Double and Inverted Images on the Retina. 475 



rence of the sensation excited, from that which is usually felt, is 

 at once perceived, and the association, by which its unity had 

 been made known to us, is broken. 



But this answer does not apply to the cases stated on this 

 point by Dr Reid, of persons who had squinted from infancy, 

 and in whom, therefore, the association of single objects must 

 have been formed with images on dissimilar points of the 

 retina?, and must have been broken when the images of an ob- 

 ject were formed on corresponding points. These persons, ac- 

 cording to the theory in question, should have seen objects single 

 when they squinted in their usual way, and not when the axes 

 were brought to bear on the same point, in a way quite unusual 

 to them. But the reverse was the fact. They, saw double when 

 they squinted (excepting in particular positions of the eyes, when, 

 as Dr Reid supposed, one of the images was formed on the well 

 known insensible spot on the retina) ; and " when they learned 

 to direct both eyes to an object, they saw it single." 



I can myself confirm the observations of Dr Reid from pretty 

 numerous trials on persons who habitually squinted ; in which it 

 always appeared, if the vision of both eyes was tolerably good, 

 that, when the attention was fairly fixed on the sensations of both 

 eyes, single objects held directly before the face were seen double, 

 and again, that different and distant objects held carefully in the 

 direction of the axes of the two eyes, seemed to coincide. 



Again, says Dr Reid, " from the time we are capable of ob- 

 serving the phenomena of single and double vision, custom makes 

 no change in them. I have amused myself," he adds, " with such 

 observations for more than thirty years ; and in every^case, where- 

 in I saw the object double at first, I see it so to this day, not- 

 withstanding the constant experience of its being single. In 

 other cases, where I know there are two objects, there appears 

 only one, after thousands of experiments. 



" Effects produced by habit must vary, according as the acts 

 by which the habit is acquired are more or less frequent ; but 



VOL. XIII. PART II. 3 P 



