COMPLEMENTARY COLORS. 519 



green object, red ; that of violet, yellow ; that of yellow, violet, 

 and so on. The reason of this is obvious. The part of the 

 retina which receives, say, a red image, is wearied by that 

 particular color, but remains sensitive to the other rays which 

 with red make up white light ; and, therefore, these by them- 

 selves reflected from a white object produce a green hue. If, 

 on the other hand, the first object looked at be green, the 

 retina, being tired of green rays, receives a red image when 

 the eye is turned to a white object. And so with the other 

 colors; the retina while fatigued by yellow rays will suppose an 

 object to be violet, and vice versd; the size and shape of the 

 spectrum corresponding with the size and shape of the original 

 object looked at. The colors which thus reciprocally excite 

 each other in the retina are those placed at opposite points 

 of the circle in Fig. 187. 



Of the Reciprocal Action of different Parts of the Retina on 

 each other. 



Although each elementary part of the retina represents a 

 distinct portion of the field of vision, yet the different elemen- 

 tary parts, or sensitive points, of that membrane have a certain 

 influence on each other ; the particular condition of one in- 

 fluencing that of another, so that the image perceived by one 

 part is modified by the image depicted in the other. The 

 phenomena, which result from this relation between the dif- 

 ferent parts of the retina, may be arranged in two classes ; the 

 one including those where the condition existing in the greater 

 extent of the retina is imparted to the remainder of that mem- 

 brane ; the other, consisting of those in which the condition of 

 the larger portion of the retina excites, in the less extensive 

 portion, the opposite condition. 



1. When two opposite impressions occur in contiguous parts 

 of an image on the retina, the one impression is, under certain 

 circumstances, modified by the other. If the impressions oc- 

 cupy each one-half of the image, this does not take place ; for 

 in that case, their actions are equally balanced. But if one 

 of the impressions occupies only a small part of the retina, and 

 the other the greater part of its surface, the latter may, if long 

 continued, extend its influence over the whole retina, so that 

 the opposite less extensive impression is no longer perceived, 

 and its place becomes occupied by the same sensation as the 

 rest of the field of vision. Thus, if we fix the eye for some 

 time upon a strip of colored paper lying upon a white surface, 

 the image of the colored object, especially when it falls on the 



