886 



THE SENSE OF SIGHT 



furnish an adequate explanation for this function, but simply leads to 

 certain assumptions which have been embodied in the theories now 

 to be discussed. l It must be emphasized, however, that the latter are 

 really mere hypotheses lacking a sound experimental basis. All of 

 them assume the existence in the retina of certain fundamental sub- 

 stances which are instrumental in effecting the primary sensations of 

 color, and the only difference between them really lies in the manner 

 in which these pigments are distributed. 



The Young-Helmholtz theory which was first advocated by Young but has later 

 on been greatly elaborated by Helmholtz, 2 assumes the presence of three primary 

 color sensations, designated as red, green and violet (Fig. 484, 1, 2 and 3). These 

 sensations arise in consequence of the activation of three separate photo-chemical 



Y <T 6 V 



FIG. 485. DIAGRAM TO ILLUSTRATE THE HERING THEORY OF COLOR VISION. 

 The curves indicate the relative intensities of stimulation of the three color substances 

 by different parts of the spectrum. Ordinates above the axis, X-X, indicate catabolic 

 changes (dissimilation), those below anabolic changes (assimilation). Curve a 

 represents the conditions for the black-white substance. It is stimulated by all the 

 rays of the visible spectrum with maximum intensity in the yellow. Curve c represents 

 the red-green substance, the longer wave lengths causing dissimilation (red), the 

 shorter ones assimilation (green). Curve 6 gives the conditions for the yellow- blue 

 substance. (Foster.) 



substances which, on being struck by the rays of light, undergo a decomposition 

 and generate nerve impulses peculiar to each of them. The red substance is 

 reduced by the rays of long wave-length, the green substance by rays of medium 

 wave-length, and the violet substance by rays of short wave-length. When these 

 chemical elements are excited in an equal measure, the result is the sensation of 

 white or gray, while no stimulation at all yields black. The other sensations of 

 color are compound in their nature, i.e., they are dependent upon the joint stimu- 

 lation of all three substances in different proportions. Thus, yellow is the result 

 of an excitation of the red and green elements and blue, the result of an activation 

 of the green and violet substances. 



The Hering theory of color vision assumes the presence of four primary sensa- 

 tions of color, namely, red, yellow, green and blue. These sensations, however, are 

 supposed to be produced by two groups of photo-chemical substances, namely, red- 



1 Calkins, Archiv fur Physiol., 1902, 244. 



2 Handb. der physiol. Optik, Berlin, 1896. 



