568 PHYSIOLOGY 



shows that this result can be achieved. Because of the importance of these 

 properties of the after image we may with advantage recapitulate as follows : 

 As a result of a stimulus the region of the retina affected gives a response 

 which is followed by a second or after image. During this after image 

 this area is incapable of reacting with the normal intensity to a like 

 stimulus, but shows increased excitability to a stimulus of the opposite 

 kind. For example, after a green stimulus the retina is unable to 

 respond fully to another green stimulus unless it falls either before 

 or after the period of the after image. Therefore if during that period a white 

 stimulus be caused to fall on the retina, it will cause a purple sensation (purple 

 being white minus green)- in that part of the retina first stimulated. The 

 duration of the after image is variable, but is found to correspond roughly 

 with the intensity and duration of the stimulus. Thus after a few seconds' 

 exposure to a bright light the after image may be noticeable for two or three 

 minutes, its intensity waxing and waning in an irregular manner. Successive 

 images are often found to show a series of colours, a common series is bluish- 

 green, violet, rose, and finally orange or green ; the phenomenon is however 

 very variable. The colours may be explained by assuming a difference in 

 the rate of oscillation for the after images of the different colours. For 

 example the above series would point to green being more rapid and red less 

 rapid than blue. 



FLICKER AND VISUAL PERSISTENCE. A study of the character- 

 istics of the sensation curve provides 

 an explanation of a number of the 

 subjective phenomena of vision. For 

 example, if a cardboard disc marked 

 as shown in Fig. 293 be caused 

 to rotate slowly, while the black 

 and white sectors are readily recog- 

 nised, their radial margins appear 

 blurred. This blurring is due to the 

 slow rise and fall of the primary image 

 of the sensation curve. If the speed 

 be increased, a point is reached at 

 which the disc gives an unpleasant- 

 glittering appearance ; this would 



appear to be due to one stimulus occurring during the after image of the 

 previous one and thus becoming suppressed, but being followed in its turn 

 by a fresh stimulus which is caused by contrast (see later) to have a greatly 

 increased intensity. If the rate of rotation of the disc be still further increased, 

 a point is reached at which a stimulus falls during the primary image of 

 the previous one. The persistence of the primary image after the cessation 

 of the stimulus causes the stimuli to fuse to give a uniform sensation without 

 flicker, which may be compared to the complete tetanus of a muscle. Since 

 the primary response is more abrupt the greater the intensity of the 

 stimulus, a more rapid rate of rotation is required to produce fusion at high 

 intensities than at low. 



