THE SENSE OF VISION. 787 



trarily chosen standard. The curves from A to H represent the distribution 

 of the intensity of light in the spectrum with eight different grades of illumi- 

 nation. This shifting of the maximum of luminosity in the spectrum 

 explains the so-called " Purkinje's phenomenon " viz. the changing rela- 

 tive values of colors in varying illumination. This can be best observed 

 at nightfall, the attention being directed to a carpet or a wall-paper 

 the pattern of which is made up of a number of different colors. As 

 the daylight fades away the red colors, which in full illumination are 

 the most intense, become 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. The layer of rods and cones has thus far 

 been spoken of as if all its elements had one and the same function. There 

 is, however, 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 

 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 that 

 the rods are a comparatively rudimentary form of visual apparatus taking 

 cognizance of the existence of light with special reference to its varying 

 intensity, and that the cones are organs specially modified for the localization 

 of stimuli and for the perception of differences of wave-lengths. The view 

 that the rods are specially adapted for the perception of luminosity and the 

 cones for that of color derives support from the fact that in the retina of cer- 

 tain nocturnal animals e. g. bats and owls rods alone are present. This 

 theory has been further developed by Von Kries, 1 who in a recent article 

 describes the rods as differing from the cones in the following respects : (1) 

 They are color-blind i. e. they produce a sensation of simple luminosity 

 whatever be the wave-length of the light-ray falling on them ; (2) they are 

 more easily stimulated than the cones, and are particularly responsive to light- 

 waves of short wave-lengths ; (3) they have the power of adapting themselves 

 to light of varying intensity. 



On this theory it is evident that we must get the sensation of white or 



1 Zeitschrift fur Psychologic und Physiologic der Sinnesorgane, ix. 81. 



