Shut one eye, direct the other to any fixed point, as the head of a
pin, and you will indistinctly see all other objects. Suppose one
of these to be a strip of white paper, or a pen lying upon a table
covered with a green cloth: either of them will disappear altogether,
as if taken off the table; for the impression of the green cloth will
entirely extend itself over that part of the retina which the image of
the pen occupied. The vanished pen will, however, shortly re-appear,
and again vanish; and the same effect will take place when both eyes
are open, though not so readily as with one eye.