crown glass

  1. window glass made in flat, circular plates by blowing and whirling, with a small knot in the center left by the blower's rod
  2. a very clear optical glass with a low index of refraction

See crown glass in American Heritage Dictionary 4

noun
  1. A soda-lime optical glass that is exceptionally hard and clear, with low refraction and low dispersion.
  2. 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.
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