On Sun, 03 Feb 2013 15:03:05 -0500, Yousuf Khan
wrote:

On 30/01/2013 7:22 PM, RichA wrote:
The Kodak Ektamate and Ektar lenses all use thorium glass. Pretty
harmless just sitting around but I wouldn't want to press my eye to
one for any length of time.

http://www.surplusshed.com/pages/item/l3759.html

What was the purpose of having a glass made of thorium? What was the
advantage?

"In designing optical lenses, it is often desirable to employ glass
with a high index of refraction. The greater the index of refraction,
the greater the bending of the light. Since this reduces the necessary
curvature of the glass, the lens can be made thinner and lighter.
Unfortunately, glass with a high refractive index can also have a high
dispersion. By adding thorium to the glass, a high refractive index
(over 1.6) can be achieved while maintaining a low dispersion."

http://www.orau.org/ptp/collection/c...cameralens.htm