APPLY the points of a pair of compasses, distant from each other
one or two lines, to the cheek, just before the ear; then move them
successively to several other parts of the cheek, and you will find,
on approaching the mouth, that the points will appear to recede from
each other; this effect being produced by the great difference of the
sense of touch in these parts, It is a general law, that in the more
sensitive portions of the skin, any two points appear to be further
asunder from each other, than points of equal distance appear to be to
a less sensitive portion. The same experiments may be made by holding
together the extremities of the fore-finger and thumb, and then passing
the tips of both in a line from the ear to either the upper or the
under lip; as they approach the latter, they will feel to the cheek as
if they were becoming more and more distant from each other.
If the skin be touched with the points of a pair of compasses, one
inch asunder, the person so touched, while he shuts his eyes, will
instantly be aware that his skin is touched in two places; but by
continually drawing the two points closer, a degree of nearness may be
reached at which the person will imagine his skin to be touched by only
one body: he will, however, describe this body, or the compasses, to
be a little longer in one direction than another; and it appears that
this difference of length corresponds with the distance between the two
points of the compasses. When these points are brought still nearer
together, the inequality will no longer be felt, and the person will
fancy he is being touched by one body only.
Handle a pea: it is _one_—place it between the first and second fingers
of the right hand, in their natural position, and you will still feel
the pea but as _one_. Then cross the two fingers, bringing the second
over the first, and place the pea in the fork between them, so as to
feel the left side of the pea with the right side of the second finger,
and the right with the left of the first The impression will then be
that you have _two_ peas touching the fingers, especially if the eyes
be shut, and the fingers be placed by another person. The illusion will
be equally strong if the two fore-fingers of both hands be crossed, and
the pea placed between them.