SINGLE VISION. 39 



to terminate at the left eye, disappears ; and if 

 the left eye be closed, then the other wire dis- 

 appears, whose termination was at the right 

 eye. The real wire, therefore, in the common 

 axis, appears to the right eye to be situated in 

 the axis of the left, and to the left eye to be 

 situated in the axis of the right, agreeably to 

 what the proposition asserts. 



The following experiments will illustrate and 

 confirm both this arid the preceding proposition. 

 Through a piece of card, or pasteboard, let two 

 small holes be made, the interval between which 

 is such, that while a very remote object is seen 

 through one of them by the right eye, the same 

 object may be seen through the other by the 

 left eye. Make afterward another hole in the 

 card, or pasteboard, exactly in the middle be- 

 tween the two former ; and let the object be 

 viewed through them as before. These, or 

 the outer holes, will now appear one, precisely 

 where the sense of feeling indicates the middle 

 hole to be ; while the middle hole will appear 

 as two, which seemingly occupy the places of 

 the real outer ones. The two appearances of 

 the middle hole, which is placed by construction 

 in the common axis, are therefore seen in the 

 optic axes ; and as the left is not seen when the 

 right eye is shut, nor the right when the left 

 eye is shut, each appearance is observed in the 



