"It has been stated that Thomas Young propounded a theory of color-vision which assumes that there exists three separate color sensations, corresponding to red, green and violet, each having its own special organs, the excitement of which causes the perception of the corresponding color, other colors being due to the excitement of two or more of these simple sensations in different proportions. Maxwell adopted blue instead of violet for the third sensation, and showed that, if a particular red, green, and blue were selected and placed at the angular points of an equilateral triangle, the colors formed by mixing them being arranged as in Young's diagram, all the shades of the spectrum would be ranged along the sides of this triangle, the center being neutral grey. For the mixing of colored lights, he at first employed the color top; but instead of painting circles with colored sectors, the angles of which could not be changed, he used circular discs of colored paper slit along one radius. Any number of such discs can be combined so that each shows a sector at the top, and the angle of each sector can be varied at will by sliding the corresponding disc between the others. Maxwell used discs of two different sizes, the small discs being placed above the larger on the same pivot, so that one set forked a central circle and the other set a ring surrounding it. He found that, with discs of five different colors, of which one might be white and another black, it was always possible to combine them so that the inner circle and the outer ring exactly matched. From this he showed that there could be only three conditions to be satisfied in the eye, for two conditions were necessitated by the nature of the top, since the smaller sectors must exactly fill the circle and so must the larger. Maxwell's experiments, therefore, confirmed, in general, Young's theory. They showed, however, that the relative delicacy of the several color sensations is different in different eyes, for the arrangement which produced an exact match in the case of one observer, had to be modified for another; but this difference of delicacy proved to be very conspicuous in color-blind persons, for in most of the cases of color-blindness examined by Maxwell the red sensation was completely absent, so that only two conditions were required by color-blind eyes, and a match could therefore always be made in such cases with four discs only. Holmgren has since discovered cases of color-blindness in which the violet sensation is absent. He agrees with Young in making the third sensation correspond to violet rather than blue. Maxwell explained the fact that persons color-blind to the red divide colors into blues and yellows, by the consideration that, although yellow is a complex