ON VISION. 449 



focus of each oblique pencil to fall either accurately or very nearly on the 

 concave surface of the retina, throughout its extent. 



Opticians have often puzzled themselves, without the least necessity, in 

 order to account for our seeing objects in their natural erect position, while 

 the image on the retina is in reality inverted: but surely the situation of a 

 focal point at the upper part of the eye could be no reason for supposing the 

 object corresponding to it to be actually elevated. We call that the lower 

 end of an object which is next the ground ; and the image of the trunk of a 

 tree being in contact with the image of the ground on the retina, we may 

 naturally suppose the trunk itself to be in contact with the actual ground: 

 the image of the branches being more remote from that of the ground, we 

 necessarily infer that the branches are higher and the trunk lower: and 

 it is much simpler that we should compare the image of the floor with the 

 image of our feet, with which it is in contact, than with the actual situation 

 of our forehead, to which the image of the floor on the retina is only acci- 

 dentally near, and with which indeed it would perhaps be impossible to com- , 

 pare it, as far as we judge by the immediate sensations only. 



We might indeed call in experience to our assistance, ahdhabitually correct 

 the errors of one sense by a comparison with the perceptions of another. 

 But it appears that some philosophers have been too hasty in supposing, that 

 the use of all our senses is derived from experience alone, and in disbelieving 

 the existence of instinct independent of it. Without any other authority 

 than that of their own imaginations, they have denied the observation re- 

 corded by Galen, on the instincts of a kid, which is sufficiently credible to 

 counterbalance much more than bare assertion. The instant after its birth, 

 accompanied by the loss of its mother, the little animal ran to some green 

 vegetables, and having first smelt them, chewed and swallowed them. The 

 kid could have been taught by no experience to be tempted by the sight, 

 to act with the proper muscles of locomotion, to go near and smell, and to 

 be induced by the smell to masticate, and by the taste to swallow and digest 

 its food, had it not been provided with some fundamental instinct, by the 

 same intelligence, which so calculated the adjustments of the eye, that the 

 lens should be able to produce a perfect image of every object, and that the 



VOL. I. 3 M 



