🚀 go-pugleaf

Hah! I found the time and the article.

Scientific American Dec 1980 (vol 243 iss 6) pps 206 - 228
Mirror Images by David Emil Thomas:

  "Consider, 1st, a puzzling and fundamental question about the plane
mirror.  Why does such a mirror exchange right for left but not up and
down?  In other words, why is it that when a book  is held up to a plane
mirror each letter appears reversed  and the sequence of letters
run from right to left, whereas the order of the lines from the top
of the page to the bottom is unchaned?	This paradox  can be traced
to the confusion  between the handedness of an image and it orientation.
Actually, for an observer facing a plane mirror the directions right
and left are not changed any more than	the directions up and down.
This fact can be demonstrated by comparing a clock face with its image
in a plane mirror.

  "The transparent clock face is particularly useful in that its num-
erals serve to define lateral directions in the plane tangent to the
mirror at its center of curvature.  For example, 11:00 identifies the
direction from the center of the clock face to the numeral 11.	Simi-
larly, 5:30 identifies the direction from the center of the clock to
the point midway between the numerals 5 and 6.	(It is assumed through-
out this discussion that the observer and the object  are close to
the axis of the mirror the line through the mirror's center of curv-
ature that is pendicular to the tangent plane at that point).

  "This coordinate system provides a means of explicitly describing
the elusive distinction between handedness and orientation.  The
numerals in the plane-mirror image of a transparent clock face run
counterclockwise order, just as the numerals viewed through the back
of the clock face do.  Comparing the real object with the image 'face-
to-face' then, it is apparent that neither the direction 9:00 (right)
nor 3:00 (left) nor 12:00 (top) nor 6:00 (bottom) is changed by the
plane mirror.  Because orientation is not changed along any direction
in the plane of the mirror, the numerals in the image of a clock face
run in counterclockwise sequence.  But plane mirrors (and all other
mirrors) do invert the directions forward and backward: Whereas the
actual clock's front faces away from the observer, the image clock's
front faces toward the observer.  This single inversion  creates
an image with reversed handedness. It is for the same reason that
the image of a right handed glove is left handed."

[There follows more of the explanation with many pictures and
diagrams that lead through many different types of mirrors and
various inversions, reversions, and distortions that may be of
interest but I am not talented enough to convey in ascii chars]

I hope that this clears up the discussion.

Rodger Coghlan

PS All typos are mine and not in the original
PpS ditto any poorfeading problems (sigh)