on Muscular Motion. 
9 
eornea of the eye which had undergone the operation being 
rendered flatter than the other, and giving a different direction 
to the rays of light, so as to form an image on a part of the 
retina not corresponding with the part impressed in the other 
eye. 
If the crystalline lens be extracted from both eyes, and the 
person applies a convex glass to one eye only, and looks at an 
object, it will appear double ; but if the convex glass is moved 
in different directions before the cornea, there will be found 
one situation in which it makes the object single. In this in- 
stance the corneas and muscles of the two eyes are under 
exactly the same circumstances ; and when the centre of the con- 
vex glass is directly in the axis of vision, the image on the retina 
of that eye is formed on parts that correspond with those im- 
pressed in the other ; but whenever the centre of the convex 
glass is out of the axis of vision this does not take place, and 
the object appears double. 
The experiments of which these observations are the result, 
were made upon the eyes of a lady who had lost the sight of 
both, by opacities in the crystalline lenses ; but by submitting 
to have the lenses extracted recovered her sight, and had af- 
terwards an uncommon degree of distinct vision ; which made 
her a very favourable subject for experiments of this kind. 
Having explained the two different modes by which double 
vision may take place in consequence of operations that render 
the refracting media of the eye imperfect, I shall now consider 
it when produted by a morbid action of the muscles. 
Several cases of this kind have come within my own know- 
ledge, and I am induced to dwell upon the subject, because 
some of them had been considered as arising from a defect in 
MDCCXCVII. C 
