28 COLOR SENSITIVITY OF THE PERIPHERAL RETINA. 



mixture of red and violet the desired tone was obtained. This latter tone 

 was found to be complementary to the stable green; and the stable 

 yellow turned out to be complementary to the stable blue. 



In his more accurate investigation of the behavior of the stable 

 colors in indirect vision, Hess employed the Hering color-mixer.* In 

 this apparatus the color-stimulus is observed through an aperture in a 

 gray screen. By moving the fixation-point in a lateral direction, the 

 image of the stimulus is made to fall upon the peripheral retina. The 

 brightness of the background may be brought to equality with that of 

 the stimulus by an appropriate inclination of the screen. By this means 

 it is possible to secure accurate judgments as to the exact point of ap- 

 pearance or disappearance of the color, since in indirect vision the 

 stimulus may be made to fuse into the gray background. 



With stable stimuli whose white-values and color-values had pre- 

 viously been equalized, Hess proceeded to determine the limits of retinal 

 sensitivity. 



The following shows the extension of the four color-zones :f 



A survey of these results reveals two interesting facts a coinci- 

 dence on the one hand of the retinal limits of sensitivity to red and to 

 green, and on the other of those to yellow and to blue ; a considerably 

 wider extension of the yellow-blue zone than of the red-green zone. 

 Hess also found that an increased saturation of stimulus gives an in- 

 creased extension of zonal limits ; he further demonstrated that the 

 zonal limits are widened by an increase in the area of the color stimulus. 



*This apparatus is figured in Titdhener's Experimental Psychology, Vol. I, 

 Part 2, 1901, p. 20. 



fHess, 1. c. S. 45ff. Cf. also his tables, S. 4Qf. Hess 'expresses his results in 

 cm. ; for the sake of uniformity we have reduced his readings to degrees. 

 (i cm. = about 2, S. 46.) 



