OPTICS 273 



each place where it actually is, but as a fiery stripe corre- 

 sponding to the path it describes. In this case, new points 

 of the retina are stimulated before the sensation of the 

 previously stimulated points has entirely disappeared. 



The individual visual sensations produced by a series of 

 light stimulations rapidly following each other blend into 

 one visual sensation. Each individual stimulation increases 

 and each interval between the stimulations decreases, to a 

 certain extent, the retinal stimulation ; but if the light 

 stimuli follow each other rapidly enough the variations in 

 the sensations are so small that they are no longer per- 

 ceived. The intensity of the visual sensation in this case is 

 as great as that produced by a correspondingly feebler light 

 acting continuously (Talbof s law]. 



(c) Fatigue of the retina. Negative after-image or suc- 

 cessive contrast. By long duration of the light, the intensity 

 of the sensation is decreased because of the fatigue of the 

 retina. A fatigued retina is less irritable than one not 

 fatigued. If one looks for some time at a light field placed 

 on a dark background and then at a uniformly light field, 

 this does not appear uniformly light, but upon it is seen a 

 dark area corresponding to the light field first looked at. 

 This phenomenon is called negative after-image or successive 

 contrast. 



Adaptation or the adjustment of the retina to various in- 

 tensities of light depends upon fatigue and restoration. If 

 one enters from a light into a dark room, nothing is clearly 

 seen at first because the retina is fatigued; gradually the 

 retina becomes more irritable, being rested, and one sees 

 much better in the dark. If one enters from a dark into a 

 lighted room, the light at first blinds because of the great 

 irritability of the retina, which is gradually decreased by 

 fatigue. 



4. TJie illumination of the surroundings of the observed 

 object. A light upon a dark background appears lighter 

 than upon a bright background (simultaneous contrast). 

 This contrast is greatest at the limit between the light and 



