[Leica] white dot and metering

["Dan Honemann" <ddh@home.com>]

Ok, here's a dumb question (so those of you who have low tolerance for dumb
questions can skip to the next msg now).  The M6 TTL meters off of a white
dot on the shutter curtain.  Why isn't the dot 18% grey?  Does the meter
test light reflected off both the white dot and some of the black curtain so
that the composite represents grey?

I'm assuming the meter works by comparing the reflected light through the
lens to the light reflected off the white dot.

And why do we use 18% grey cards anyway when "scientific studies now
indicate that an average scene actually reflects 13% (not 18%) of the light
that falls on it" (Peter Burian, _National Geographic Photography Field
Guide_, p.137)?

Curious Dan