762 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



we look at the picture formed by a Camera with the bodily eye ; and that 

 our consciousness must be therefore impressed by its discordance with the 

 information which we receive through our sense of Touch. Some philoso- 

 phers, indeed, have actually gone so far as to assert that the Infant must at 

 first see everything inverted, and that the erectness of visual objects is only 

 learned by the corrective experience gained by touching and handling them. 

 But such is clearly not the case ; for the visual perception is obviously not a 

 mere transfer of the seusorial impression, but is a mental state excited by it, 

 and therefore related to it as an effect to its cause ; and we know no reason 

 why it should be less natural for the retinal picture to suggest to the mind the 

 notion of erect position, than for it to have the contrary effect. Moreover, it 

 will appear from investigations to be hereafter detailed ( 635), that there is 

 in the eye a common "centre of direction," through which all lines must 

 pass, that are drawn from any points of an external object to the correspond- 

 ing points of its retinal image; 1 and that we immediately refer the cause of 

 the excitation of any spot of the retina by a luminous impression, to an 

 objective source in the "line of direction" which passes from that spot 

 through the centre of direction ; so that, in virtue of this " law of visible 

 direction," as all the lines of direction cross each other both vertically and 

 laterally, the formation of an inverted image upon our retina suggests to our 

 minds the representation of the object in its erect position, 'and the same re- 

 versal takes place also in regard to its two sides, which are transposed in the 

 retinal picture. A peculiar arrangement of the receptive apparatus, which 

 seems to be subservient to this mental appreciation of direction, has been 

 already noticed ( 615). 



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

 the subject of much discussion ; and here, too, the difficulty is rather appar- 

 ent than real, having for its foundation the idea that the mind looks at the 

 two retinal pictures as at two separate objects, instead of being impressed by 

 a certain state of the Sensoriutn, which may be excited through the instru- 

 mentality of either eye, or through that of both in combination. Some 

 have even asserted, under the influence of this idea, that we do not really 

 employ both eyes simultaneously, but that the mind is affected by the image 

 communicated by one only; which might seem to be confirmed by the fact 

 sufficiently well ascertained respecting the alternate use of the two eyes, 

 when they are looking through two differently-colored media. But of this 

 assertion a complete disproof is afforded by the knowledge we now possess 

 ( 623), that our appreciation of the solid forms of bodies depends on the com- 

 bination by the Mind, of the images simultaneously suggested by the two pic- 

 tures; and that our knowledge of distances is in great part obtained in like 

 manner. Attempts have been made to explain the phenomena of Single 

 Vision by the peculiar decussatiou of the Optic Nerves formerly described 

 ( 518) ; it being supposed that only one Optic Ganglion would be affected 

 by an impression made upon both Retinse. 1 This explanation, however, 



i With regard to the precise situation of this "contre of direction," there is a want 

 of accordance among those who have attempted to determine it; some having placed 

 it in the centre of the pupil, others in the centre of the crystalline lens, others at 

 various distances between this and the. centre of the globe, and others (among them 

 Sir D. Brewster) in the centre of the globe. This last notion, with the " law of visi- 

 ble direction " founded upon it, which affirms that every object is seen in the direc- 

 tion of the perpendicular (or radius) to every point of the retina which i< impressed 

 is so manifestly wrong, that it is difficult to conceive how it could ever have been 

 entertained by men of science. The experimental investigations of Listing indicate 

 a point near the centre of the crystalline as the " centre of direction " (% 609). 



8 This decussation seems to be even more complete than was formerly supposed, for 

 in various experiments made by Dr. Waller upon the effects of section of one optic 



