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



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 posi- 

 tion 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 repre- 

 sented to the mind as double. Hence, too, it seems clear that singleness of 

 vision in an object that is looked at, is also dependent in part upon the con- 

 vergence of the optic axes in that object ( 613) ; and that this is the case 

 appears further from a curious experiment devised by Prof. Wheatstoue, in 

 which two similar objects are made to seem as one, when they are placed in 

 the line of convergence. This is accomplished by looking through two 

 tubes, placed before the right and left eyes respectively, at two similar 

 objects of any description, placed near the farther extremities of the tubes; 

 if, now, these objects be slightly approximated, so that the axes of the tubes 

 (still directed towards them) meet in a point beyond, the mind is impressed 

 with the image of only a single object, and this appears to be removed back 

 to the point of convergence. 



622. On the mode in which our notion of the solid forms and relative pro- 

 jection of objects is acquired, great light has been thrown by the interesting 

 experiments of Professor Wheatstone. 1 It seems perfectly evident, both from 

 reason and experience, that the flat picture upon the retina, which is the 

 immediate source 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 view is fully confirmed by the case already re- 

 ferred to ( 619), as recorded by Cheselden. The first visual idea entertained 

 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 

 Professor Wheatstoue, that they concur in exciting the perception of solidity 

 or projection, which arises from the mental combination of the two dissimilar 

 pictures formed upon the two retinae. It is easily shown, that any near ob- 

 ject is seen in two different modes by the two eyes. Thus let the reader 

 hold up a thin book, in such a manner that its back shall be exactly in front 

 of his nose, and at a moderate distance from it ; he will observe by closing 

 first one eye and then the other, that his perspective view of it (or the man- 

 ner in which he would represent it on a plane surface) is very different, ac- 



states that the interior system of the Horoptcr is an hyperbole of equal dimensions 

 lor each eye ; one of the radii of which passes through the t'ovca contrails and never 

 passes through the punctum c;vcum ; its summit is found at the side of the fovea. The 

 real axes have the same angle between them as that of t.he two retinal horizons ; their 

 inclination on this does not dill'er much from half a right angle. In individual cases 

 it is found that for a point of fixation situated at an infinite distance the Horopter 

 becomes a spherical surface with an infinite radius. Thus in looking at a star all the 

 stars are found on the Hornpter. Fur a point of fixation situated in the primary 

 position of the retinal plane the Horopter becomes reduced to a circle, the circle of 

 the Horopter of Miiller passing through the centre of each globe and the fixed point. 

 For n point of fixation situated in t.he median plane, the Horopter is an ellipse situ- 

 ated in a plane which bisects the angle formed by the visual plane and the primary 

 position. 



1 Philosophical Transactions, 1838 and 1852. 



