on Muscular Motion. 
1 3 
jects are made to appear double ; so that in this view, double 
vision is an intermediate state between single vision with both 
eyes, and squinting. Squinting has been very generally believed 
to arise entirely from an inability in the muscles to direct the eye 
properly to the object. There is, however, probably no original 
defect in the muscles; certainly none sufficient to sanction such 
an opinion; since the muscles of a squinting eye have the 
power of giving it any direction, but cannot do it without some 
degree of effort. The defect, therefore, appears to be princi- 
pally in the eye itself, which is too imperfect to assist the other 
in producing distinct vision. From this imperfection, the mus- 
cles have not the same guide to direct them as those of the 
other eye ; and, therefore, although perfectly formed, cannot 
make their actions exactly correspond with them. 
In a squinting person, both eyes certainly do not see the 
object looked at. This is evident to a by-stander, who is able 
to determine, that the direction of one of the eyes differs so 
much from that of the other, that it is impossible for the rays 
of light from any object to fall upon the retinas of both; and, 
therefore, that one eye does not see the object. 
The same thing may be proved in another way; for since a 
small deviation in the direction of either eye from the axis of 
vision, produces double vision, any greater deviation must have 
the same effect, only increasing the distance between the two 
images, till it becomes so great that one eye only is directed to 
the object. In squinting there is evidently a greater deviation 
from the axis of vision than in double vision, and the object 
does not appear double; it is therefore not seen. by both eyes. 
The circumstance of those who squint having an imperfect 
eye, is corroborated by all the well authenticated observations 
