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From: bl@hplabsb.UUCP (Bruce T. Lowerre)
Newsgroups: sci.misc
Subject: Re: Glass flow
Message-ID: <3786@hplabsb.UUCP>
Date: Wed, 5-Nov-86 14:09:09 EST
Article-I.D.: hplabsb.3786
Posted: Wed Nov  5 14:09:09 1986
Date-Received: Wed, 5-Nov-86 23:06:07 EST
References: <17@wjh12.HARVARD.EDU> <666@faron.UUCP> <18@wjh12.HARVARD.EDU> <703@chinet.UUCP>
Distribution: net
Organization: Hewlett-Packard Laboratories
Lines: 18
Summary: Telescope mirrors and lenses

In article <703@chinet.UUCP>, magik@chinet.UUCP (Ben Liberman) writes:
> 
> I have also heard that the large store front windows on the downtown Chicago
> stores (Marshall Fields, etc.) would be removed and rotated every few decades
> to prevent breakage because the glass would get thicker at the bottom and 
> thinner at the top.

There has been some comments on the net about glass windows flowing over
long periods of time.  If glass really does flow over time then what about
old telescope mirrors and lenses.  The Mt. Wilson observatory has a telescope 
with a 100 inch mirror (made of plate glass) which is about a 100 years old.
Also, the Yerkes (SP?) observatory has the largest refractor telescope with
a lens of 40 inches including a double element, one made of flint glass and
the other made of crown glass. One element has a thick center with thin edge.
I have not heard of either of these glass optical instruments suffering
distortion due to glass flow.  Even the SLIGHEST glass flow (1/4 wave length
of light) would distort the objective element beyond use.  Does glass really
flow at room temperatures?