226 HAND-BOOK OF PHYSIOLOGY. 



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 versa; 

 the size and shape of the spectrum corresponding with the size and shape 

 of the original object looked at. The colors which thus reciprocally ex- 

 cite each other in the retina are those placed at opposite points of the 

 circle in Fig. 384. 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 ex- 

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

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

 oped, 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 por- 

 tion of the field of vision, yet the different elementary parts, or sensitive 

 points of that 'membrane, have a certain influence on each other; the par- 

 ticular 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 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. 



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 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 



