VISUAL PERCEPTIONS. 523 



Many other examples might be given of similar 

 fallacies in our visual perceptions. 



All impressions made on the nerves of sensa- 

 tion have a definite duration, and continue for a 

 certain interval of time after the action of the 

 external agent has ceased. The operation of this 

 law is most conspicuous in those cases where the 

 presence or absence of the agent can readily be 

 determined. Thus we retain the sensation of a 

 sound for some time after the vibrations of the 

 external medium have ceased; as is shown by 

 the sensation of a musical note being the result 

 of the regular succession of aerial undulations, 

 when the impression made by each continues 

 during the whole interval between two consecu- 

 tive vibrations. Whether light be caused by the 

 emission of material particles, or the undulations 

 of an etherial fluid, its impulses on the retina 

 are unquestionably consecutive, like those of 

 sound ; but being repeated at still shorter inter- 

 vals, they give rise to a continuous impression. 

 A familiar instance of the same principle occurs 

 in the appearance of an entire luminous circle, 

 from the rapid whirling round of a piece of lighted 

 charcoal ; for the part of the retina which re- 

 ceives the brilliant image of the burning char- 

 coal, retains the impression with nearly the same 

 intensity during the entire revolution of the 

 light, when the same impression is renewed. 

 For the same reason a rocket, or a fiery meteor, 



