portion will lie somewhere on a line drawn between the two points. For instance, a fifty-fifty mixture would appear halfway between them, and 1/4 of one and 3/4 of the other would appear 1 4 of the way from one point to the other, and so on lmost all of the colors that we can ever see, because all the colors that we can ever off If we use a blue and a green and a red, as primaries, we see that all the colors that we can make with positive coefficients are inside the dotted triangle, which contains see are enclosed in the oddly shaped area bounded by the curve. Where did this of we can see against three special ones. But we do not have to check all colors that f-o area come from? Once somebody made a very careful match of all the colors that we can see, we only have to check the pure spectral colors, the lines of the spectrum Any light can be considered as a sum of various positive amounts of various pure spectral colors-pure the physical standpoint. A given light will ha tain amount of red, yellow, blue, and so on--spectral colors. So if we know how components, we can calculate how much of each is needed to make our given colg e a2 much of each of our three chosen primaries is needed to make each of these pur so, we find out what the color coefficients of all the spectral colors are for any。。。,。d, An example of such experimental results for mixing three lights together is given in Fig. 35-5. This figure shows the amount of each of three different particular Fig. 35-4. The standard chromaticity primaries,red,green and blue, which is required to make each of the spectral diagram. colors. Red is at the left end of the spectrum, yellow is next, and so on, all the way to blue. Notice that at some points minus signs are necessary. It is from such ta that it is possible to locate the position of all of the colors on a chart, where the x-and the y-coordinates are related to the amounts of the different primaries hat are used. That is the way that the curved boundary line has been found. It is the locus of the pure spectral colors. Now any other color can be made by adding spectral lines, of course, and so we find that anything that can be produced by connecting one part of this curve to another is a color that is available in nature. a The straight line connects the extreme violet end of the spectrum with the extreme red end. It is the locus of the purples. Inside the boundary are colors that can be made with lights, and outside it are colors that cannot be made with lights, and -oz 10 6 wAVeLENGTH m nobody has ever seen them(except, possibly, in after-images 35-5 The mechanism of color vision e color coel Now the next aspect of the matter is the question, why do colors behave in this pure spectral colors in terms of a certain set of standard primary colors way? The simplest theory, proposed by Young and Helmholtz, supposes that in the eye there are three different pigments which receive the light and that the have different absorption spectra, so that one pigment absorbs strongly, say, the red, another absorbs strongly in the blue, another absorbs in the green. Then when we shine a light on them we will get different amounts of absorptions in the three regions, and these three pieces of information are somehow maneuvered in the brain or in the eye, or somewhere, to decide what the color is. It is easy to demonstrate that all of the rules of color mixing would be a consequence of this proposition. There has been considerable debate about the thing because the next problem, of course, is to find the absorption characteristics of each of the three pigments. It turns out, unfortunately, that because we can transform the color coordinates in any manner we want to, we can only find all kinds of linear combinations of absorption curves by the color-mixing experiments, but not the curves for the individual pigments. People have tried in various ways to obtain a specific curve which does describe some particular physical property of the eye. One such curve is called a brightness curve, demonstrated in Fig. 35-3. In this two curves, one for eyes in the dark, the other for eyes in the light the latter is the cone brightness curve This is measured by finding what is the smallest amount of colored light we need in order to be able to just see it. This is in diff very interesting way to measure this. If we take two colors and make them appear in an area, by flickering back and forth from one to the other, we see a flicker if the frequency is too low. However, as the frequency increase 35-7