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From: tjr@ihnet.UUCP (Tom Roberts)
Newsgroups: net.physics
Subject: colors
Message-ID: <152@ihnet.UUCP>
Date: Mon, 20-Aug-84 10:35:33 EDT
Article-I.D.: ihnet.152
Posted: Mon Aug 20 10:35:33 1984
Date-Received: Tue, 21-Aug-84 00:16:37 EDT
Organization: AT&T Bell Labs, Naperville, IL
Lines: 21

I am puzzled by the recent discussions of color vision, and the following:

A few years ago I attended a lecture/demonstration by Dr Land,
inventor of the Polaroid camera. During this demonstration he
displayed two (monochrome) slide photographs of the same image, one taken 
with a red (I think) filter, one with a green (I think) filter. He
displayed the two images side-by-side, using red and white
projectors. When he caused the two images to EXACTLY superimpose,
a FULL COLOR image appeared (suddenly!).

I find this striking demonstration hard to understand using a theory
that color vision is strictly frequency dependent - I have not made
any effort to do so (e.g. measuring the frequency spectrum of his white
projector). Superficially, it seems as though some sort of intereference
effect is being interpreted by the eye as "color" (though both projectors
were incoherent incandescent sources).

Does anyone know what is really going on?

		Tom Roberts
		ihnp4!ihnet!tjr