440 Dynamic Theory. 



hausted and disintegrated by the light, and renewed by fresh accessions 

 of organic matter from the blood. The proof of exhaustion of any of 

 the tissues comes with the fact of such tissues losing their mobility un- 

 der their usual stimulus. Such mobility is lost for all the rods and 

 cones after a short exposure to direct sunlight or electric light, and this 

 means total blindness for the time. A brief period of rest in darkness 

 allows the blood vessels of the retina to replace the wasted tissues and 

 restore their mobility. If the eye be allowed to rest upon a colored 

 spot, as a red wafer, for example, until fatigued, and the vision then 

 transferred suddenly to a piece of white paper, a spot the size of the 

 wafer will appear on the paper, but it will be greenish instead of red. 

 And generally, when the eye is exhausted by looking at one color, up- 

 on looking at a white object an image of the same form but of the com- 

 plementary color will be seen. This proves that some of the cones are 

 sensitive to one sort of rays, while mingled with them are others sensi- 

 tive only to other rays. Thus, if all the cones in the tract covered by 

 the image of the wafer on the retina were alike, they would all be fa- 

 tigued, and when turned to look at white paper would see a black spot 

 instead of a green one. But only those sensitive to red being fatigued, 

 the rest are able to see their several colors reflected from the white 

 sheet, the resultant of their mixture being green. Outside of the spot, 

 all the cones being fresh, all the white will be seen, because each cone is 

 agitated by its own ray, and the complete union of all the color sensa- 

 tions is the sensation of white. It is not necessary to infer from this 

 that there are cones for every possible shade of color, or for every 

 shade of color that we can recognize. White light can be produced by 

 the combination of red, green and violet, and by unequal combinations 

 of these three, or of two of them, every possible shade of color may be 

 produced. Based on this fact, a theory has been proposed by Thos. 

 Young, and accepted by Helmholtz and others, that there are three 

 kinds of cones susceptible to these three colors severally, and that their 

 agitation in varying degrees produces, when the sensations are inter- 

 mingled, a sense of some one of the thousand possible shades. 



Thus, yellow light shining into the eye would agitate strongly the 

 cones sensitive to red and green, and slightly those sensitive to violet. 

 The several agitations thus derived from one ray would produce in the 

 brain the single sensation of yellow after the reconsolidation and mix- 

 ture of the three. In whatever manner any shade of color should be 

 broken up in contact with these three kinds of cones, when the three 

 sensations caused by them are thrown together they will reproduce some 

 sort of a sensation of that shade of color. Such sensation would prob- 

 ably be uniform for each individual, but might not be the same for any 

 two. Indeed, we have no means of knowing whether the sensation 



