Archived posting to the Leica Users Group, 1999/07/31

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Subject: Re: [Leica] Warm Looking LTM Lenses
From: LEICAMAN56@aol.com
Date: Sun, 1 Aug 1999 00:54:33 EDT
 
Stan,

Hold your LTM Summicron over a white piece of paper and look through it.  If 
the lens has a yellowish cast, this is most certainly one of the early 
Summicrons in which radioactive thorium was one of the "rare" earth elements 
used in making the glass.  Apparently, this warm yellowish cast is part of 
the decaying process of the thorium.  The radioactivity can actually be 
measured with a Geiger counter.  I don't think its hazardous to your health, 
but it may fog film over time.  And it will certainly impart a warmish look 
to your pictures.

Bill Rosauer