crown glass

Short definition from Wiktionary

Noun

1. 1
   An early type of window glass made by blowing glass into a hollow globe (crown) and then flattening it on a punty, reheating and spinning out into a flat disk by centrifugal force.
2. 2
   A variety of glass used in lenses and other optical components due to its low refractive index and dispersion. Originally crown glass was made from alkali-lime silicates with approximately 10% potassium oxide, but similar properties may be attained with other recipes.