408 PROCEEDINGS OF THE AMERICAN ACADEMY 



It is, of course, yet possible that retinal purple is merely protective when 

 the retina is most sensitive ; viz., after rest. It is possibly connective 

 tissue between the disks. It is, again, quite possible that its changes 

 are connected with the perception of light and shade, and not at all 

 with that of color. On the one hand, the fact that pressure causes the 

 same sort of bleaching as light; that the color is chiefly confined to 

 the external members of the outermost retinal layer, while near the 

 vitreous body the cones and rods are nearly colorless ; the fact that 

 fluid solutions of retinal purple can be made only by a substance which 

 causes the disks to fall apart, in Kiihne's language, like a roll of coin 

 suddenly unfastened; its high refractive power, — all th^e focts point 

 decidedly toward the photo-physical explanation. On the other hand, 

 we must not forget that the fact that retinal purple can be dissolved 

 and filtered ; the fact that perception probably takes place long before 

 any sensible bleaching of the retina occurs ; that colors are represented 

 only by more or less paling, and never by changes of hue in the red 

 substance ; that it seems to be confined to the rods, and that pigment 

 cells and the colored oil drop in the base of the outer members seem 

 to undergo concomitant changes, perhaps regenerative, perhaps partici- 

 patory ; the fact that vinegar changes the red to instant yellow, — these 

 facts, while they do not seem, as Kiihne urges, to compel the belief 

 that the retina is a photo-chemical workshop, or that the red is inde- 

 pendent of all structural changes, do show us that we are here on the 

 boundary line between chemistry and physics, and that the interpreta- 

 tion of each may be partial. Observations on retinal purple, and the 

 fact that continued pressure on one eye causes all objects for a time to 

 seem tinged with violet, would incline us to believe that the action of 

 light on the cone-disks causes increased tension, possibly shortening 

 instead of, or more probably along with, sympathetic vibration.* 



* On the photo-pliysical liypotliesis, the ahnost constant action of red rays 

 from the blood would tend to relax tension, — perhaps bringing a large number 

 of disks into distances from each other corresponding to the lengtli of the red 

 instead of to tliat of the shorter waves, — and the retinal purple miglit then be 

 due to the same cause as tlie color of thin plates ; while the white wliicli occurs 

 with protracted illumination of a fixed eye would be physiologically analogous 

 to tetanus ; and the observer would see all the colors superimposed, produc- 

 ing the impression of white. This, again, would indicate that the phenomena 

 of fatigue are to be explained, in part at least, by nervous exhaustion, and not 

 entirely by failure of mechanical response in the terminal apparatus ; and 

 thus we are brought, by a very direct, scientific path, to the old question of 

 phosphorescent effects in the eye. 



