COLOR-BLINDNESS AND COLOR-PERCEPTION. 89 



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 simul- 

 taneously 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 in- 

 stance, 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 differ- 

 ence 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 

 theorv. Professor Hering assumes that there are three chemical vis- 

 ual substances in the retina, which he calls the black-icJiite, the red- 

 green, and the Mue-ydlow. 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 regen- 

 eration 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 dis- 

 posed to regard red as the dissimilating color of the red-green sub- 

 stance, and green the assimilating color. Blue, he thinks, causes dis- 

 similation 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 vary- 

 ing degrees of intensity, yellow acting with the greatest power, the 

 strength of action diminishing toward the two ends of the spectrum. 



