248 OF SENSATION, AND THE ORGANS OF THE SENSES. 



For some time after the sight was tolerably clear, the lad preferred finding his 

 way through his father's house, to which he, had been quite accustomed when 

 blind, by touch rather than by sight, the use of the latter sense appearing to 

 perplex rather than to assist him ; but, when learning a new locality, he 

 employed his sight, and evidently perceived the increase of facility which he 

 derived from it. 



335. The question has been proposed, whether a person born blind, who 

 was able by the sense of touch to distinguish a cube from a sphere, would, on 

 suddenly obtaining his sight, be able to distinguish them by the latter sense. 

 This question was answered by Locke in the negative ; and probably with 

 justice. It is no real objection to such a reply, that a netv-born animal seeks 

 the nipple of its mother, when informed of its proximity by sight ; for all that 

 is indicated by this fact is, that the sensation excites an intuitive feeling of 

 desire, which gives rise to movements adapted to gratify it. Such instinctive 

 actions, founded upon intuitive perceptions, are, as already pointed out, much 

 more numerous in the lower animals than in the higher, and in the young of 

 the Human species than in the adult ( 259) ; and they do not afford any proof 

 that definite notions, such as we acquire, of the forms and properties of exter- 

 nal objects, are possessed by the animals which exhibit them. We shall now 

 examine, a little more in detail, into the means by which we gain such notions, 

 and the data on which they are founded. 



336. The first point to be determined, is one which has been a fruitful 

 source of discussion, the cause of erect vision, the picture upon the retina 

 being inverted. Many solutions of it have been attempted ; but they are for 

 the most part rather specious than really satisfactory. That which has been 

 of late years the most in vogue, is founded upon what was styled the Law of 

 Visible Direction, which has been supported by Sir D. Brewster and other 

 eminent Philosophers. This law affirms, that every object is seen in the 

 direction of the perpendicular to that point of the retina, on which its image 

 is formed ; or, in other words, that, as all the perpendiculars to the several 

 points of the inner surface of a sphere meet in the centre, the line of direction 

 of any object is identical with the prolonged radius of the sphere, drawn from 

 the point at which its image is made upon the retina. Upon close examina- 

 tion, however, it is found that this law cannot be optically correct ; since the 

 lines of direction cross each other at a point much anterior to the centre of the 

 globe; as may be determined by drawing a diagram upon a large scale, and 

 laying down the course of the rays received by the eye, according to the cur- 

 vatures and refractive powers of its different parts/ In this manner it has 

 been determined by Volkmann, that the lines of direction cross each other in 

 a point a little behind the crystalline lens; and that they will thus fall at such 

 different angles on different points of the retina, that no general law can be 

 laid down respecting them. It may be questioned, moreover, whether any 

 such law would afford any assistance in explaining the phenomenon; since, 

 after all, it is requisite to assume an intuitive applicatjon of it, in supposing 

 the mind to derive its ideas of the relative situations of objects, from the 

 imagined line of direction -A much simpler and more, direct explanation 

 may be given. We must remember that which we have had occasion to 

 notice in regard to all the other senses, the broad line of distinction between 

 the sensation and the perception or elementary notion; and this is still more 

 clearly shown by the complete absence of any relation, but such as experience ' 

 develops, between the perceptions derived through the sight and those 

 acquired from the touch. Hence there is no more difficulty in understanding 

 that an inverted picture upon the retina should convey to us a notion of the 

 external world, which harmonizes with that acquired through the sense of 

 touch, than there is in comprehending the formation of any of those intuitive 



