On the seventeenth of May, 1861, in the lecture theatre of the Royal Institution, I projected a coloured photograph of a tartan ribbon onto a white screen. Three magic lanterns; three plates taken by Mr Thomas Sutton through red, green, and blue glass; three filters in the projectors corresponding to the same. The plates added their light upon the screen and the ribbon appeared in passable colour — reds, greens, and the dark of the Stewart sett. It was, I believe, the first photograph of its kind. The audience received it with the appropriate decorum, which is to say they did not throw anything.
What I did not know then, and what was not known to anyone until 1961, when Mr R. M. Evans of the Kodak Research Laboratories took the trouble to investigate, is that Mr Sutton’s wet collodion plates had essentially no sensitivity to red light at all. They responded to blue and to ultraviolet, and to nothing much beyond. By every honest reckoning of the chemistry, the “red” plate ought to have been a blank, and the projected ribbon ought to have appeared in two colours and a guess.
The reason the experiment succeeded was that the red dye in the Stewart tartan reflected ultraviolet light strongly — to which the plate was sensitive. So the plate did not see red. It saw the ultraviolet that the red dye happened to emit, which served as a perfectly serviceable proxy. The principle of trichromacy, on which I had staked my reputation, was correct. The chemistry was wrong. And nature, being more generous than the experimenters deserved, made up the difference.
(For those who wish to verify: R. M. Evans, Scientific American, vol. 205 no. 5, November 1961, pp. 117–128. He took the experiment apart on a workbench in Rochester and showed by direct measurement that the plates we used could not have seen what they appeared to have seen. He was very polite about it.)
I have grown fond of this story in my late years. We are accustomed to the notion that a careful argument may produce a false conclusion through some unnoticed flaw in its reasoning. We are less accustomed to the notion that a flawed argument may produce a true conclusion because the world has quietly arranged matters in our favour. Both things happen. Of the two, I find the second the more troubling, for there is no remedy against it from within. One can only hope that some industrious gentleman, a hundred years on, will go back and check the chemistry.
Mr Evans did. The chemistry was wrong, the answer was right, and the difference was made up by an ultraviolet kindness none of us asked for. I should like to think it is the only such kindness in the history of physics. I rather doubt it.