Focus of sight.
If the eye is required to look at an object placed too near to it,
it cannot judge of it well--as happens to a man who tries to see the
tip of his nose. Hence, as a general rule, Nature teaches us that an
object can never be seen perfectly unless the space between it and
the eye is equal, at least, to the length of the face.
Differences of perception by one eye and by both eyes (26-29).
26.