30fi PROCEEDINGS OE THE ACADEMY OP [1886. 



that of the body. Our knowledge that the retinal image is 

 reversed is not gained by perception, but by our study of the 

 laws of optics. We conceive of a picture upon the retina whose 

 direction is reverse to that of tlie body, and are puzzled to under- 

 stand why the mind does not perceive it thus reversed. In this 

 we are comparing sensations with ideas. The image of the body 

 is also reversed upon the retina, and thus its direction, as optically 

 recognized, agrees with that of all other objects. Our accepted 

 standard of direction therefore fails us in this particular. The 

 universe, as perceived by the mind through the medium of the 

 e^^e, is relatively in harmony in all its particulars of direction 

 and position, and unless the mind had some innate conception of 

 absolute direction, it would be impossible for it to discover that 

 a reversal of the image had taken place. We have already shown 

 that it has no such innate ideasof direction, and is entirely depend- 

 ent upon its perceptions in this particular. The body, our accepted 

 standard of direction, is perceived to be erect as compared with 

 our perception of the earth's surface. All other objects upon 

 this surface bear the same relation to it, and we have no means 

 of discovering that we are optically deceived, except by a second- 

 ary process of reasoning, based upon the laws of optics. No 

 tree, for instance, could seem to us upside down unless the earth's 

 surface seemed upside down, which is impossible. 



The mystery in which this question has been so long involved, 

 therefore, vanishes when considered from this point of view. 

 The mind fails to discover that its optical image is reversed 

 simpl}' from the fact that this reversal includes all things, nothing 

 retaining its erect position to serve as a standard of comparison, 

 and from the second fact that the mind has no innate idea of 

 direction to make it aware of its error. There is no error in our 

 perception. Relatively it is in ever}- particular correct, and of 

 the absolute we have no knowledge except through the process of 

 reasoning.^ 



^ Since forming these theoretical views, the writer has become aware that 

 the same theory has been previously presented. Dr. William Mackenzie 

 "Physiology of Vision," London, 1841 says: "In the image on the 

 retina, the relative position of the parts of the object remain unchanged, 

 as well as its relations to surrounding objects. The images of all objects, 

 even those of our own bodies, are equally inverted on the retina, and there- 

 fore maintain the same relative position. Even the image of our hand, 



