SENSE OF VISION. 549 



as he had been formerly accustomed to do, than by his newly-acquired 

 sense of Sight ; being evidently perplexed, rather than assisted by the 

 sensations which he derived through it. But when learning a new 

 locality, he employed his sight and evidently perceived the increase 

 of facility which he derived from it. Among the many interesting 

 particulars recorded of the youth on whom Cheselden operated with 

 equal success, it is mentioned that, although perfectly familiar with a 

 dog and a cat by feeling them, and quite able to distinguish between them 

 by his sight, it was long before he associated his visual with his tactual 

 sensations, so as to be able to name either animal by sight alone. The 

 question was put by the celebrated Locke, whether a person born blind, 

 who was able by his touch to distinguish a cube from a sphere, would, 

 on suddenly obtaining his sight, be able to recognise each by the latter 

 sense; the reply was given in the negative ; and the experience of the 

 cases just referred to, as well as of many others, fully justifies such an 

 answer. 



963. Still there are, even in Man, certain intuitive perceptions, 

 which afford great assistance in the formation of ideas regarding ex- 

 ternal objects through the visual sense. And the first of these is the 

 power by which we recognise their erect position, notwithstanding the 

 inversion of the image upon the retina. This is certainly not a matter 

 of experience ; nor is it capable of explanation (as some have thought) 

 by a reference to the direction in which the rays fall upon the retina. 

 It is the mind which rectifies the inversion ; and as already remarked, 

 it is just as difficult to understand how the inverted image on the retina 

 should be taken cognizance of by the mind at all, as it is to comprehend 

 how it should be thus rectified. In fact, there is no real connexion 

 whatever, between the inversion of the image upon the retina, and that 

 wrong perception of external objects, which some have thought to be 

 its necessary consequence. Any distortion of the picture, giving a 

 wrong view of the relative positions of the objects represented, would be 

 attended with a different result. The same may be said of the cause of 

 the singleness of the sensation perceived by the mind, although an 

 image is formed upon the retina of each ye, of those objects at least, 

 which lie in the field of vision that is common to both. This blending 

 of the pictures, formed upon the two retinae, into a single perception, 

 appears to be, in part at least, the effect of habit. For when the images 

 do not fall upon those parts of the two retinae, which are accustomed to act 

 together, double vision is the result. Thus if, when looking steadily at 

 an object, we press one of the eyeballs sideways with the finger, we see 

 two representations of the object ; and the same thing frequently occurs, 

 as a result of an affection of the nerves or muscles of one or both eyes 

 (as in ordinary strabismus or squinting), or from some change in the 

 nervous centres, as in various disorders of the Encephalon, and in in- 

 toxication. If this condition should be permanent, however, we usually 

 find that the individual becomes accustomed to the double images, or 

 rather ceases to perceive that they are double ; probably because the 

 mind becomes habituated to receive the impressions from the two parts 

 of the retinae which now act together. And if, after the double vision 

 has passed away, the conformity of the two eyes be restored (as by the 



