_Color-Mixture._ If, on a rotating motor, disks of different colors--say red and yellow--are placed and rotated, one sees on looking at them not red or yellow but orange. This phenomenon is known as _color-mixture_. The result is due to the simultaneous stimulation of the retina by two kinds of ether vibration. If the colors used are a certain red and a certain green, they neutralize each other and produce only gray. All the pairs of complementary colors mentioned above act in the same way, producing, if mixed in the right proportion, no color, but gray. If colored disks not complementary are mixed by rotation on a motor, they produce an intermediate color. Red and yellow give orange. Blue and green give bluish green. Yellow and green give yellowish green. Red and blue give violet or purple, depending on the proportion. Mixing pigments gives, in general, the same results as mixing by means of rotating the disks. The ordinary blue and yellow pigments give green when mixed, because each of the two pigments contains green. The blue and yellow neutralize each other, leaving green. _Visual After-Images._ The stimulation of the retina has interesting after effects. We shall mention here only the one known as _negative after-images_. If one will place on the table a sheet of white paper, and on this white paper lay a small piece of colored paper, and if he will then gaze steadily at the colored paper for a half-minute, it will be found that if the colored paper is removed one sees its complementary color. If the head is not moved, this complementary color has the same size and shape as the original colored piece of paper. The negative after-image can be projected on a background at different distances, its size depending on the distance of the background. The after-image will be found to mix with an objective color in accordance with the principles of color-mixture mentioned above. After-image phenomena have some practical consequences. If one has been looking at a certain color for some time, a half-minute or more, then looks at some other color, the after-image of the first color mixes with the second color. _Adaptation._ The fact last mentioned leads us to the subject of adaptation. If the eyes are stimulated by the same kind of light for some time, the eyes become adapted to that light. If the light is yellow, at first objects seem yellow, but after a time they look as if they were illuminated with white light, losing the yellow aspect. But if one then goes out into white light, everything looks bluish. The negative after-image of the yellow being cast upon everything makes the surroundings look blue, for the after-image of yellow is blue. All the other colors act in a similar way, as do also black and white. If one has been for some time in a dark room and then goes out to a lighter place, it seems unusually light. And if one goes from the light to a dark room, it seems unusually dark.