362 THE SPECIAL SENSES. 



sensations result, and as this is the only reaction possible in the 

 rods these elements can furnish us only sensations of this quality. 

 The molecules of gray substance in the cones, on the other 

 hand, have undergone a development such that certain portions 

 only of the molecule may become dissociated by the action of light 

 of certain periods of vibration. This development may be sup- 

 posed to have taken place in two stages: first, the formation of 

 two groupings within the molecule, one of which is dissociated by 

 the longer waves and gives a sensation of yellow, and one of which 

 is dissociated by the shorter waves and gives the sensation of 

 blue. This stage remains still on portions of the periphery of the 

 retina, and is the condition present in the fovea also in the eyes 

 of the red-green blind. The second stage consists in the division 

 of the yellow component into two additional groupings in one 

 of which the internal movements are of such a period as to be 

 affected b}^ the longest visible waves, the red of the spectrum, 

 while the other is dissociated by rays corresponding to the green 

 of the spectrum and gives rise to the sensation of green. If 

 the red and green groupings are dissociated together the resulting 

 effect is the same as follows from the dissociation of the entire yellow 

 component, while the complete dissociation of the red, green, and 

 blue groupings gives the stimulus obtained originally from the disso- 

 ciation of the whole molecule, and causes gray sensations. The idea 

 of this subdivision or differentiation in structure of the original gray 

 substance is indicated diagrammatically in Fig. 155. The theory 

 accounts admirably for many phenomena in vision, and is perhaps 

 especially adapted to explain the facts of color blindness and the 

 variations in quality of our visual sensations in the peripheral areas 

 of the retina. An extension and modification of this theory 

 has been published by Schenck.* He assumes that each of the 

 three-color perceiving substances is composed of two parts. 

 One part which acts as a receiver for the stimulus, a sort of an 

 optical resonator, in fact, and a second part which is set into 

 activity by the receiver and gives rise to the corresponding 

 color sensation. The theory is very elastic in its adaptability 

 to the various kinds of color blindness. 



The two latter theories seem to imply that a number of different kinds 

 of impulses may be transmitted along the optic fibers. Hering's theory re- 

 quires apparently the possibiUty of six qualitatively different impulses, — 

 namely, white, black, red, green, yellow, and blue, — while the Franklin theory 

 assumes impulses corresponding to white (gray), red, green, yellow, and blue. 

 Black is not specifically accounted for except as a part of the gray series. At 

 present in physiology there is no proof that nerve impulses can differ quali- 

 tatively from each other, although it may be urged, perhaps with equal force, 

 that there is no proof that they can not so differ. The doctrine of specific 

 nerve energy assumes that nerve impulses are, as regards quality, always 



* Schenck, "Archiv f. d. gesammte Physiologic, " 118, 129, 1907. 



