As we have two eyes, and a separate image of every external object
is formed in each, it may be asked, why do we not see double? The
answer is, it is a matter of habit. Habit alone teaches us, that
the sensations of sight correspond to any thing external, and shows
to what they correspond. Thus, place a wafer on a table before you;
direct your eyes to it, that is, bring its image on both retinæ to
those parts which habit has ascertained to be the most sensible, and
best situated for seeing distinctly, and you will see only the _single
wafer_. But, while looking at the wafer, squeeze the upper part of one
eye downwards, by pressing on the eyelid with the finger, and thereby
forcibly throw the image on another part of the retina of that eye,
and double vision will be immediately produced; that is, _two wafers_
will be distinctly seen, which will appear to recede from each other
as the pressure is stronger, and approach, and finally blend into one,
as it is relieved. The same effect maybe produced without pressure, by
directing the eyes to a point nearer to, or farther from them, than the
wafer; the optic axes, in this case, being both directed away from the
object seen.