S6 PSYCHOLOGY. 



equilibrium of the retina becomes gradually restored. When 

 successive contrast is carefully guarded against, the simul- 

 taneous contrast, whether seen directly or through the tube, 

 never lasts for an instant on removal of the colored field. 

 The physiological exj^lanation applies throughout to all the 

 phenomena presented by colored shadows. * 



If we have a small field whose illumination remains con- 

 stant, surrounded by a large field of changing brightness,, 

 an increase or decrease in brightness of the latter results 

 in a corresponding apparent decrease or increase respect- 

 ively in the brightness of the former, while the large field 

 seems to be unchanged. Exner says : 



" This illusion of sense shows that we are inclined to regard as con- 

 stant the dominant brightness in our field of vision, and hence to refer 

 the changing difference between this and the brightness of a limited field 

 to a change in brightness of the latter." 



The result, however, can be shown to depend not on 

 illusion, but on actual retinal changes, which alter the sen- 

 sation experienced. The irritability of those portions of 

 the retina lighted by the large field becomes much reduced 

 in consequence of fatigue, so that the increase in brightness 

 becomes much less apparent than it would be without this 

 diminution in irritability. The small field, however, shows 

 the change by a change in the contrast-effect induced upon, 

 it by the surrounding parts of the retina, f 



The above cases show clearly that physiological processes, 

 and not deception of judgment, are responsible for contrast of 

 color. To say this, however, is not to maintain that our 

 perception of a color is never in any degree modified by 

 our judgment of what the particular colored thing before us 

 may be. We have unquestionable illusions of color due to 

 wrong inferences as to what object is before us. Thus Von 

 Kries;}: speaks of wandering through evergreen forests cov- 

 ered with snow, and thinking that through the interstices of 

 the boughs he saw the deep blue of pine-clad mountains, cov- 



*Hering: Archiv f. d. ges. Physiol., 3d. xl. S. 173 ff. ; Delabarrer 

 A.merican Journal of Psychology, ii. 636. 



t Hering : Archiv f. d. ges. Physiol., Bd. XLi. S. 91 ff. 

 X D'.e Gesichtsempfindungen u. ihre Analyse, p. 128. 



