570 PHYSIOLOGY 



on the retina in its different parts, and that the relative luminosities of 

 different parts of the spectrum bear no relation 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 yellow ' 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 excita- 

 tion 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. 



THE RELATION OF THE INTENSITY OF SENSATION TO 



THE STRENGTH OF STIMULUS 



Weber's law, viz. that the increase of stimulus necessary to give an in- 

 crease of sensation always bears the same ratio to the whole stimulus, holds 

 good for visual sensations. This ratio in the case of white light is about T - o . 

 We can thus distinguish between two lights of 20 and 20 J candle-power, both 

 of them at the same distance from the eye, or between two of 99 and 100 

 candle-power. If the illumination be excessive the law no longer holds good, 

 and we should be unable to tell the difference between two lights of the 

 latter power if held close to the eye, or between two arc lamps at a con- 

 siderable distance, even though one might be much stronger than the other. 

 According to some authors our power of distinguishing differences in lumin- 

 osity varies with different colours. 



