336 THE SPECIAL SENSES. 



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. 146. 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. 



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

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

 quires apparently the possibility 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 

 the same, and differ from one another only in intensity, the qualitative differ- 

 ences that exist among sensations being referred to a difference in reaction 

 in the end-organ in the brain. 



Entoptic Phenomena. L T nder the term entoptic phenomena 

 is included a number of visual sensations due to the shadows of 

 various objects within the eyeball itself. Ordinarily these shadows 

 are imperceptible, owing to the diffuse illumination of the interior 

 of the eye through the relatively wide opening of the pupil. By 

 means of various devices the illumination of the eye may be so 

 controlled as to make these shadows more distinct and thus bring 

 the retinal images into consciousness. Some of these entoptic ap- 

 pearances are described briefly, but for a detailed description the 

 reader is referred to the classical work of Helmholtz.* 



The Blood-corpuscles. The entoptic images that are most easily 

 recognized perhaps are those of the moving corpuscles in the capil- 

 laries of the retina. If one looks off into the blue sky he will have 

 no difficulty in recognizing a number of minute clear and dark specks 

 that move in front of the eye in definite paths. The character of 

 the movement leaves no doubt that these sensations are due to the 

 shadows of the blood-corpuscles. In fact, the shadows often show 

 a rhythmic acceleration in velocity synchronous with the heart- 

 beats, a pulse movement. By projecting the moving images upon 

 a screen at a known distance from the eye the velocity of the capil- 

 lary circulation has been estimated in man. 



The Retinal Blood-vessels. The blood-vessels of the retina lie 

 in front of the rods and cones and must necessarily throw their 

 shadows upon these sensitive end-organs. The shadows may be 



* Helmholtz, "Handbuch der phvsiologischen Optik," second edition,. 

 I, 184. 



