SINGLE VISION. 11 



% 



appear to either eye, it certainly cannot be seen 

 in the same place by both, except at some point 

 common to the two directions. Dr. Smith ac- 

 knowledges this, and says*, that when an ob- 

 ject is perceived single with both eyes, it is 

 seen at the mutual intersection of the two visual 

 rays ; the visible direction of any object coin- 

 ciding, according to him, with the visual ray, 

 or the principal ray of the pencil which flows 

 from it to the eye. Should we then even 

 allow, that all we know by sight of the places 

 of bodies has been borrowed from feeling, it 

 will still be easy to show, that the rule of vision 

 for each eye, which he has derived from such 

 experience, 'that of our seeing objects in the 

 directions of their visual rays, is inconsistent 

 with many of the phenomena of sight with two 

 eyes ; and, consequently, that he has left un- 

 removed the chief difficulty of his subject, which 

 was to explain the single appearance of objects 

 to both eyes, from those laws, or rules of vision, 

 which affect each of them singly. For it is a 

 well-known fact, that if two bodies of the same 

 shape, size, and colour, be placed, one in each 

 optic axis, they appear but as one body, pro- 

 vided they be at equal distances from the eyes. 

 Agreeably to the theory of our seeing objects 



* Vol. II. Remarks, p. 86. 



