negative from a (I) Sutton, Thomas. " Photographic Notes," 6, # 125, June 15 ( 1861). different positive. There was less separation between (2) The Scient~fic Papers of , J. Hermann , Pari s. the green and blue than between the blue and red I, pp. 445, (1_890) . . (3) Mees, C. E. K. The Theory of the Photographic Proces>, 1st ed. as we would expect. Somewhat ironically considering The Macmillan Co., N.Y. ( 1942). Maxwell's main thesis, the yellow filter negative was (4) Spencer, D . A. " The First Colour Photograph," Penrose Ann ual, p. 9 ~ (1940), (with accompanying colour plate repro duction of the essentially the same as the green, and probably could Maxwell picture).