884 OF SENSATION, AND THE ORGANS OF THE SENSES. 



everything is seen reversed, the relative position of the objects remains un- 

 changed. Hence it is, also, that no discordance arises between the sensations 

 of inverted vision and those of touch, which perceives everything in its erect 

 position; for the images of all objects, even of our own limbs, "on the retina, 

 are equally inverted, and therefore maintain the same relative position. Even 

 the image of our hand, when used in touch, is inverted." From what has been 

 stated, it would appear quite conceivable, that a person just end owed with sight, 

 should not at first know, by his visual powers, whether a pyramid placed before 

 his eyes is the same body, and in the same position, as one with which he has 

 become acquainted by the touch ; and, if this be admitted, the inference neces- 

 sarily follows, that the notion of erectness, which we form by the combined use 

 of our eyes and our hands, is really the product of experience in ourselves, 

 whilst it is probably innate or intuitional in the lower Animals. 



886. The cause of Single Vision with the two Eyes has in like manner been 

 the subject of much discussion, since the mode in which we are affected by 

 the two simultaneous impressions is quite different from that in which we 

 derive our knowledge of external things through the other senses. Some 

 have even asserted that we do not really employ both eyes simultaneously, but 

 that the mind is affected by the image communicated by one only ; and this 

 idea might seem to be confirmed by the fact heretofore mentioned ( 861), re- 

 specting the alternate use of the two eyes, when they are looking through two 

 differently colored media. But it is easily disproved in other ways. It will 

 presently be shown that all our estimates of the forms of bodies depend on 

 the combination, by the mind, of the images simultaneously transmitted by the 

 two eyes ; and our knowledge of distances is in great part obtained in like 

 manner. One condition of Single Vision, however, seems to be this, that the 

 two images of the object should be formed on parts of the two retinae which 

 are accustomed to act in concert ; and habit appears to be the chief means by 

 which this conformity is produced. There can be no doubt, however, that 

 double images are continually being conveyed to the Sensorium ; but that, from 

 their want of force and distinctness, and from the attention being fixed on 

 something else, we do not take cognizance of them. This may be shown by a 

 very simple experiment. If two fingers be held up before the eyes, one in 

 front of the other, and vision be directed to the more distant, so that it is seen 

 singly, the nearer will appear double ; while, if the nearer one be regarded more 

 particularly, so as to appear single, the more distant will be seen double. A 

 little consideration will show, therefore, that our minds must be continually 

 affected with sensations, which cannot be united into the idea of a single image; 

 since, whenever we direct the axes of our eyes towards any object, everything 

 else will be represented to us as double ; but we do not ordinarily perceive this, 

 from our minds being fixed upon a clear and distinct image, and disregarding, 

 therefore, the vague undefined images formed by objects at a different focus. 

 Of this it is very easy to convince one's self. It is, moreover, evident, from this 

 experiment, that double vision cannot result from want of symmetry in the 

 position of the images upon the retina, to which some have attributed it ; for it 

 answers equally well if the line of the two fingers be precisely in front of the 

 nose, so that the inclination of both eyes towards either object is equal; 

 the position of the images of the second object must then be at the same 

 distance on either side from the central line of the retina, and yet they are 

 represented to the mind as double. Hence it seems clear that singleness of 

 vision is also dependent upon the convergence of the optic axes in the object to 

 which our gaze is directed. Attempts have been made to explain the pheno- 

 mena of Single Vision by the peculiar decussation of the Optic Nerves formerly 

 described ( 742), it being supposed that only one Optic Ganglion would be 

 affected by an impression made upon both Retinae. This explanation, however, 



