SINGLE VISION. 63 



perpendicular to the visual base toward the left; 

 and the muscular actions, which regulate the 

 mutual distances of the pupils, by increasing 

 the inclinations of the axes to each other, 

 moved it afterward, from the left to the right, 

 back again to a perpendicular to the visual base. 

 Let these two causes act together, and it is 

 plain, that no observable effect will be pro- 

 duced by either, as long as they are thus pro- 

 portioned. When they are not so, only the 

 difference of their forces will be exhibited by 

 the phenomena. 



But farther ; to show the extent of this theory 

 of visible direction being dependant upon the 

 actions of the muscles of the eyes, I shall now 

 apply it to the explanation of an instance of 

 apparent motion, which at first may be thought 

 to furnish an argument against it. Look with 

 one eye, the other being closed, at any remote 

 object through a small hole in a card. If you 

 should afterward suddenly attempt to view the 

 hole itself accurately, with the same eye, you 

 will observe both it and the distant object, par- 

 ticularly the latter, to move from left to right, 

 if the right eye be used ; but; if the left eye be 

 the one employed, then from right to left. 

 Shift now your attention as suddenly back from 

 the hole to the object seen through it, and 

 both will return to the places they formerly 



