SINGLE VISION. 61 



force will be required to retain the right eye in 

 its original position, as is necessary to give to 

 the left eye its motion toward the right ; and 

 hence, that, in every different inclination of 

 the left axis to the right, an object placed in 

 the latter, though its real position be un- 

 changed, will, nevertheless, appear in a dif- 

 ferent direction, in consequence of the dif- 

 ferent state of action in the muscles of the right 

 eye, which accompanies every new degree of 

 inclination of the axes to each other. As the 

 object must always appear in the common axis, 

 the alteration, in this example, of its visible di- 

 rection, from an increase of the mutual inclina- 

 tions of the optic axes, will be from left to 

 right ; but when the inclination decreases, 

 from right to left. If the right axis be the one 

 which is moved, and the left fixed, the altera- 

 tions of visible direction in an object placed 

 in the latter, from similar changes in their in- 

 clinations, will be contrary to those which have 

 just been mentioned. 



The reason also can now be made to appear, 

 why an object, preserving constantly its place 

 in the optic axis, may, in a considerable variety 

 of its real positions, possess but one visible 

 direction. For, in such cases, the change of 

 its visible direction, which might be expected 

 to accompany the motion of the eye in the axis 



