the ficker will ultimately disappear at a certain frequency that depends on the brightness of the light, let us say at 16 repetitions per second. Now if we adjust the brightness or the intensity of one color against the other, there comes an intensity where the flicker at 16 cycles disappears, To get ficker with the brightness so adjusted, we have to go to a much lower frequency in order to see a flicker of the color. So, we get what we call a flicker of the brightness at a higher frequency and It a lower frequency, a flicker of the color. It is possible to match two colors for qual brightness"by this flicker technique. The results are almost, but not exac the same as those obtained by measuring the threshold sensitivity of the eye for seeing weak light by the cones. Most workers use the ficker system as a definition Now, if there are three color-sensitive pigments in the eye, the problem is to determine the shape of the absorption spectrum of each one. How? We know there are people who are color blind--eight percent of the male population, and one-half of one percent of the female population. Most of the people who are color blind or abnormal in color vision have a different degree of sensitivity than others to a variation of color, but they still need three colors to match. However, there are some who are called dichromats, for whom any color can be matched using only two primary colors. The obvious suggestion, then, is to say that they are missing one of the three pigments. If we can find three kinds of color-blind dichro mats who have different color-mixing rules, one kind should be missing the red another the green, and another the blue pigmentation. By measuring all these types we can determine the three curves! It turns out that there are three types of dichro matic color blindness; there are two common types and a third very rare type and from these three it has been possible to deduce the pigment absorption spectra radar Fig. 35-6. Loci of colors confused by Fig. 35-7. Loci of colors confused euterano pes. otanopes Figure 35-6 shows the color mixing of a particular type of color-blind person called a deuteranope. For him, the loci of constant colors are not points, but certain lines, along each of which the color appears to him to be the same. If the theory that he is missing one of the three pieces of information is right, all these lines should intersect at a point. If we carefully measure on this graph, they do intersect perfectly. Obviously, therefore, this has been made by a mathematician and does not represent real data! As a matter of fact, if we look at the latest paper with real data, it turns out that in the graph of Fig. 35-6, the point of focus of all the lines is not exactly at the right place. Using the lines in the above figure, we cannot find reasonable spectra; we need negative and positive absorptions in difTerent regions. But using the new data of Y ustoa, it turns out that each of the absorption curves is everywhere positive