60 AN ESSAY ON 



by that of another, made by myself, both of 

 which have been already related. 



To explain, therefore, why an object in the 

 optic axis appears at different times in different 

 directions, though the axis be kept fixed, it 

 is only necessary to show, that, whenever this 

 happens, a change, notwithstanding, occurs in 

 the actions of the muscles which move the eye. 

 With this view, I observe, that the motions of 

 that organ maybe divided into two sets; the 

 first, consisting of those, by which one eye 

 is carried along with the other, upward and 

 downward, to the right and to the left, and in 

 every oblique direction, the interval between 

 the pupils remaining constantly the same ; the 

 second, of the motions of the pupils, or the an- 

 terior parts of the eyes, to and from each other. 

 Supposing now, that both the optic axes are 

 perpendicular to the visual base ; should the 

 left axis be afterward inclined to the right side, 

 the natural tendency of the right axis is to in- 

 cline equally to the same side, so as to preserve 

 its former parallelism to the left. This ten- 

 dency, however, in the right axis to follow the 

 left, may be counteracted by an effort of the 

 muscles, which regulate the interval of the 

 pupils, until the two axes intersect each other 

 within two or three inches of the face. But it 

 is evident, that the same degree of muscular 



