If a hair be drawn between the finger and thumb, from the end to
the root, it will be distinctly felt to give a greater resistance
and a different sensation to that which is experienced when drawn
the opposite way: in consequence, if the hair be rubbed between the
fingers, it will only move one way (travelling in the direction of a
line drawn from its termination to its origin from the head or body),
so that each extremity may thus be easily distinguished, even in the
dark, by the touch alone.
The mystery is resolved by the achromatic microscope. A hair viewed on
a dark ground as an _opaque_ object with a high power, not less than
that of a lens of one-thirtieth of an inch focus, and dully illuminated
by a _cup_, the hair is seen to be indented with teeth somewhat
resembling those of a coarse round rasp, but extremely irregular and
rugged: as these incline all in one direction, like those of a common
file, viz. from the origin of the hair towards its extremity, it
sufficiently explains the above singular property.
This is a singular proof of the acuteness of the sense of feeling, for
the said teeth may be felt much more easily than they can be seen. We
may thus understand why a razor will cut a hair in two much more easily
when drawn against its teeth than in the opposite direction.--_Dr.
Goring._