PROPERTIES OF THE RETINA. 



359 



rays act more or less on all of the substances, and the resulting red 

 or violet sensation, is, therefore, mixed to some extent with white, — 

 that is, is not entirely saturated. The theory, as stated by Helm- 

 holtz, held strictly to the doctrine of specific nerve energy, in assuming 

 that each photochemical substance serves simply as a means for the 

 excitation of a nerve fiber, and that the quality of the sensation 

 aroused depends on the ending of this fiber in the brain. The phe- 

 nomenon of negative after-images finds a simple explanation in terms 

 of this theory. If we look fixedly at a green object, for example, 

 the corresponding photochemical substance is chiefly acted upon, and 

 if subsequently the same part of the retina is exposed to white light, 

 the red and violet substances, having been previously less acted 

 upon, now respond in greater proportions to the white light, and 



Fig. 153.— Schema to illustrate the Young-Helmholtz theory of color vision.— (/^eZm- 

 holtz.) The spectral colors are arranged in their natural order, — red to violet. The curves 

 represent the intensity of stimulation of the three color substances: 1, The red perceiving 

 substance; 2, the green perceiving; 3, the violet perceiving. Verticals drawn at an" 

 point of the spectrum indicate the relative amount of stimulation of the three substances, 

 for that wave length of the spectrum. 



the after-image takes a red- violet — that is, purple — color. Many 

 objections have been raised to the Young-Helmholtz theory. It 

 has been urged, for instance, that we are not conscious that white 

 or yellow sensations are blends or compounded color sensations; 

 we perceive in them none of the supposed component elements as 

 we do in such undoubted mixtures as the blue-greens or the purples. 

 The theory explains poorly or not at all the fact that on the periphery' 

 of the retina we are color blind and yet can perceive white or gray, 

 and it breaks down also in the face of the facts of partial and com- 

 plete color blindness. The explanation given for black is also 

 unsatisfactory in that it assumes an active state of consciousness 

 associated with a condition of rest in the visual mechanism. 



//. Hering's Theory of Color Vision.— 1\Ab theory also assumes 

 the existence in the retina of three photochemical substances, but 

 of such a nature as to give us six different qualities of sensation. 

 There is a white-black substance which when acted upon by the 



