VISUAL SENSATIONS 571 



TIME RELATIONS OF THE EXCITATORY PROCESS 

 When a stimulus of short duration, such as an induction shock, is applied 

 to a muscle, the response of the latter bears no likeness in its intensity and 

 its time-relations to the exciting stimulus. The muscle after a short latent 

 period begins to contract when the stimulus has already ceased to act. It 

 contracts rapidly at first, then more slowly, and then relaxes. In the same 

 way the sensation evoked by a momentary light stimulus takes a certain 

 time to attain its maximum and then dies away slowly, persisting, that is to 

 say, during a certain interval after the stimulus has been entirely withdrawn. 

 This persistence of the visual sensation is experienced whenever we look at a 

 brilliant source of light, such as a candle or lamp, and then either shut the 

 eyes or direct the gaze on to a blackened surface. We then see on the dark 

 background a bright image of the candle or lamp, which gradually fades 

 This phenomenon is often spoken of as a ' positive after-image.' If the 

 object has been very bright the image in fading becomes coloured. It first 

 appears greenish blue, which changes later to violet, rose colour, and finally 

 orange or green. 



The slow rise and fall of the sensation evoked by a momentary stimulus, 

 or by a change in the intensity of light falling on the retina, are responsible 

 for the blurred outline of any object 

 that is regarded while in rapid motion. 

 If a disc with alternate sectors of black 

 and white, such as is shown in Fig. 295, 

 be caused to rotate slowly, it is easy to 

 distinguish both black and white sectors. 

 As the speed of rotation increases the 

 margins of the sectors become blurred, 

 and the fact that both margins of each 

 white sector are blurred shows that the 

 sensations produced both by the appli- 

 cation and by the shutting off of the 

 stimulus, and due to the white light 



reflected from the surface, are gradual JT IG 295. 



and not instantaneous. With further 



increase in rate the whole disc takes a grey colour, which, however, oscillates 

 or ' flickers/ and with a still further increase the ' flicker ' disappears and 

 the disc has a uniform grey appearance. The phenomenon is analogous to 

 the phenomenon observed in muscle as the result of intermittent excitation. 

 Each single shock gives rise to a contraction which is more prolonged than 

 the shock itself. When the shocks are repeated we obtain, according to 

 their frequency, a series of single contractions, a partial fusion of contrac- 

 tions, so that an imperfect tetanus is produced (' flicker '), or finally with 

 a certain rate of interruption of the stimulus complete fusion of contrac- 

 tion, i.e. complete tetanus. 



We see therefore that when a portion of the retina is excited for a certain 



