Archived posting to the Leica Users Group, 1996/04/04
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]At 01:43 PM 4/4/96 -0800, Charles E. Dunlap wrote: >I remember that someone mentioned Thorium as an additive to optical glass. >100% of natural Th is unstable. Although it also has a half life over a >billion years, all of it is decaying, greatly increasing the radioactivity >of a sample. Perhaps this was the scare back then. Alas, but "back then" it wasn't an issue. The US and British military and, I suppose, the military forces of other nations as well, used thorium as a common element in tank gunsights, rangefinders, and the like. The radioactivity didn't pose much of a threat to the wartime users as they were but rarely exposed to it during actual combat or training. However, these scopes were surplused after the war, and many a photographer, birdwatcher, and amateur astronomer built a nifty addition to his or her optical gear from such, and this would be used for thirty years or so. THIS was the problem. The danger was identified, almost casually, in the late 1970's, and a lot of warnings were sent through the photographic and astronomical media to alert folks to the problem. I don't believe birdwatchers have ever been alerted, though I suppose their use of such surplus optics was far smaller than among photographers and amateur astronomers. I remember vividly the catalogues of Jaegers and Edmund Scientific being filled with these scopes, and every surplus store in every town had a slew more. Marc msmall@roanoke.infi.net FAX: +540/343-7315 Cha robh bas fir gun ghras fir!