58 AN ESSAY ON 



must be such, that the direction, in which it is 

 seen by one eye, coincides with that in which 

 it is seen by the other. But, if we push one 

 eye from its place, no change is hereby made 

 in the action of its muscles ; for the state of 

 action in those of the free eye is confessedly 

 the same as it was ; and it will be attended 

 with a corresponding state in those of the dis- 

 torted eye ; in proof of which it may be ob- 

 served, that, whenever the pressure is removed, 

 the distorted eye immediately returns to its 

 former position, without the aid of any new 

 muscular effort. The conclusion then is, that, 

 since there has been no alteration in the action 

 of its muscles, neither ought there to be any in 

 the direction of the spot seen by it, which is 

 the fact to be explained. 



Hence also is to be derived the true reason, 

 why objects appear double, when one eye is 

 pushed from its place. For as their pictures 

 must fall upon points of the retina in this eye, 

 different from what they formerly possessed ; 

 and as no change is made, by the distortion, 

 upon the visible direction, suggested by any 

 part of the retina, the objects will be seen by 

 the pressed eye, exactly in the same directions 

 as they would have been, before it was pressed, 

 had the pictures then fallen upon the points of 

 the retina, which they now occupy. They must 



