24 AN ESSAY ON 



when the strong eye is closed, the weaker is 

 pointed to objects,, exactly as the former would 

 be in the same situation ; and that it likewise 

 perceives them in similar directions. Let now 

 the ordinary position of the person's eyes, upon 

 whom the experiment is made, be such, that 

 the optic axes intersect each other about an 

 inch or two from the face ; and while the other 

 is closed, let the flame of a candle be placed in 

 the axis of the weak eye, which I shall call the 

 left, at the distance of some feet from it, and on 

 the right side of the body. The flame will con- 

 sequently appear in the same direction, as if his 

 eye had no fault, and will be seen on his right, 

 where it is in reality situated. Both eyes re- 

 taining the same position with respect to his 

 head and each other, let the weak eye be after- 

 ward shut, and the right opened, and let another 

 object be placed in the axis of the latter, an 

 opake body being at the same time so disposed, 

 as to hide from it the candle which is in the 

 axis of the left eye. This object in the right 

 axis will consequently appear on the left side* 

 Now, since the two objects, which have been 

 thus viewed separately, are situated, one to the 

 right, and one to the left ; and since they have 

 been also seen in those positions, their visible 

 places must be two, as well as their tangible, 

 and must be remote from each other. How 



