636 PHYSIOLOGY 



has been observed that after extraction of the lens for cataract the 

 visibility of the spectrum, which in the normal eye only extends to 

 the line H with a wave length of 397, is increased on the violet side so 

 that the spectrum may be seen as far as a point corresponding to a 

 wave length of 313. It is evident from this that in the absorption 

 of the ultra-violet rays by the eye the lens takes a preponderating part. 



LUMINOSITY OF DIFFERENT PARTS OF THE SPECTRUM 

 The physiological nature of our conception of light is shown by 

 the fact that the spectrum differs in its luminosity, i.e. in its total 

 stimulating effect on the retina in its different parts, and that the 

 relative luminosities of different parts of the spectrum bear no rela- 

 tion to the amount of radiant energy represented by each kind of wave 

 length. The greatest energy is attained by the ultra-red rays and 

 there is a gradual diminution from here to the violet end of the spectrum. 

 The yellow part of the spectrum, however, appears much brighter 

 than any other part. The relative luminosity of different parts is 

 shown in the following Table by Vierordt : 



Part of spectrum Luminosity 



Red 'B' 22 



Orange ' C ' 128 



Reddish yeUow ' D ' 780 



Yellow ' D ' to ' E ' 1000 



Green ' E ' 370 



Bluish green ' F ' 128 



Blue ' G ' 8 



Violet ' H ' 1 



The limited excitability of the retina and its special sensitivity 

 to rays in the middle of the spectrum present considerable advantages 

 for the normal functioning of the optical apparatus. No provision 

 is made for securing achromatism. The dispersion of the ultra-red 

 and ultra-violet rays is so great in passing through the refracting 

 surfaces of the eyeball that, if they all arrived at the retina, and this 

 organ were sensitive to both kinds of rays, it would be impossible to 

 obtain any clear image of external objects. The image formed by the 

 ultra-red rays would be far behind the retina when the ultra-violet 

 rays were focused on the retina and vice versa. As it is, the retina 

 is unstimulated by the two ends of the spectrum, and its stimulation 

 by the red as well as by the blue rays is only minimal, so that, for the 

 excitation of a mosaic of spots on the retina in spatial extension and 

 arrangement corresponding to that of the objects from which the light 

 reaches the retina, practically only the middle part of the spectrum is 

 of importance, and the distance of the image formed by the reddish- 

 yellow rays from that formed by the green rays will not be great 

 enough to cause any appreciable distortion of the exciting image. 



