THE SENSE OF VISION. 



341 



the most intense, becomes gradually darker, and are scarcely to be distin- 

 guished from black at a time when the blue colors are still very readily 

 distinguished. 



Function of Rods and Cones. — There is, as mentioned on p. 337, 

 some reason to suppose that the rods and cones have different functions. 

 That color sensation and accuracy of definition are most perfect in the 

 central portion of the retina is shown by the fact that when we desire to 

 obtain the best possible idea of the form and color of an object we direct 



670 050 625 605 590 575 555 

 B C D 



520 505 490 

 E F 



470 



430 



a 



Fig. 152.— Diagram showing the distribution of the intensity of the spectrum as dependent upon the 



degree of illumination (Konig). 



our eyes in such a way that the image falls upon the fovea centralis of the 

 retina. The luminosity of a faint object, however, seems greatest when we 

 look not directly at it, but a little to one side of it. This can be readily 

 observed when we look at a group of stars, as, for example, the Pleiades. 

 When the eyes are accurately directed to the stars so as to enable us to count 

 them, the total luminosity of the constellation appears much less than when 

 the eyes are directed to a point a few degrees to one side of the object. Now, 

 an examination of the retina shows only cones in the fovea centralis. In the 

 immediately adjacent parts a small number of rods are found mingled with 

 the cones. In the lateral portions of the retina the rods are relatively more 

 numerous than the cones, and in the extreme peripheral portions the rods alone 

 exist. Hence this phenomenon is readily explained on the supposition, 

 which is supported by Kainon yCajal's 1 recent observations, that the rods 

 are a comparatively rudimentary form of visual apparatus, taking cognizance 



1 Zeitschriftfiir Psychologic und Physiologic dcr Sinnesorganr, xvi. S. 161. 



