SINGLE VISION. 



voluntary motions of the eye, it must, I think, 

 be necessarily owing to the action of the mus- 

 cles, by which these motions are performed. 

 Assuming then as true, that the apparent di- 

 rection of an object, 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 

 axis, which were mentioned in the beginning 

 of this part of my Essay. 



The thing itself is universally acknowledged, 

 though a dispute has arisen whether custom or 

 an original property be the cause, that every 

 voluntary motion of one eye, in persons who 

 do not squint, is attended with a corresponding 

 motion in the other. Now as all voluntary 

 motions are produced by muscular action, it 

 follows, that every state of action, in the 

 muscles of one eye, has its corresponding state 

 in those of the other, and that the two are 

 constantly conjoined. When, therefore, the 

 spot appears single to both eyes in their free 

 positions, the states of action in the muscles 



