To: 
Reply-To: "Scott Berfield" 


I was showing my wife the cool stuff about the Foucault test this weekend
(touch the mirror and immediately see raised areas, heat shimmer, etc...)
and she asked me how big the differences we were seeing in the surface
really were. While I know how to measure the changes in focal length using
the tester, I never really thought about the absolute size of the changes
in the glass. So how much deviation from a sphere are we seeing when we see
things like surface roughness, ripple, etc? How about a faint scratch? How
deep is it really when it looks so darned big in the tester image? Or the
bumps left after you touch the mirror with your fingertips for a second? I
know we are talking about REALLY small numbers, but just HOW small?

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