74 VISUAL SENSATIONS. [BOOK in. 



holds good within certain limits only ; it fails when the stimulus 

 is either above or below a certain range of intensity. 



Hence, if we take the smallest difference w^hich we can 

 appreciate in a stimulus as a measure of our sensibility to 

 differences in the stimulus, we may say that on the one hand in 

 respect to absolute differences, such as that between two lamps 

 and that between two rushlights, our sensibility varies inversely 

 as the magnitude of the stimulus ; we are more sensible of the 

 same absolute difference when that is a difference between two 

 rushlights than when it is a difference between two lamps. On 

 the other hand, in regard to relative differences, our sensibility is 

 independent of the magnitude of the stimulus ; the difference of 

 which we are sensible in the case of the lamp bears the same 

 proportion to the whole luminosity of the lamp as the difference 

 of which we are sensible in the case of the rushlight bears to the 

 whole luminosity of the rushlight. 



748. Eeturning now to consider the duration of the sti- 

 mulus, as distinguished from its intensity, we find that a stimulus 

 of extremely brief duration may give rise to a distinct sensation ; 

 the flash of an electric spark, for instance, is readily visible. 

 There is probably a limit in respect to duration within which the 

 stimulus fails to produce a sensation ; it is probable, for instance 

 that a certain number of undulations in succession must fall on 

 the retina in order to give rise to a visual sensation, and that a 

 single undulation of the ether falling on the retina, if such a 

 thing were possible, would produce no visual effect ; but the 

 exact limit will depend on the intensity and nature of the light, 

 and we need not enter upon these details here. 



It is of more importance to note that the visual sensation 

 caused by a very brief stimulus lasts a considerable time ; the 

 sensation has a duration much greater than that of the stimulus. 

 The sensation of a flash of light, for instance, lasts for a much 

 longer time than that during which luminous vibrations are 

 falling on the retina. In this respect, we may roughly compare a 

 visual sensation to a simple muscular contraction caused by such 

 a stimulus as a single induction shock. We might indeed con- 

 struct a " visual sensation curve " very much after the fashion of 

 a " muscle curve." We should find that after a very obvious 

 latent period the sensation began, rose to a maximum and then 

 declined. This \atent period forms an important part of the 

 " reaction period," on which we dwelt in a former part of this 

 work ( 691). As we have said, in all the sensations with which 

 we are now dealing, we have to distinguish at least two parts, 

 the peripheral part, the events taking place in the retina, and 

 the central part, the events taking place in the brain, the two 

 being united by means of the visual impulses passing along the 

 optic nerve. And within the latent period are comprised the 

 changes in the retina which start the visual impulses, the passage 



