The most powerful psycho-physical technique in color judgment is to use the eye as a null instrument. That is, we do not try to define what consititutes a green sensation, or to measure in what circumstances we get a green sensation because it turns out that this is extremely complicated. Instead, we study the conditions under which two stimuli are indistinguishable. Then we do not have to decide whether two people see the same sensation in different cir cumstances, but only whether, if for one person two sensations are the same, they are also the sanhe for another. We do not have to decide whether, when one sees something green, what it feels like inside is the same as what it feels like inside someone else when he sees something green; we do not know anything about that. To illustrate the possibilities, we may use a series of four projector lamps which ave filters on them, and whose brightnesses are continuously adjustable over vide range: one has a red filter and makes a spot of red light on the screen, the next one has a green filter and makes a green spot, the third one has a blue filter, and the fourth one is a white circle with a black spot in the middle of it. Now if we turn on some red light, and next to it put some green, we see that in the area of overlap it produces a sensation which is not what we call reddish green, but a new color, yellow in this particular case. By changing the proportions of the red and the green, we can go through various shades of orange and so forth. If we have set it for a certain yellow, we can also obtain that same yellow, not by mixing these two colors but by mixing some other ones, perhaps a yellow filter with white ight, or something like that, to get the same sensation. In other words, it is possible to make various colors in more than one way by mixing the lights from various filters What we have just discovered may be expressed analytically as follows. A particular yellow, for example, can be represented by a certain symbol Y, which is the"sum""of certain amounts of red-filtered light(R)and green-filtered light(G) By using two numbers, say r and g, to describe how bright the(r)and(G)are,we can write a formula for this yellow: Y=rR+gG. (351) The question is, can we make all the diferent colors by adding together two or hree lights of different, fixed colors? Let us see what can be done in that connec tion. We certainly cannot get all the different colors by mixing only red and green, because,for instance blue never appears in such a mixture. However, by putting some blue the central reg appear to be a fairly nice white. By mixing the various colors and looking at this central region, we find that we can get a considerable range of colors in that region by changing the proportions, and so it is not impossible that all the colors can be made by mixing these three co lights. We shall discuss to what extent this is true; it is in fact essentially correct, and we shall shortly see how to define the proposition better In order to illustrate our point, we move the spots on the screen so that they all fall on top of each other, and then we try to match a particular color which appears in the annular ring made by the fourth lamp. What we once thought was white"coming from the fourth lamp now appears yellowish. We may try to match nd blue as be by a kind of trial a error, and we find that we can approach rather closely this particular shade of " cream"color. So it is not hard to believe that we can make all colors. We shall try to make yellow in a moment, but before we do that, there is one color that might be very hard to make. People who give lectures on color make all the"bright colors, but they never make brown, and it is hard to recall ever having seen brown light. As a matter of fact, this color is never used for any stage effect, one never sees a spotlight with brown light; so we think it might be impossible to make brown In order to find out whether it is possible to make brown, we point out that brown light is merely something that we are not As a matter of fact, ke it by red and yellow. To prov that we are looking at brown light, we merely increase the brightness of the annular