DURATION OF PERCEPTIONS. 46-3 



the outer corner of the orbit, the luminous appear- 

 ance excited by the pressure is immediately re- 

 ferred to the opposite or inner side of the eye. 



If we place a card perpendicularly between the 

 two eyes, and close to the face, the card will appear 

 double, because, although each surface is seen by 

 the eye which is adjacent to it, in the direction in 

 which it really is with regard to that eye, yet, 

 being out of the limits of distinct vision, it is referred 

 to a much greater distance than its real situation ; 

 and consequently, tlie two sides of the object ap- 

 pear separated by a wide interval, and as if they 

 belonged to two different objects. Many other 

 examples might be given of similar fallacies in our 

 visual perceptions. 



All impressions made on the nerves of sensation 

 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 a regular succession 

 of aerial undulations, when the impression made 

 by each continues during the whole interval be- 

 tween two consecutive 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 

 intervals, they give rise to a continuous impression. 

 A familiar instance of the same principle occurs 



VOL, II. H H 



