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Young's Lucky Guess
 

Hermann von Helmholtz was not yet eight years old when Thomas Young died, yet their names are joined for all time as the progenitors of the classical Young-Helmholtz theory of human color vision. More than six decades after Young published On the Theory of Light and Colors Helmholtz expanded the hypothesis into a more detailed theory of color vision.

Looking back from our viewpoint, Young's theory seems inspired, indeed. He achieved it principally through experimentation with color mixture. And he worked prior to the invention of the ophthalmoscope, so he never was able to examine a functioning retina.

In his 1802 treatise, Young considered how the retina might be constructed: "As it is almost impossible to conceive each sensitive point of the retina to contain an infinite number of particles, each capable of vibrating in perfect unison with every possible undulation [color], it becomes necessary to suppose the number limited, for instance to the three principal colors." He went on to speculate that the retina might consist of receptors tuned to red, green, and violet light. He had no way of experimentally determining the validity of his assertion. Nonetheless, he was tantalizingly close to the truth.

And how does Young's guess fare today? It does pretty well in one respect, but not so well in another. We've learned that there are three kinds of cone cells in the eye. (Cones are responsible for our color vision.) We deduce their sensitivity by measuring how they absorb various colors of light. And those three kinds of cones do indeed appear to have three distinctly different ranges of color sensitivity. Give Thomas Young a gold star!

What's the problem, then? Well, it is in Young's guess about the specific colors to which the cones are sensitive. Young guessed peak sensitivities to red, green and blue (violet) light. Modern investigations suggest peak sensitivities to yellow , green and blue light. None of the three types of cones absorbs very much red light.

You may not be surprised that Young missed the exact sensitivities of the three types of receptors. After all, it was 1802. Doesn't he get some points for two out or three? And you may wonder if the entire question is even important. But the difference in Young's guess and the actual sensitivities raises some profound questions.

Young's version was quite logical. He had each type of receptor being sensitive to about one third of the available spectral light. Very neat. The reality isn't so tidy, though, and the real situation seems to fail in accounting for some phenomena of human vision.

It's problematic that none of the cones has a peak sensitivity to the red region of the spectrum, because we see red light rather well. Our eyes are more sensitive to yellow light than to other colors. They are least sensitive to blue. The high sensitivity to yellow makes sense, because two of the types of cones have their peak sensitivities in the yellow-green region of the spectrum. But the fact that we see red light much better than we see blue light seems to be in conflict with how the cones actually absorb light energy. Hmmm.

Does the discrepancy between Young's guess and the actual spectral sensitivity of the cones create problems for the Young-Helmholtz theory? Well, it begins to.

And, what is the significance of Young's lucky guess, anyway? Where did it take us? We'll see that it set the stage for three important occurrences. First, it prepared James Clerk Maxwell to misunderstand what was happening in his photographic experiments , a half-century later. Second, it allowed Helmholtz to perceive Young's and Maxwell's two near-misses as big hits in postulating what became the Young-Helmholtz theory. And third, it all fits so neatly together that it may have delayed our learning the truth about human color vision for a hundred years or more.

Young would probably have been appalled to find his almost casual assertion accepted so blindly. I think he would have expected us to be far more skeptical, and to probe much deeper for an understanding of the miracle of color vision.

© J. C. Adamson, 1997