LIGHT. *43 The accompanying figure represents a chromatrope with an arrange¬ ment for quickly replacing one disk by another of different pat¬ tern. Rotation is given by fric¬ tion pulleys. With this form there is a disk with the so-called seven primary colors to illus¬ trate Newton’s theory of colors, 3. & <3 4 one to illustrate Brewster’s theory, two to illustrate Young’s theory, and a chameleon top, designed by President Morton, of Stevens Institute, Hoboken. The effects with all the forms of chromatropes are due to persistence of vision. Interesting subjective effects may be observed by projecting in the ordinary way bits of colored glass an inch or two square, so as to have upon the screen a large patch of color with a boundary of white light. The eyes must be fixed attentively upon the colored patch for about half a minute, when the colored piece must be quickly removed, the eyes to be kept meanwhile upon the screen. To prevent the eyes from unconsciously wandering while looking, it will be found advisable to pin a large black button or a piece of black paper to the screen in the middle of the disk. This is to be kept in the centre of vision. The effects observed will of course depend upon the color upon the screen, and