sensation corresponding to a mixture of red and green, yet in nature, yellow tints are so much brighter than greens, that they excite the green sensation more than green objects themselves can do; and hence greens and yellows are called yellow by such color-blind persons, though their perception of yellow is really the same as perception of green by normal eyes. Later on, by a combination of adjustable slits, prisms, and lenses arranged in a 'color box,' Maxwell succeeded in mixing, in any desired proportions, the light from any three portions of the spectrum, so that he could deal with pure spectral colors instead of the complex combinations of differently colored lights afforded by colored papers. From these experiments, it appears that no ray of the solar spectrum can affect one color sensation alone, so that there are no colors in nature so pure as to correspond to the pure simple sensations, and the colors occupying the angular points of Maxwell's diagram affect all three color sensations, though they influence two of them to a much smaller extent than the third. A particular color in the spectrum corresponds to light which, according to the undulatory theory, physically consists of waves, all of the same period; but it may affect all three of the color sensations of a normal eye, though in different proportions. Thus yellow-light of a given wave-length affects the red and green sensations considerably and the blue (or violet) slightly, and the same effect may be produced by various mixtures of red or orange and green." For his researches on the perception of color, the Royal Society awarded Clerk Maxwell the Rumford Medal in 1860. Besides this more or less theoretic work, however, Maxwell made some interesting and important discoveries and inventions in optics. For instance, he noted the great differences that exist in the eyes of dark and fair complexions to different colors when the light falls upon the center of the yellow spot, the so-called fovea centralis, or central pit of the retina. His researches with regard to this led him to the discovery that this portion of the retina is largely lacking in sensibility to blue light. He was able to demonstrate this by his experiment of looking through a glass vessel containing a solution of chrome alum, when the central portion of the field of vision appears of a light red color for the first second or two. He was also the inventor of an ingenious optical apparatus, a real image stereoscope. A still more important discovery was that of the double refraction which is produced for the time in viscous liquids when they are stirred and their motion is not as yet stopped. Maxwell showed that Canada balsam, for instance, when stirred, acquired a distinct power of double refraction, which it retained so long as the stress in the fluid produced by stirring remained.