Double and Inverted Images on the Retine. 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 Rex, 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 
retine, 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 Rem 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 Rx1p 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 Rein, “ from the time we are capable of ob- 
serving the phenomena of single and double vision, custom makes 
no change inthem. 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. 3P 
