curmudgeon1, I believe that with a decent loupe on the calipers placed over the gauging rings on its glass plate, one can easily discriminate within a couple ten-thousandths where a blackened edge is relative to the gauging ring. A 10X loupe makes a .001" gap look like a .010" gap to the naked eye.
Here's a link to a popular one used these days:
http://bulletin.accurateshooter.com/...il-jones-tool/
You can see light coming through caliper jaws when they're only .0002" apart.
It does help to have exceptional vision.