SINGLE VISION. 23 



the direction of a line passing from its picture 

 on the retina, through the centre of the eye. 

 If, therefore, this direction be the same as that 

 suggested by the original property so often men- 

 tioned, the latter law is merely another ex- 

 pression for the former, and ought to be rejected 

 as superfluous. If it be different, and should 

 the two laws exist together, objects seen with 

 both eyes might sometimes appear quadruple, 

 sometimes triple, but never single. Were they 

 to exist successively, one when we employ one 

 eye, the other when both, an object, though at 

 rest, should always appear to move when viewed 

 alternately by one and by both eyes ; neither of 

 which conclusions is agreeable to experience. 



Thirdly ; To show in a different way, and one 

 perhaps more easily understood, that the opinion 

 of Dr. Reid is not consistent with the pheno- 

 mena of vision it ought to explain, I shall sup- 

 pose an experiment to be made upon a person 

 who squints. But I must premise, that it ap- 

 pears, from the observations of Dr. Jurin* and 

 himselff, that all such persons have one eye of 

 a weaker sight than the other ; that when both 

 eyes are open, the weaker is turned away from 

 objects, which are attentively viewed \ but that 



* Smith's Optics, Vol. II. Remarks, p. 3O 

 f Inquiry, chap. vi. sect. 10. 



