SENSE OF VISION. 417 



attributed it ; for it answers equally well, if the line of the two fingers be pre- 

 cisly 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 mind as double. It is, moreover, easily shown, 

 that, in the lower animals whose orbits are not directed forwards as in us, but 

 sideways in a greater or less degree, whenever an object is so situated as to 

 be seen by both eyes, the points of the two retina? on which its images are 

 formed, must be very far from possessing this symmetry. 



545. Many attempts have been made to explain the phenomena of Single 

 Vision by the peculiar decussation of the Optic nerves ( 445) ; and an inte- 

 resting correspondence between the varieties in the degree of decussation, and 

 the position of the eyes, in several animals, has been pointed out by Mr. Solly 

 and Mr. Mayo. From these and other data, it has been concluded, that each 

 nerve is used in looking towards the opposite side. This is evidently true of 

 the Osseous Fishes, whose two eyes, being directed sideways, have two en- 

 tirely different spheres of vision. And it is also true of Man, if Mr. Mayo's 

 account of the distribution of the nerve be correct ; since, when we look at 

 an object held directly in front of the face, at the level of the eyes, and at the 

 nearest point for distinct vision, almost the whole of that portion of the right 

 retina, which lies to the outside of the entrance of the optic nerve, is directed 

 to the left; and the exactly different, complementary, or inner portion of the 

 left retina, which is supplied by the same nerve, is likewise directed to the 

 left. On this supposition, all the rays entering the two eyes from any one 

 point, will be brought to a focus on fibrils belonging to the nerve of the same 

 side ; though these are in Man, as in other animals whose spheres of vision 

 are nearly or partly coincident, distributed to distinct visual organs.* It is 

 obvious, however, that this or any similar explanation, must be insufficient to 

 explain the phenomenon of single vision ; since the images formed upon the 

 two retinae are necessarily different, and must be combined or harmonized by 

 an act of the mind, as will be shown in the succeeding paragraphs. 



546. We shall next consider the mode, in which our notion of the solid 

 forms and relative projection of objects is acquired ; on which great light has 



recently been thrown by the interesting experiments of Mr. Wheatstone.t It 

 is perfectly evident, both from reason and experience, that the flat picture upon 

 the retina, which is the only object of our sensation, could not itself convey to 

 our minds any notion, but that of a corresponding plane surface. In fact, any 

 notion of solidity, which might be formed by a person, who had never had 

 the use of more than one eye, would entirely depend upon the combination of 

 his visual and tactile sensations. This idea is fully confirmed by the case 

 already referred to, as recorded by Cheselden. The first visual idea formed 

 by the youth was, that the objects around him formed a flat surface, which 

 touched his eyes, as they had previously been in contact with his hands ; and 

 after this notion had been corrected, through the education of his sight by his 

 touch, he fell into the converse error of supposing that a picture, which was 

 shown to him, was the object itself represented in relief on a small scale. But 

 where both eyes are employed, it has been ascertained by Mr. Wheatstone, 



The late Dr. Wollaston was subject to a curious affection of vision, which consisted in 

 his not being able to see more than half an object, the loss being sometimes on one side, 

 and sometimes on the other. The Author has met with several cases of this disorder, which 

 has been termed hemiopia. Dr. W. thought that they might be explained by the decussation 

 of the optic nerve; but Mr. Mayo states that he has known instances of a parallel affection, 

 involving alternately the centre and the circumference of the retina, and therefore not attribut- 

 able to any such structural arrangement, 

 -j- Philosophical Transactions, 1838. 



