SINGLE VISION. 



must be rejected, together with all its conse- 

 quences. To Aguilonius, however, the merit 

 is due, of being the first who so far generalised 

 the phenomena of single and double vision, as 

 to observe, that those objects alone are seen 

 single, which are really situated in the plane of 

 the horopter. 



The opinion of Dr. Smith is the next in the 

 order of time. * " If it be asked (says that 

 author) why, in seeing with both eyes, we do 

 not always see double, because of a double sensa- 

 tion, I think it is sufficient to say, that in the 

 ordinary use of our eyes, in which the pictures 

 of an object are constantly painted upon | corre- 

 sponding places of the retinas, the predominant 

 sense of feeling has originally and constantly 

 informed us that the object is single. By this 



* Complete System of Optics. Vol. I. p. 48. 



f Dr. Smith gives the following definition of corresponding 

 points. " When the optic axes are parallel, or meet in a 

 point, the two middle points of the retinas, or any points 

 which are equally distant from them, and lie on the same 

 sides of them, either towards the right hand or left hand, 

 or upwards or downwards, or in any oblique direction, are 

 called corresponding points." Vol. I. p. 46. According to 

 this definition, points correspond which have a certain agree- 

 ment in situation. No contradiction is, therefore, implied 

 in this system, by saying, that an object may appear single, 

 though its pictures should fall upon points which do not 

 correspond. Dr. Reid's definition of the same term is very 

 different. 



