Re: ATM [Fwd: BOUNCE atm@shore.net: Non-member submission from [Kurt Hillig <khillig@Chem.LSA.umich.edu>]]

[tomlinson <etomlins@rohan.sdsu.edu>]

Your message dated: Tue, 15 Oct 1996 08:09:37 PDT
>
>It's easier on larger mirrors, but even on a 4" you should be able to
>measure the depth to 0.001", maybe half that of the gauges include some
>leaves with half-thousandth thicknesses (probably 0.0015" and 0.0025", as 
>0.0005" is a tad on the fragile side).
>
>                            Dr. Kurt Hillig

Why bother to measure the sagitta at all?  For that small a sagitta,
an error of even 0.0005" in measurement will result in a large error
in calculated radius--about 2.5 inches, for a sagitta of 0.02" on
a mirror of diameter 4".

No, I'd spray some water on the mirror and use the spit test, with
a small light (cigarette lighter or penlight) held near the eye.
If you grind a wet or two with #400 or similar before spit-testing,
the mirror will retain a nice, thin film of water, and you could
probably measure the radius to the nearest half-inch.

Cheers,
-et