Passage IV

NATUARAL SCIENCE: This passage is adapted from An Anthropologist on Mars by Oliver Snacks (©1995 by Oliver Sacks). Johann Wolfgang von Goethe was an eighteenth-century German poet and philosopher; Hermann von Helmholtz was a nineteenth-century scientist and philosopher.

Goethe’s color theory, his Farbenlehre (which he regarded as the equal of his entire poetic opus), was, by and large, dismissed by all his contemporaries and has remained in a sort of limbo ever since, seen as the whimsy, the pseudoscience, of a very great poet. But science itself was not entirely insensitive to the “anomalies” that Goethe considered central, and Helmholtz indeed, gave admiring lectures on Goethe and his science, on many occasions—the last in 1892. Helmholtz was very conscious of “color constancy”—the way in which the colors of objects are preserved, so that we can categorize them and always know what we are looking at, despite great fluctuations in the wavelength of the light illuminating them. The actual wavelengths reflected by an apple, for instance, will vary considerably depending on the illumination, but we consistently see it as red, nonetheless. This could not be ,clearly, a mere translation of wavelength into color. There had to be some way, Helmholtz thought, of “discounting the illuminant”—and this he saw as an “unconscious inference” or “an act of judgement” (though he did not venture to suggest where such judgement might occur). Color constancy, for him, was a special example of the way in which we achieve perceptual constancy generally, make a stable perceptual world form a chaotic sensory flux—a world that would not be possible if our perceptions were merely passive reflections the unpredictable and inconstant input that bathes out receptors.

Helmholtz’s great contemporary, James Clerk Maxwell, had also been fascinated by the mystery of color vision from his student days. He formalized the notions of primary colors and color mixing by the invention of a color top (the colors of which fused, when it was spun, to yield a sensation of grey), and a graphic representation with three axes, a color triangle, which showed how any color could be created by different mixtures of the three primary colors. These prepared the way for his most spectacular demonstration, the demonstration in 1861 that color photography was possible, despite the fact that photographic emulsions were themselves black and white. He did this by photographing a colored bow three times, though red, green , and violet filters. Having obtained three “color-separation” images, as he called them, he now brought these together by superimposing them upon a screen, projecting each image through its corresponding filter (the image taken though the red filter was projected with red light, and so on). Suddenly, the bow burst forth in full color. Clerk Maxwell wondered if this was how colors were perceived in the brain, by the addition of color-separation images or their neural correlates [what functions in the brain as a color-separation image], as in his magic-lantern demonstrations.

Clerk Maxwell himself was acutely aware of the drawback of this additive process: color photography had no way of “dis counting the illuminant,” and its colors changed helplessly with changing wavelengths of light.

In 1957, ninety-odd years after Clerk Maxwell’s famous demonstration, Edwin Land—not merely the inventor of the instant Land camera and Polaroid, but an experimenter and theorizer of genius—provided a photographic demonstration of color perception even more startling. Unlike Clerk Maxwell, he made only two black-and-white images (using a split-beam camera so they could be taken at the same time from the same viewpoint, through the same lens) and superimposed these on a screen with a double-lens projector. He used two filters to make the images: one passing longer wavelengths (a red filter), the other passing shorter wavelengths (a green filter). The first image was then projected though a red filter, the second with ordinary white light, unfiltered. One might expect that this would produce just an overall pale-pink image, but something “impossible” happened instead. The photograph of a young woman appeared instantly in full color— “blonde hair, pale blue eyes, red coat, bluegreen collar, and strikingly natural flesh tones,” as Land later described it. Where did these colors come from, how were they made ? They did not seem to be “in” the photographs or the illuminants themselves. These demonstrations, overwhelming in their simplicity and impact, were color “illusions” in Goethe’s sense, but illusions not “out there” in the world, nor (as classical theory held) an automatic correlate of wavelength, but, rather, are constructed by the brain.