26 AN ESSAY ON- 



the optic axes cross each other near to the fare, 

 was actually made by Dr.. Reid, with this result, 

 that the two objects appeared in different places, 

 when seen by both eyes together ; and that the 

 other experiment, in which the optic axes are 

 supposed to diverge, was made by myself, with 

 a similar event. Dr. Reid, however, instead of 

 being led, by the termination of his experiment, 

 to impute a fault to the principle from which 

 he had expected a different one, concluded from 

 it, that there was something unnatural, beside 

 the squinting, in the person's eyes, upon whom 

 it was made ; though it had been previously 

 ascertained, that objects appeared in the ordi- 

 nary manner to each of them, when separately 

 employed. 



My examination of the second class of opi- 

 nions, respecting the cause of the single appear- 

 ance of objects to two eyes, being finished, some 

 person, perhaps, will now say ; Granting that no 

 error can, at first sight, be shown in your argu- 

 ments against those of Dr. Smith and Dr. Reid, 

 is it not a sufficient reason for believing them 

 fallacious, that they prove too much ? If objects 

 appear single neither from custom, nor an ori- 

 ginal property of the eyes, have we not an effect 

 without a cause, and must there not be some- 

 thing wrong in the facts or reasoning which lead 

 to such a conclusion ? The answer I make is 



