VISUAL SENSATIONS 637 



THE RELATION OF THE INTENSITY OF SENSATION TO 

 THE STRENGTH OF STIMULUS 



Weber's law, viz. that the increase of stimulus necessary to 

 give an increase of sensation always bears the same ratio to the whole 

 stimulus, holds good for visual sensations. This ratio in the case of 

 white light is about T iny. We can thus distinguish between two 

 lights of 20 and 20 1- candle-power, both of them at the same distance 

 from the eye, or between two of 99 and 100 candle-power. If the 

 illumination be excessive the law no longer holds good, and we should 

 be unable to tell the difference between two lights of the latter power 

 if held close to the eye, or between two arc lamps at a considerable 

 distance, even though one might be much stronger than the other. 

 According to some authors our power of distinguishing differences in 

 luminosity varies with different colours. 



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 interval after the stimulus has been entirely withdrawn.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 per- 

 sistence 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. 293, be caused to rotate slowly, it is easy to dis- 

 tinguish 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 application and by the shutting off of the stimu- 

 lus, and due to the white light reflected from the surface, are gradual 



