Re: flat glass

At 04:33 17/01/01 +0000, you wrote:

>Guess someone should tell the restorers of old houses! I've taken
>windows out of houses less than 100 years old, where the glass is at
>least twice as thick at the bottom (compared to the top)!

Does this mean that if you store a camera lens for a long time, you'd
better rotate it regularly to equalise the "flow"? Would be interested to
know if the effect is at all noticeable (or even detectable) in the optical
performance of vintage cameras that have been stored in one position for an
extended period.

As pointed out elsewhere in this thread, 100 years is not long enough (by a
few orders of magnitude) for the glass to flow sufficiently to show a
factor of 2 difference in thickness top to bottom - if that was really
happening, the glass would probably have drooped well below the top of the
frame as well. My own house is over 100 years old, and the front window
frames and glass are all original; there's no sign of any "flow" in the
glass & happily the panes are still the same size as the frames ;-)

(Interesting to note though that the old frames show no sign of rot or
deterioration, despite a North facing aspect in the wet climate of the
English Northwest, whereas two new frames on the south-facing back of the
house needed rot treatment after only 10 years. They don't make wood like
they used to...)

Actually, I suspect that the real problem with very old window glass was
that the process of manufacture (for example, early processes involved
blowing a large glass bubble & then opening it out to form a large disc,
then chopping the disc up into suitable shaped pieces) inherently results
in uneven thicknesses of the resulting material & that it may well have
been standard practice for the tradesmen of the day to put the thick bit at
the bottom of the window frame.

Regards,
Tony