30 AN ESSAY ON 



know, only two theories offered respecting the 

 apparent directions of objects. One is, that 

 they are perceived in the direction of lines pass- 

 ing from their pictures on the retina, through 

 the centre of the eye ; the other, that their 

 apparent directions coincide with their visual 

 rays *. But both of these theories are inconsist- 

 ent with the phenomena of single vision with 

 two eyes. For according to neither of them can 

 an object, placed at the concourse of the optic 

 axes, be seen single, unless we have a most ac- 

 curate knowledge of its distance ; nor will either 

 admit two objects to be seen as one, which are 

 situated in the optic axes, whether on this side, 

 or beyond where they meet, unless the united 

 object be referred by sight to their very point 

 of intersection ; both of which conclusions are 

 contradicted by experience. It is evident, 

 therefore, that some other theory of visible di- 

 rection is required, which shall not be liable to 



* Mr. D'Alembert has said (Opuscules Mathematiques, 

 Tom. I. p. 265) that all optical writers before him had re- 

 garded it as an axiom, that every visual point is seen in the 

 direction of its visual ray. But the assertion is not well 

 founded. For Kepler long ago taught (Paralipomena in 

 Vitellionem, p. 173), that objects are perceived in lines pass- 

 ing from their pictures upon the retina, through the centre of 

 the eye j in which he was followed by Dechales and Doctor 

 Porterfield ; to the latter of whom Dr. Reid improperly at- 

 tributes the discovery of the same supposed law. 



