14 AN ESSAY ON 



example, if a right line be any where placed in 

 the plane of the optic axes, it follows, from 

 what he has said in one part of his book, that 

 those points of it, through which the axes pass, 

 must be seen united at the mark we look at, 

 the axes crossing each other there ; and from 

 what I have just quoted, that every other point 

 must be seen by each eye between its real place 

 and that mark. The appearances, therefore, of 

 all the points, if they do not lie disjoined, but 

 are connected together in some orderly manner, 

 will be arranged in the forms, either of two 

 curves, both passing through the intersection of 

 the optic axes, or of four right lines meeting 

 one another at that point. If the right line be 

 placed nearer to the face than the mark we 

 look at, the apparent lines, whether curved or 

 straight, will approach toward us from their 

 common point, but recede from us, if the real 

 line be situated beyond the mark. Such are 

 the phenomena which ought to follow upon the 

 admission of these two parts of Dr. Smith's 

 theory of vision with two eyes, but which are 

 not found to exist in nature. Aguilonius was 

 at least consistent when he maintained, that all 

 objects are seen in the plane of the horopter ; 

 while Dr. Smith, by deserting that opinion in 

 part, seems only to have involved himself the 

 more deeply in error. 



