792 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



colored disk be placed upon a sheet of white paper, looked at attentively for a 

 few seconds, and then withdrawn, the eye will perceive in its place a spot of 

 light of a color complementary to that of the disk. If, for example, the disk 

 be yellow, the yellow-perceiving elements of the retina become fatigued in 

 looking at it. Therefore when the mixed rays constituting white light are 

 thrown upon the portion of the retina which is thus fatigued, those rays which 

 produce the sensation of yellow will produce less effect than the other rays for 

 which the eye has not been fatigued. Hence white light to an eye fatigued for 

 yellow will appear blue. 



If the experiment be made with a yellow disk resting on a sheet of blue 

 paper, the negative after-image will be a spot on which the blue color will 

 appear (1) more intense than on the neighboring portions of the sheet, owing 

 to the blue-perceiving elements of that portion of the retina not being fatigued ; 

 (2) more saturated, owing to the yellow-perceiving elements being so far 

 exhausted that they no longer respond to the slight stimulation which is pro- 

 duced when light of a complementary color is thrown upon them, as has been 

 explained in connection with the subject of saturation. 



Contrast. As the eye wanders from one part of the field of vision to 

 another it is evident that the sensation produced by a given portion of the 

 field will be modified by the amount of fatigue produced by that portion on 

 which the eye has last rested, or, other words, the sensation will be the result 



FIG. 245. To illustrate the phenomenon of contrast. 



of the stimulation by the object looked at combined with the negative after- 

 image of the object previously observed. The effect of this combination is to 

 produce the phenomenon of successive contrast, the principle of which may be 

 thus stated : Every part of the field of vision appears lighter near a darker 



