crown glass

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A soda-lime optical glass that is exceptionally hard and clear, with low refraction and low dispersion.
2. n. A form of window glass made by whirling a glass bubble to make a flat circular disk with a lump left in the center by the glass blower's rod.

Century Dictionary and Cyclopedia

1. n. A good quality of common blown window-glass. It is used in connection with flint-glass for dioptric instruments, in order to destroy the effect of chromatic aberration. Now largely superseded by cylinder-glass. See glass.

GNU Webster's 1913

1. n. See under Glass.
2. n. a variety of glass, used for making the finest plate or window glass, and consisting essentially of silicate of soda or potash and lime, with no admixture of lead; the convex half of an achromatic lens is composed of crown glass; -- so called from a crownlike shape given it in the process of blowing.

WordNet 3.0

1. n. a glass blown into a globe which is later flattened and spun to form a disk
2. n. optical glass of low dispersion and low refractive index