THE SENSE OF VISION. 781 



painted upon different sectors. This is most readily accomplished by using 

 a number of disks, each painted with one of the colors to be experimented 

 with, and each divided radially by a cut running from the centre to the circum- 

 ference. The disks can then be lapped over each other and rotated together, and 

 in this way two or more colors can be mixed in any desired proportions. This 

 method of mixing colors depends upon 

 the property of the retina to retain an 

 impression after the stimulus causing 

 it has ceased to act a phenomenon of 

 great importance in physiological optics, 

 and one which will be further discussed 

 in connection with the subject of " after- 

 images." 



The physiological mixing of Colors FIG. 239. Diagram to illustrate color mixture by 



cannot be accomplished by the mixture reflected and transmitted "* (Heimhoitz). 

 of pigments or by allowing sunlight to pass successively through glasses of 

 different colors, for in these cases rays corresponding to certain colors are 

 absorbed by the medium through which the white light passes, and the phe- 

 nomenon is the result of a process of subtraction and not addition. Light 

 reaching the eye through red glass, for instance, looks red because all the rays 

 except the red rays are absorbed, and light coming through green glass appears 

 green for a similar reason. Now, when light is allowed to pass successively 

 through red and green glass the only rays which pass through the red glass 

 will be absorbed by the green. Hence no light will pass through the combi- 

 nation of red and green glass, and darkness results. But when red and green 

 rays are mixed by any of the three methods above described the result of this 

 process of addition is not darkness, but a yellow color, as will be understood 

 by reference to the color diagram on p. 780. In the case of colored pigments 

 similar phenomena occur, for here too light reaches the eye after rays of cer- 

 tain wave-lengths have been absorbed by the medium. This subject will be 

 further considered in connection with color-theories. 



Color-theories. From what has been said of color-mixtures it is evident 

 that every color sensation may be produced by the mixture of a number of 

 other color sensations, and that certain color sensations viz. the purples can 

 be produced only by the mixture of other sensations, since there is no single 

 wave-length corresponding to them. Hence the hypothesis is a natural one 

 that all colors are produced by the mixture in varying proportions of a certain 

 number of fundamental colors, each of which depends for its production upon 

 the presence in the retina of a certain substance capable of being affected 

 (probably through some sort of a photo-chemical process) by light of a certain 

 definite wave-length. A hypothesis of this sort lies at the basis of both the 

 Young-Helmholtz and the Hering theories of color sensation. 



The former theory postulates the existence in the retina of three substances 

 capable of being affected by red, green, and violet rays, respectively i. e. by 

 the three colors lying at the three angles of the color diagram given on p. 780 



