538 



VISION 



turned off the sensation does not vanish instantly, but persists for a meas- 

 urable time. 



The latter statement is very easily proved. When one looks for a moment 

 at a bright lamp flame, then suddenly closes the eyes and covers them with the 

 hand, or looks at an absolutely dark background, he still sees a bright image 

 of the same form as that of the bright object itself. The image gradually 

 disappears and as it does so changes its color. This phenomenon in which 

 the bright parts of the object remain bright and the dark parts dark is called 

 a. positive after-image. Such an after-image has at first the proper color of 

 the object, and very often it reproduces very exactly the separate parts of the 

 object in their proper form and shade. 



Proof that the rise of a visual sensation also requires a certain time is 

 not much more difficult. We need only consider for a moment the effect of 





A B 



FIG. 232. Section through the retina of a frog's eye, after Engelmann. A, after the eye was kept 

 for from one to two days in complete darkness ; B, kept for 24 hours in the dark, then exposed 

 for one-half hour to diffuse bright daylight (cf. page 537). 



rotating a circular disk composed of black and white sectors (as in Fig. 233) 

 to be convinced of this. So long as the rate of rotation is low, the black and 

 white sectors remain perfectly distinct. But as the rate becomes higher the 

 edges of the sectors are obliterated, and this is true as well of the edges going 

 before as of those coming after, whichever the direction of rotation. If the 

 excitation of the retina were instantaneous, the leading edges of the white 

 sectors ought to continue sharply defined, while from what has just been 

 said above, it is evident that the edges coming after ought to be indistinct 

 (cf. the dotted line in Fig. 234). 



With, a high rate of speed the rays coming from the white sectors no 

 longer have sufficient time to produce the maximum excitation and on the 

 other hand the light is never completely shut out by the black sectors; con- 



