background against which we see the very same light, and we see that that is, in fact, what we call brown! Brown is always a dark color next to a lighter back ground. We can easily change the character of the brown. For example, if we take ut we get a reddish brown, apparently a chocolatey reddish and if we put more green into it, in proportion, we get that horrible color which all the uniforms of the Army are made of, but the light from that color is not so horrible by itself; it is of yellowish green, but seen against a light background Now we put a yellow filter in front of the fourth light and try to match that (The intensity must of course be within the range of the various lamps; we cannot match something which is too bright, because we do not have enough power in the lamp. But we can match the yellow; we use a green and red mixture, and put in a touch of blue to make it even more perfect. Perhaps we are ready to believe that, under good conditions, we can make a perfect match of any given color. Now let us discuss the laws of color mixture. In the first place we found that diferent spectral distributions can produce the same color; next, we saw that any"color can be made by adding together three special colors, red, blue, and green. The most interesting feature of color mixing is this: if we have a certain light, which we may call X, and if it appears indistinguishable from Y, to the eye (t may ectral distrib but it appears indistinguishable), call these colors"equal, " in the sense that the eye sees them as equal, and we write X= Y Here is one of the great laws of color: if two spectral distributions are indistinguish able, and we add to each one a certain light, say Z (if we write X+ Z, this means that we shine both lights on the same patch), and then we take y and add the same amount of the same other light, Z, the new mixtures are also indistinguishable X+Z=Y+Z (353) We have just matched our yellow; if we now shine pink light on the whole thing, it will still match. So adding any other light to the matched lights leaves a match In other words, we can summarize all these color phenomena by saying that once ye have a match between two colored lights, seen next to each other in the same circumstances, then this match will remain . and one light can be substituted for the other light in any other color mixing situation. In fact, it turns out, and it is very important and interesting, that this matching of the color of lights is not dependent upon the characteristics of the eye at the moment of observation: we know that if we look for a long time at a bright red surface or a bright red light, and then look at a white paper, it looks greenish, and other colors are also dis- torted by our having looked so long at the bi d if we now have a match between, say, two yellows, and we look at them and make them match, then we look at a bright red surface for a long time and then turn back to the yellow, it may not look yellow any more; i do not know what color it will look, but it will not look yellow. Nevertheless the yellows will still look matched, and so, as the eye adapts to various levels of intensity, the color match still works, with the obvious exception of when we go into the region where the intensity of the light gets so low that we have shifted from cones to rods then the color match is no longer a color match, because we are different system The second principle of color mixing of lights is this: any color at all can be made from three different colors, in our case, red, green, and blue lights. By suitably mixing the three together we can make anything at all, as we demonstrated with our two examples. Further, these laws are very interesting mathematically. For those who are interested in the mathematics of the thing, it turns out as follows Suppose that we take our three colors, which were red, green, and blue, but label them A, B, and C, and call them our primary colors. Then any color could be made by certain amounts of these three: say an amount a of color A, an amount b of color B, and an amount c of color C makes x: X=aA+ bb+ cC