SINGLE VISION. * 



existence of which it is impossible, from the 

 nature of the parts, either to demonstrate, or 

 to refute by experiments ; and that no one of 

 them, though admitted to be true, is yet suf- 

 ficient to explain the phenomena on account 

 of which it was framed. 



The opinions of the second class being built, 

 as their authors think, upon experiments and 

 observations, both allow and demand a more 

 accurate investigation. I shall proceed, there- 

 fore, to examine such of them as I am acquainted 

 with, beginning with that of Aguilonius ; and 

 what I shall observe concerning it will apply 

 also to those of Dechales and Dr. Porterfield, 

 who have done little more than copy what he 

 has said. 



If a line be drawn through the point of the 

 mutual intersection of the optic axes, parallel 

 to the interval between the eyes, Aguilonius 

 calls it, from its office, the horopter-, and if 

 through this line, a plane be made to pass at 

 right angles to that of the optic axes, he names 

 it the plane of the horopter. After defining 

 these terms, he asserts, that, by a law of our 

 constitution, all bodies which we see with one 

 glance or look, whatever are their real places, 

 appear to each eye to be situated in this plane. 

 And if this be granted to him, he easily and 

 satisfactorily shows, why some should be seen 



B 2 



