THE SENSES. 617 



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 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 

 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 correspond- 

 ing 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. 415. The peripheral parts of the' 

 retina have no perception of red. The area of the retina which is 

 capable of receiving impressions of color is slightly different for each 

 color. 



Color Blindness or Daltonism. Daltonism or color-blindness is a 

 by no means uncommon visual defect. One of the commonest forms is 

 the inability to distinguish between red and green. The simplest expla- 

 nation of such a condition is, that the elements of the retina which 

 receive the impression of red, etc., are absent, or very imperfectly de- 

 veloped, or, according to the other theory, that the red-green substance 

 is absent from the retina. Other varieties of color-blindness in which 

 the other color-perceiving elements are absent have been shown to exist 

 occasionally. 



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 sensi- 

 tive points of that membrane, have a certain influence on each other; 

 the particular condition 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 relation between the dif- 

 ferent parts of the retina, may be arranged in two classes: the one in- 

 cluding those where the condition existing in the greater extent of the 

 retina is imparted to the remainder of that membrane; the other, con- 

 sisting 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 occupy 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 lat- 



