SENSE OF VISION. 249 



perceptions of animals which are so much more removed from the teachings 

 of our own experience ( 290). 1 It is justly remarked by Miiller that, " if we 

 do see objects inverted [or rather, if the picture on the retina is inverted] the 

 only proof we can possibly have of it, is that afforded by the study of the laws 

 of Optics ; and, if every thing is seen reversed, the relative position of the 

 objects remains unchanged. Hence it is, also, that no discordance arises 

 between the sensations of inverted vision and those of touch, which perceives 

 every thing 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 endowed 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 necessarily 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. 



337. 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 ( 313) 

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

 two differently-coloured 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. / The condition of Single Vision has been already stated ( 253) to 

 be probably this, that the two images of the object should be formed on parts 

 of the two retina? which are accustomed to act in concert ; and reasons were 

 given for the belief, that habit is the chief means by which this conformity is 

 produced. ; There can be no doubt, however, that double images are continu- 

 ally being conveyed to our minds ;' but that, from their want of force and dis- 

 tinctness, and from the attention being fixed on something else, we do not take 

 cognizance of them. This may be sho\vn 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, every thing 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 each 

 side from the central line of the retina, and yet they are represented to the 



