of Mr Charles Bell. 267 



" Now it is evident, from these two last experiments, that the situation 

 of the spot does not depend upon the bare position of the eyes, or else in 

 the former of them it would have appeared double, and in the latter it 

 would have been moved from the middle of the paper, when the only eye 

 by which it was seen, was pushed from its place. Neither can it depend 

 upon the bare position of the muscles of the eye, as these were also moved 

 in the same experiments, nor upon any affections whatever of the optic 

 nerve. For since this last substance is altogether passive, even in those 

 motions of the eyes which do occasion a change of the spot's situation, every 

 alteration induced upon the nerve by those motions, must be ultimately 

 ascribed to a change of its position ; and we have seen, that similar changes 

 of its position, have been produced by external violence without any alter- 

 ation of the spot's situation. The apparent situation of the spot being, 

 therefore, dependent upon none of these circumstances, and being at the 

 same time affected by the voluntary motions of the eye, it must, I think, be 

 necessarily owing to the action of the muscles by which these motions are 

 performed. Assuming, then, as true, that the apparent direction of an ob- 

 ject, which sends its picture to any given point of the retina, depends upon 

 the state of action existing at the same time in the muscles of the eye, and, 

 consequently, that it cannot be altered, except by a change in the state of 

 that action, I shall proceed to trace to this principle several phenomena 

 of vision, particularly the uniform singleness of the spot already described, 

 and the two facts respecting the visible directions of objects in the optic 

 axes, which were mentioned in the beginning of this part of my essay." 



Having thus quoted Dr Wells's own account of his experi- 

 ments, we shall give a brief, and we trust an irrefragable de- 

 monstration of their fallacy. 



1. When we push up the one eye by the force of the fin- 

 ger, and keep it in its new position, it has not performed an an- 

 gular movement in its socket, but merely a small vertical as- 

 cent, by which, in an upright position of the body and head, 

 it is raised into a more elevated horizontal plane. In this ele- 

 vated position, the eye can execute horizontal, and even verti- 

 cal angular movements, and can direct itself to contemplate 

 either one or other of the double images of the objects before 

 it. The consequence of this is (as we have explained in this 

 Journal No. iii. p. 5,) that the spectral impression ascends only 

 through a very small space, which it requires nice observation 

 to appreciate, but which increases with the force applied to the 

 eye. 



2. It is impossible, by the pressure of the finger, to fix the 

 left eye in a different angular position from the right eye ; 



