Archived posting to the Leica Users Group, 1998/05/05

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Subject: Re: [Leica] 6.8 Telyts
From: "Patrick G. Sobalvarro" <>
Date: Tue, 05 May 1998 22:33:31 -0700

Henning J. Wulff wrote:
> The explanation given to me was that some glasses (glass being a fluid) can
> indeed flow enough in a normal span of years to cause this. Leica has used
> a number of exotic glasses, and I was told that this might have been the
> cause. Separation of the two elements had not taken place. One other person
> has related the same thing (re: a 400/6.8) to me.

I think the people who gave you this explanation were not very up-to-date in
their knowledge about glasses.  According to most modern materials scientists,
silicate glass at room temperature (or any temperature up to about 600 C) is
not a liquid, but an amorphous solid.  It can be bent, but after the stress is
removed it will return to its original shape over time and it will not flow in
real-world conditions at room temperature.  As a schoolchild I, too, was told
stories of windows in old houses being thicker at the bottom due to glass
flow.  Modern scientists regard these claims of glass flow as incorrect.

There's even some information about this available on the Web -- see 
<> and

- -Patrick