1886,] NATURAL SCIENCES OF PHILADELPHIA. 303 



What we really perceive is the impression produced upon the 

 retina, which is transmitted by the optic nerve to the brain. The 

 theory that the brain o.r mind, by some secondary action, physically 

 refers this sensation back again to the nerve extremities, or even 

 projects it beyond them into space, along reversed lines, has no 

 warrant in known physiological facts. Far more probably the 

 seeming projection is a mental action only. Natural selection 

 may have adapted each nerve to yield a sense of the distance 

 and direction of its impressions and, therefore, of their external 

 location. An animal destitute of such sense would be unable to 

 estimate the exact point of a threatened danger, and only those 

 capable of localizing their sense impressions could survive. Yet 

 if the brain estimates the distance from which any sensation 

 comes only through conditions hereditarily existing in the trans- 

 mitting nerve, the idea of referred sensations, with all theories 

 based upon it, falls to the ground. The reference of a pain to 

 the natural location of an amputated foot would form a constituent 

 portion of the impression conveyed to the brain by the nerve 

 which formerly passed to that foot. In like manner the optic 

 nerve may, among its hereditaril3'-gained powers, have that of 

 refei'ring its impressions to a point beyond the nerve extremity 

 or in external space, the locality from which danger from visible 

 objects comes. But this would have no bearing upon the ques- 

 tion of the character of its impression, the retinal image as a 

 whole being mentally transferred to an external region, but in no 

 sense changed in character. The question seems to be solely one 

 of a certain power or strength possessed by the nerves, through 

 which each of them indicates that location of its impressions best 

 adapted to the efficiency of protective activity. In the case of 

 sight this distance would necessarily be beyond the actual posi- 

 tion of the retina, and external to the body. The same rule 

 holds good in the case of sound. In every case it is very prob- 

 ably a resultant of long-continued natural selection. 



We do not actually see objects. We simply perceive the 

 images of them which are impressed upon the retina. This 

 stands as a picture-plane between our mind and the universe. 

 We perceive the impressions with which it is affected incor- 

 rectly, if this affection is in any sense an incorrect one. But the 

 conception gained from these impressions is subject to mental 

 accommodations, the result of experience and of hereditary influ- 



