colored wool. Of course, the same mistakes will be made here as in the preceding method.
Another method of examination rests on the phenomena of what are called contrast colors. When a white surface is illuminated simultaneously by a red and a white light—as by two lamps, for example, before one of which a red glass is held—an object, a pencil, for instance, held midway between the two will cast two shadows, one from the red light and another from the white light. To one of normal color-perception, one of these shadows (that cast by the white light) will be red, while the other (that cast by the red light) will be green; to any one blind for either one of these colors, there will be no difference in the color of the shadows. If rings cut from black or gray paper are laid upon red or green paper and the whole is covered with tissue-paper, the rings will have a reddish tinge if the ground is green, and green if the ground is red. If, however, the individual is blind to either of these colors, no such difference will be noted; and, if letters cut from black or gray paper are used instead of rings, they can not be distinguished when laid on the colored ground and covered with the tissue-paper.
Another method is to make letters of certain colors on different colored grounds—shades of red letters, for instance, on a green ground. When these are of the requisite tints, the color-blind person is not able to distinguish them.
There are other methods, but they are all modifications to a greater or less extent of the foregoing, and any one who is color-blind to any considerable degree can be detected by any one, or at least by any two, of the methods indicated.
There is another theory of colors brought forward within the last few years by Professor Hering, of Prague, which is adhered to by many physiologists, and is a vigorous rival of the Young-Helmholtz theory. Professor Hering assumes that there are three chemical visual substances in the retina, which he calls the black-white, the red-green, and the blue-yellow. Light acts upon these substances by what he calls assimilation (A), and dissimilation (D). When light acts in a dissimilating or decomposing manner on the black-white substance, the sensation of white is produced; when there is an assimilation or regeneration of this substance, the sensation is black. Hering is by no means certain which are the A-and which the D-colors, but he is disposed to regard red as the dissimilating color of the red-green substance, and green the assimilating color. Blue, he thinks, causes dissimilation of the blue-yellow substance, while its regeneration is caused by yellow. All colors, he supposes, act in a dissimilating manner on the black-white substance—that is, they produce the sensation of white in addition to their own peculiar color. They act, however, in varying degrees of intensity, yellow acting with the greatest power, the strength of action diminishing toward the two ends of the spectrum.