66z THE SENSE OF SIGHT. 



their colour is not that of the object, or of the image pro- 

 duced directly by the object, but the opposite, or comple- 

 mental colour.' The spectrum of a red object is, therefore, 

 green; that of a 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 colour, but remains 

 sensitive to the other rays which with red make up white 

 light ; and, therefore, these by themselves 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 colours ; the 

 retina while fatigued by yellow rays will suppose an object 

 to be violet, and vice versa ; the size and shape of the 

 spectrum corresponding with the size and shape of the 

 original object looked at. The colours which thus recipro- 

 cally excite each other in the retina are those placed at 

 opposite points of the circle in fig. 1 80. 



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 

 elementary parts, or sensitive points, of that membrane 

 have -a certain influence on each other ; the particular con- 

 dition of one influencing 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 rela- 

 tion between the different 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 membrane ; 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. 



