Richard Gregory's elucidation of mirror images (Books, THES , October 4) becomes clear when we imagine a fly crawling over a vertical mirror. If it crawls to our left, the fly "in the mirror" crawls left. But if it flies away from the mirror, its image moves further into the mirror.
One dimension - the depth perpendicular to the surface - is reversed by the mirror. The mirror doesn't rotate anything, though that is what we mistakenly feel it is doing.