From: "Bob May" 
To: "atmlist" 
Reply-To: "Bob May" 


Actually, the polarizers don't need to be in perfect alignment to do the
test.  Anything close to the crossed condition will show quite dramatic
shadows of a tempered piece of glass. If you are doing a lens and want to
see if the glass is stressed in some way by a minor (relative to the
stresses of a piece of tempered glass) stress, then the need does become
where you insert the glass between the polarizers and THEN get the maximum
darkening so that the maximum sensitivity of the test can be obtained.
Also note that you do need to have the surfaces somewhat polished as opal
surfaces don't transmit the polarization very well.  Then again, plate
glass is normally nicely polished for this test.  In addition, the glass
doesn't need to have plane parallel surfaces nor even a semiflat surface.
Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net

--- BBBS/NT v4.01 Flag-4