VISUAL PURPLE AND VISUAL YELLOW. 701 



If the choroid and the pigmentary layer of the retina be removed, the rods 

 are bleached, and the color is restored in the dark when the choroid is re- 

 placed. In the eye of the frog, kept in the dark, the hair-like processes 

 which extend from the pigmentary layer of the retina downward between the 

 rods and cones are retracted, and the pigment is then contained chiefly in the 

 cells themselves. After prolonged exposure of the retina to light, these pro- 

 cesses, loaded with pigment, extend between the cones as far as the limitary 

 membrane (Kiihne). 



The fact that visual purple has never been found in the fovea centralis is 

 opposed to the theory that its existence is directly essential to distinct vision ; 

 nevertheless, certain phenomena observed in passing from a bright light to 

 comparative obscurity, and the reverse, show that the purple has, at least, an 

 important indirect action. In passing from the dark to bright light, the eye 

 is dazzled and distinct vision is difficult. It may be assumed that this is due 

 to unusual general sensitiveness of the retina to light, on account of the ex- 

 cessive quantity of visual purple which has accumulated in the dark, and 

 that distinct vision is restored when the retina is bleached to a yellow, which 

 seems to be the most favorable condition for the exact appreciation of visual 

 impressions, under full illumination. On the other hand, it requires time for 

 the eye to become accustomed to a dim light ; and during this time the yel- 

 low is changing to purple. These changes in the color of the retina have 

 been actually observed (Ayres). Investigations of the absorption-spectra of 

 the purple and yellow have shown that the purple allows the actinic rays to 

 pass perfectly, while the yellow completely absorbs these rays (Kiihne). The 

 existence of visual purple seems to be most favorable to the imperfect and 

 shadowy vision which occurs under dim illumination, when the exact appre- 

 ciation of minute details is impossible. In the condition known as night- 

 blindness, it is probable that the visual purple has become exhausted beyond 

 the possibility of prompt restoration such as is normal ; and persons so affected 

 can not see at night, although minute vision under a bright light may not be 

 affected. In certain cases of this kind, the normal conditions may be re- 

 stored by a few days' seclusion in the dark. What is called functional night- 

 blindness frequently occurs in sailors during long, tropical voyages, and is 

 due to the excessive action of diffused light upon the retina. " That the 

 affection is local, is shown by the fact that darkening one eye, with a band- 

 age, during the day, has been found to restore its sight enough for the ensuing 

 night's watch on board ship, the unprotected eye remaining as bad as ever " 

 (Nettleship). 



The change of the visual purple to yellow is readily effected, but the 

 farther change to white is slower and more difficult. Conversely, the change 

 from white to yellow is slow and the change from yellow to purple is com- 

 paratively prompt. One use of the colors purple and yellow seems to be to 

 accommodate the retina for vision under different degrees of illumination. 

 The purple adapts the eye to a feeble illumination, and the yellow, to a full 

 illumination. This being the case, it is manifestly proper to speak of a 

 visual yellow (Kuhne) as well as of visual purple. 



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