SINGLE VISION. 35 



from the object to one of the eyes, it will re- 

 present all the real or tangible positions of the 

 hole, which allow the object to be seen by that 

 eye, and the whole of it will coincide with the 

 optic axis. Let a similar line be drawn to the 

 other eye, and the two must appear but as one 

 line ; for if they do not, the two holes in the 

 optic axes will not, at every distance, appear 

 one, whereas experiments prove that they do. 

 This united line will, therefore r represent the 

 visible direction of every object situated in 

 either of the optic axes. But the end of it, 

 which is toward the face, is seen by the right 

 eye to the left, and by the left eye as much to 

 the right. It must be seen then in the middle 

 between the two, and, consequently, in the 

 common axis. And as its other extremity coin- 

 cides with the point where the optic axes inter- 

 sect each other, the whole of it must lie in the 

 common axis. Hence the truth of the proposi- 

 tion is evident, that objects, situated in the optic 

 axis, do not appear to be in that line, but in the 

 common axis. 



Many other experiments might be mentioned 

 which demonstrate the same thing. If, for ex- 

 ample, the head of a pin, or of a needle, be 

 interposed between each eye, and any small 

 object to which both the optic axes are directed, 



D 2 



