Thorium was commonly used in Lens glass. Most often found in the Pentax f1.4 screw mount lenses. Turns the Glass Yellow with time, but a day or two in strong sunlight seems to bleach it back. (don't accidentally focus the light and start a fire).

yes, Thorium IS radioactive.

For more details try to find a copy of Living With Radiation: The First Hundred Years
By William Kolb and Paul Frame { }