I totaly agree with Bill on this Anold

ok, the answer is.... 1.5,1.55, 1.517...

so from this I am guessing that anywher near 1.5 is cool for glass... though as I have suggested, it would be very cool to see if this theory stands up with other glass objects under different lighting conditions, and perhaps even with different cam angles as well...

anybody game to give this a go?