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Historical and Theoretical Data 3
apparent difference will be that the copy will be more subdued, or less pure in tint than the original. Here, however, we have the process performed twice — first on the screen, and then on the retina."
It would seem legitimate to deduce from this passage that Clerk Maxwell had in mind the photographic recording of the colors according to the Young-Helmholtz sensation curves. Therefore, it may be considered as an anticipation of the theory later patented by Ives.
In a subsequent lecture8 this subject was elaborated and an attempt made to actually reproduce a colored object. Thos. Sutton carried out the photographic work for Clerk Maxwell and gives the following account of the same:9 "A bow made of ribbon, striped with various colors, was pinned upon a background of black velvet, and copied by photography by means of a portrait lens of full aperture, having various colored fluids placed immediately in front of it, and through which the light from the object had to pass before it reached the lens. The experiments were made out of doors, in a good light, and the results wTere as follows: — A plate-glass bath, containing the ammoniacal sulfate of copper, which chemists use for the blue solution in the bottles in their windows, was first placed immediately in front of the lens. With an exposure of six seconds a perfect negative was obtained. This exposure was about double that required when the colored solution was removed. 2nd. A similar bath was used, containing a green solution of chloride of copper. With an exposure of twelve minutes not the slightest trace of a negative was obtained, although the image was clearly visible on the ground-glass. It was, therefore, found desirable to dilute the solution considerably; and by doing this, and by making the green tinge of the water very much paler, a tolerable negative was eventually obtained in twelve minutes. 3rd. A sheet of lemon-colored glass was next placed in front of the lens, and a good negative obtained with an exposure of two minutes. 4th. A plate-glass bath, similar to the others, and containing a strong red solution of sulfo-cyanide of iron was next used, and a good negative obtained with an exposure of eight minutes. It is impossible to describe in words the exact shades of color, or intensity of these solutions. The thickness of the fluid through which the light had to pass was about three-quarters of an inch. The collodion was simply iodized, the bath neutral, and the developer pyrogallic acid. The chemicals were in a highly sensitive state, and good working order, producing clean and dense negatives, free from stains and streaks in all cases. The negatives taken in the manner described were printed by the tannin process upon glass, and exhibited as transparencies. The picture taken through the red medium, was at the lecture illuminated by red light — that through the blue medium, by blue light — that through the yellow medium, by yellow light, and that through the green medium, by green light; — and when these different-colored images were superimposed upon a screen, a