352 LECTURE XXXVIII. 



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 accidentally near, and with which indeed it would perhaps 

 be impossible to compare it, as far as we judge by the immediate sensa- 

 tions only.* 



We might indeed call in experience to our assistance, and habitually 

 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 recorded by Galen, on the instincts of a kid, which is suffi- 

 ciently 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 retina should be of that precise form, 

 which is exactly suited to the reception of the image to be depicted on it. 



The whole surface of the retina appears to be usually occupied by such 

 an image, but it is not all of equal sensibility ; a certain portion only, near 

 the axis, is capable of conveying distinct impressions of minute objects. 

 But the perfection of this limited distinctness is a far greater advantage to 

 us, than a more extensive field of moderately accurate vision would have 

 been ; for by means of the external muscles, we can easily so change the 

 position of the eye, that the image of any object before us may be made 

 to fall on the most sensible part of the retina. We may readily observe 

 the want of sensation at the entrance of the optic nerve, by placing two 

 candles so that the distance of each from the eye may be about four times 

 their distance from each other: then if we direct our right eye to the 

 left hand candle, the right hand candle will be lost in a confused mass of 

 faint light, its image on the retina falling on the point at which its sensi- 

 bility is deficient, t 



* Consult Berkeley on Vision, Dub. 1709. Lecat, Traite des Sens, 1767. Wal- 

 ter, Berlin Mem. 1788, p. 3. Wells, Essay on Single Vision, 1791. Wollaston, 

 Ph. Tr. 1824, p. 222. Berthold, Ueber das Aufrecht-erscheinen der Gesichtsob- 

 jecte, Gott. 1830. Bartels,"Beitrage zur Phys. des Gesichtsinnes, Berl. 1834. 

 Volkmann, doTLeipz. 1836. 



t A better way of doing this is to make two blots on a sheet of paper, about four 

 inches apart, and to look attentively with the right eye on that which lies to the left 

 hand ; the eye being placed right over it. When the eye is raised to the height of 



