Since therefore the pictures are thus inverted, it is demanded how
it comes to pass that we see the objects erect and in their natural
posture?
89. In answer to this difficulty we are told that the mind, perceiving an
impulse of a ray of light on the upper part of the eye, considers this
ray as coming in a direct line from the lower part of the object; and in
like manner tracing the ray that strikes on the lower part of the eye, it
is directed to the upper part of the object. Thus in the adjacent figure,
C, the lower point of the object ABC, is projected on C the upper part of
the eye. So likewise the highest point A is projected on A the lowest
part of the eye, which makes the representation CBA inverted: but the
mind considering the stroke that is made on C as coming in the straight
line CC from the lower end of the object; and the stroke or impulse on a
as coming in the line AA from the upper end of the object, is directed to
make a right judgment of the situation of the object ABC, notwithstanding
the picture of it is inverted. This is illustrated by conceiving a blind
man who, holding in his hands two sticks that cross each other, doth with
them touch the extremities of an object, placed in a perpendicular
situation. It is certain this man will judge that to be the upper part of
the object which he touches with the stick held in the undermost hand,
and that to be the lower part of the object which he touches with the
stick in his uppermost hand.
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