736 HANDBOOK OF PHYSIOLOGY. 



may, under certain circumstances, be reversed in the spectrum; what 

 was bright may be dark, and what was dark may appear light. This 

 occurs whenever the eye, which is the seat of the spectrum of a luminous 

 object, is not closed, but fixed upon another bright or white surface, as 

 a white wall, or a sheet of white paper. Hence the spectrum of the sun, 

 which, while light is excluded from the eye, is luminous, appears black 

 or gray when the eye is directed upon a white surface. The explanation 

 of this is, that the part of the retina which has received the luminous 

 image remains for a certain period afterward in an exhausted or less 

 sensitive state, while that which has received a dark image is in an 

 unexhausted, and therefore much more excitable condition. 



The ocular spectra which remain after the impression of colored ob- 

 jects upon the retina are always colored ; and their color is not that of 

 the object, or of the image produced directly by the object, but the oppo- 

 site, or complemental color. 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 

 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 versa; 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. 435. The peripheral parts of the 

 retina do not react to rays of red. The area of the retina which is 

 capable of receiving impressions of color, and therefore the field of 

 vision, 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 explana- 

 tion of such a condition is, that the elements of the retina which receive 

 the impression of red, etc., are absent, or very imperfectly developed, 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. 



THE RECIPROCAL ACTION OF DIFFERENT PARTS OF THE RETINA. 



Although each elementary part of the retina represents a distinct 

 portion of the field of vision, yet the different elementary parts, or sensi- 



