80 



COLOUR VISION 



This is the same ratio, 5 : 4, as for diminution of intensity of light 

 by quarters (p. 79). The diminution by each quarter of the area is thus 

 equivalent to a quarter of the intensity of light. Any a'perture subtending 

 more than 5° will give the same field. 



Guilleryi confirmed Hess's results by using the size of the area 

 stimulated as a guide to the size of the colour fields. For each colour 

 a gradual increase is necessary in passing towards the periphery. The 

 increase for yellow and blue and for red and green agree respectively, 

 but of course the red-green increase differs from the yellow-blue increase. 



50r^ 



I 2 Z '}■ 5 6 



Aperture in powers of - Z 



Fig. 33. The temporal ( ) and nasal (- ) limits of the fields 



of vision for four spectral colours for different areas of stimulation. (Abne}'.) 



The general threshold of light at the periphery is difficult to measure. 

 Abney used a small spot of luminous paint for fixation. At 10° from 

 the fovea he found that the extinction of all light from red light takes 

 place when the light is about one-third brighter than is required for 

 the fovea. With white paraffin light it is somewhat less. With green 

 light at about the E line and with blue at the Lithium line the necessary 

 reduction of the light is greater than for the centre of the eye. The 

 photochromatic interval is greater for the peripheral than for the central 

 part of the retina. 



^ Ztfich. f. Psychol, u. Physiol, d. Sinnesorg. xii. 267, 189fi 



