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.

OF THE EYE.

When both eyes direct the pyramid of sight to an object, that object
becomes clearly seen and comprehended by the eyes.

27.

Objects seen by one and the same eye appear sometimes large, and
sometimes small.

28.

The motion of a spectator who sees an object at rest often makes it
seem as though the object at rest had acquired the motion of the
moving body, while the moving person appears to be at rest.

ON PAINTING.

Objects in relief, when seen from a short distance with one eye,
look like a perfect picture. If you look with the eye _a_, _b_ at
the spot _c_, this point _c_ will appear to be at _d_, _f_, and if
you look at it with the eye _g_, _h_ will appear to be at _m_. A
picture can never contain in itself both aspects.