What is it?

The altimeter in this illustration http://en.wikipedia.org/wiki/Altimeter is
almost identical to the one in our Archer. I've been puzzling over what
that little black and white hatched area is on the dial ever since I bought
into the plane. It doesn't rotate with the Kollsman window knob, although
perhaps it rotates with the thousands hand, since it appears in this
illustration to be exactly opposite the hand. Anyone know what it's used
for? Inquiring minds want to know.

--
Bob (Chief Pilot, White Knuckle Airways)