Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 exptools 1/6/84; site ihima.UUCP Path: utzoo!linus!decvax!decwrl!amd!dual!zehntel!ihnp4!ihima!cmb From: cmb@ihima.UUCP (Christine M. Buss) Newsgroups: net.physics Subject: Re: colors Message-ID: <208@ihima.UUCP> Date: Wed, 1-Aug-84 17:04:27 EDT Article-I.D.: ihima.208 Posted: Wed Aug 1 17:04:27 1984 Date-Received: Fri, 3-Aug-84 01:59:35 EDT Organization: AT&T Bell Labs, Naperville, IL Lines: 37 >Human eyes cannot generally see "chords" of light. >Our ears are capable of doing a spectral analysis of sound, but we cannot >do the same with light. The ears can simultaneously detect a large range of >frequencies, our eyes can only detect (!) three. However, we are able to see >one "chord", the combination of blue and red which makes magenta. (Exclamation point mine) As someone who studies human color vision, I find these statements rather misleading. As you say, the ears can detect a large range of sound frequencies. They do a kind of Fourier analysis on the sound and retain information about each of the constituent frequencies in the sound. The eyes also can detect (and discriminate) a large range of light frequencies (from about 400 to 700 nm), not just three. Three light absorbing pigments with different spectral sensitivities (broad, overlapping, but not identical spectral sensitivities) report to the visual system the quantum catches they receive from any light. The visual system compares their three outputs to determine the "color" of the light. So the visual system can discriminate any two single wavelengths in the visible spectrum (except a pair very close together), but any two mixtures of wavelengths that produce the same quantum absorption in the three pigments look identical. I have no idea what to make of your use of the word "chord" with respect to vision. If a chord is simply a mixture of wavelengths, than of course we can _see_ them. Can we tell what the constituent wavelengths are? No. In the case of magenta, we can tell approximately what two wavelengths must be mixed to produce it, but only really on the basis of empirical color mixture experience. It's not at all like what the ear does, where the components of a chord are experienced as distinct in the mixture. Magenta looks like a single color, not like the superposition of two colors, red and blue. I'm not at all certain that the author of the quoted lines misunderstands color vision, but I think that one could easily read what he said and come away with some mistaken ideas of vision. I hope this makes some of the processes clearer.