[Sca-cooks] Farsighted lenses

[Stefan li Rous]

Susan asked:
<<<http://en.wikipedia.org/wiki/Magnifying_glass states that ?a magnifying 
devise? existed  in 424 BC. It does not specify whether it was glass, 
crystal or clear quartz. What is the difference? - It continues to state 
that ?glass lenses? were produced in the Middle Ages. I assume that 
magnifying lenses were available. >>>

The earliest lens were ground from rock crystal. However, finding rock crystal large enough and without cracks, occlusions and other distortions was difficult. And since they were rare and the skills needed to grind them were expensive, these were one of a kind and could only be afforded by a few emperors and such.

<<< Medieval Muslims, including those in Bagdad and Andalusia, worked with 
quartz and also they had a oops - glass or crystal factory (now that you 
question it I don?t know which) outside of Cordoba by the 9th century as 
Ziryab insisted that wine be drunk from glass  or crystal goblets. I don?t 
know enough about it to be able to say if they produced glass or crystal. >>>

Again, just being able to produce glass doesn't mean you can make acceptable lens with it. While you can drink out of something that distorts, you can't use that for looking through.

When you see comments about crystal goblets, that is what they are, goblets made out of natural chunks of crystal.

Susan gave an interesting source which said:
<<< Published evidence in the form of letters of the dukes of Milan, Francesco and Galeazzo Maria Sforza, dated 1462 and 1466 respectively, reveal the first detailed information about spectacles since their invention; namely, 1. Florence was producing in large quantities not only convex lenses for presbyopes, but also concave (diverging) lenses for myopes (i. e., about a half century before the latter were thought to have been developed); >>>

This is the first I've heard of concave lenses within our period, at all. Like Urtatim, this comes as a surprise to me. Interesting, but I think I'll hold it with a bit of skepticism until I see some more supporting evidence. Bear? Seen or heard of these letters?

I don't see that it is that more difficult to carve concave lenses if you can do convex ones. You mainly need the idea and reason to do so. Getting the clear, consistent  glass is the biggest problem.

Stefan

--------
THLord Stefan li Rous    Barony of Bryn Gwlad    Kingdom of Ansteorra
   Mark S. Harris           Austin, Texas          StefanliRous at austin.rr.com
http://www.linkedin.com/in/marksharris
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