34 AN ESSAY ON 



makes the two holes to appear one, does not 

 determine where this one is to be seen. It 

 cannot be seen in only one of the perpendicu- 

 lars to the images upon the retinas, for no reason 

 can be given why this law of visible direction, 

 which Dr. Reid thinks established beyond dis- 

 pute, if it operates at all, should not operate 

 upon both eyes at the same time ; and if it be 

 seen by both eyes in such lines, it must appear 

 where those lines cross each other, that is, in 

 the same place with the object viewed through 

 the holes, which, as I have already mentioned, 

 is contrary to experience. Nor is it seen in any 

 direction, the consequence of a law affecting 

 both eyes considered as one organ, but sus- 

 pended when each eye is used separately. For 

 when the two holes appear one, if we pay at- 

 tention to its situation, and then close one eye, 

 the truly single hole will be seen by the eye re- 

 maining open, in exactly the same direction as 

 the apparently single hole was by both eyes. 



Hitherto I have supposed the holes almost 

 touching the face. But they have the same 

 unity of appearance, in whatever parts of the 

 optic axes they are placed ; whether both be 

 at the same distance from the eyes, or one be 

 close to the eye in the axis of which it is, and 

 the other almost contiguous to the object seen 

 through them. If a line, therefore, be drawn 



