346 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



intense illumination suffices to stimulate the eye, the slowly vibrating red 

 rays produce the more permanent impression. 



After-images. — When the object looked at is very brightly illuminated the 

 impression upon the retina may be so persistent that the form and color of the 

 object are distinctly visible for a considerable time after the stimulus has ceased 

 to act. This appearance is known as a " positive after-image," and can be best 

 observed when we close the eyes after looking at the sun or other bright source 

 of light. Under these circumstances we perceive a brilliant spot of light which, 

 owing to the above-mentioned difference in the persistence of the impressions 

 produced by the various colored rays, rapidly changes its color, passing gen- 

 erally through bluish green, blue, violet, purple, and red, and then disappear- 

 ing. This phenomenon is apt to be associated with or followed by another 

 effect known as a " negative after-image." This form of after-image is much 

 more readily observed than the positive variety, and seems to depend upon the 

 fatigue of the retina. It is distinguished from the positive after-image by the 

 fact that its color is always complementary to that of the object causing it. In 

 the experiment to demonstrate the fatigue of the retina, described on p. 345, 

 the white spot which appears after the black disk is withdrawn is the " nega- 

 tive after-image " of the disk, white being complementary to black. If a 

 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 few seconds, and then withdrawn, the eye will perceive in its place a spot of 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 vellow, 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 eve 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 in/ens,' 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, in other words, the sensation will be the result 

 of the stimulation l.y tl bjecl 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 he 

 thus stated : Every pari of the held of vision appears lighter near a darker 

 part and darker near a lighter part, and its color seen near another color 

 approaches the complementary color of the latter. A contrast phenomenon 



