SINGLE VISION. 



PROPOSITION I. 



Objects situated in the Optic Axis, do not appear to be in 

 that Line, but in the Common Axis. 



EVERY person knows, that, if an object be 

 viewed through two small holes, one applied to 

 each eye, the two holes appear but as one. The 

 theories hitherto invented afford two explana- 

 tions of this fact. According to Aguilonius, 

 Dechales, Dr. Porterfield and Dr. Smith, the 

 two holes, or rather their borders, will be seen 

 in the same place as the object viewed through 

 them, and will consequently appear united, for 

 the same reason, that the object itself is seen 

 single. But whoever makes the experiment 

 will distinctly perceive, that the united hole is 

 much nearer to him than the object ; not to 

 mention, that any fallacy on this head might be 

 corrected by the information from the sense of 

 touch, that the card, or other substance, in 

 which the holes have been made, is within an 

 inch or less of our face. The other explana- 

 tion is that furnished by the theory of Dr. Reid. 

 According to it, the centres of the retinas, which 

 in this experiment receive the pictures of the 

 holes, will, by an original property, represent 

 but one. This theory, however, though it 



D 



