VISUAL PERCEPTIONS. 367 



to the object. All that we can distinguish as to the locality 

 of the visual appearance which an object produces, is ti^at 

 this appearance occupies a certain place in the field of vi- 

 sion ; and we are taught, by the experience of our other senses, 

 that this is a sign of the existence of the external object in a 

 particular direction with reference to our own body. It is not 

 until long after this association has been established, that we 

 learn, by deduction from scientific principles, that the part of 

 the retina, on which the impression causing this appearance 

 is made, is on the side opposite to that of the object itself; and 

 also that the image of a straight object is curved as well as 

 inverted. But this subsequent information can never inter- 

 fere with our habitual, and perhaps instinctive reference of 

 the appearance resulting from an impression made upon 

 the upper part of the retina, to an object situated below us, 

 and vice versa. Hence we at once refer impressions made 

 on any particular part of the retina to a cause proceeding 

 from the opposite side. Thus, if we press the eye-ball with 

 the finger applied at the outer corner of the orbit, the lumi- 

 nous appearance 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, the two sides of the object appear 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 ope- 

 ration of this law is most conspicuous in those cases where 

 the presence or absence of the agent can readily be deter- 

 mined. Thus, we retain the sensation of a sound for some 



