304 PROCEEDINGS OF THE ACADEMY OP [1886. 



ences ; and wc may, therefore, gain a correct idea from au impres- 

 sion -which is physically incorrect. 



Though the retinal image seems to be referred by the optic nerve 

 to a position external to the eye, j^et it is certainly not exactly 

 located by the sense of sight, and the mental accommodations 

 above mentioned are the locating influences. It is said that to a 

 blind man suddenly restored to sight, the surrounding objects 

 seem like the details of a picture, and within easy reach. He 

 appears to see the retinal picture, somewhat projected, but with 

 no correction except that the reversal of the image does not 

 appear. Our perception of roundness or solidity is, then, a 

 result of experience. This is followed by a knowledge of rela- 

 tive distance, comparative location being one of the most appar- 

 ent relations between objects. But the actual distance of objects 

 which are somewhat removed from the body is learned by experi- 

 ence, aided by several of the senses. Of more distant objects, 

 we seldom properly estimate the actual distance. All this, how- 

 ever, is a result of the action of the reasoning powers, guided by 

 the senses. The evidence of the blind man assures us that what 

 we actually see is the retinal image, somewhat projected into 

 space, and differing from an ordinary picture mainly in that it is 

 perfect in its lights and shades, and thus forcibly tends to pro- 

 duce that illusion of solidity which is the effect sought in all 

 pictures. The only correction is that of the reversal of the 

 image. This correction, therefore, seems a natural one, inherent 

 in the sense or the mind. 



In attempting an explanation of this phenomenon, in view of 

 the considerations above taken, the question may be asked : In 

 what way can the mind discover that there is anything incorrect 

 in the direction of the retinal image? To do so it must have 

 some standard of direction with which the impressions upon 

 the retina can be compared, and their agreement or disagree- 

 ment observed. If there be any such standard, it must exist in 

 one of three locations : on the retina itself, in external nature, or 

 in the mind. If on the retina, it could only be some line, or 

 group of lines, apparent to sight, and serving as directive guides 

 with which to compare retinal impressions. It need not be said 

 that nothing of the kind exists. 



The standard of comparison, therefore, by whose aid alone we 

 can discover that the retinal image is reversed, if it exists. 



