416 ON SENSATION, AND THE ORGANS OF THE SENSES. 



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 perceptions of animals, 

 which are so much more removed from the teachings of our own experience 

 ( 490). It is justly remarked by Muller 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 everything 

 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 everything in its erect position ; for the 

 images of all objects, even of our own limbs, on the retina, are equally in- 

 verted, 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 ne- 

 cessarily 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. 



544. 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 ( 519), 

 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, de- 

 pend on the combination by the mind, of the images simultaneously transmit- 

 ted 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 

 ( 454) to be probably 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 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 continually being conveyed to our minds ; 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 oneself. 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 



