124 COLOUR VISION 



There is therefore obviously not the same relationship between area 

 and threshold intensity that appertains to the fovea. There is, how- 

 ever, a relationship between the square root of the area (which is 

 equivalent to the visual angle subtended by circular objects) and the 

 threshold intensity. Piper states the law thus : For the dark-adapted 

 periphery the stimulation value of a luminous surface is proportional 

 to the square root of the area of the retinal image ; or in other words, 

 the product of the threshold value and the square root of the area of 

 the retinal image is constant. Henius and Fujita showed that the law 

 is valid only for mixed white light and objects subtending a visual 

 angle up to 10. Above 10 the sensitiveness of the retina increases 

 much more slowly than the visual angle increases, and still more slowly 

 for red light. Fujita showed too that the law is not valid below 1. 



For the light-adapted periphery Fujita showed that there was 

 no simple relationship between area and threshold intensity. For small 

 objects, however, subtending less than 2 the threshold value diminishes 

 with the visual angle, but not so quickly as for the fovea. 



Owing therefore to the great difference between the photopic and 

 scotopic periphery threshold values, especially for relatively large 

 luminous areas, the condition of adaptation is of great importance, 

 especially as it itself changes with the actual stimulation. 



The relationship of the region of the retina and the size of the area 

 stimulated respectively to the sensations derived from colourless 

 mixtures and equations have been studied by v. Helmholtz 1 , v. Frey 

 and v. Kries 2 , and Hering 3 . Hering observed colourless mixtures of 

 complementary monochromatic lights of a given area and studied the 

 effects of altering the size or shifting the fixation point. A mixture of 

 red and blue-green became redder and darker on diminution, greener 

 and brighter on augmentation of the area. The foveal colourless 

 mixture became greener and brighter on direct fixation ; the peripheral 

 colourless mixture became redder and darker on central fixation. A 

 colourless mixture of yellow-green and violet became yellow-green on 

 diminishing the area, rose-red on increasing the area or observation 

 by indirect fixation, but the effects were much less than in the first case. 



Hering found that colourless equations with spectral lights become 

 invalid, both with light and dark adaptation, if the area is increased or 

 fixation changed from direct to indirect (contrast p. 69). So too in a 

 match between a mixture of spectral red and blue-green (A) and yellow 



1 1st ed. p. 301. 2 Arch.f. Physiol. 330, 1881. 



3 Arch.f. d. ges. Physiol. LIV. 277, 1893. 



