The name for the
window glass used in the early
1800s. Back then, windows were formed by blowing a large
bubble of glass, then
spinning it until it was
flat. This left a sheet of glass with a "crown"
bump in the
center. This caused a wavy
refractive distortion when looking through the glass.
Today, window glass is made flat by letting it spread itself out under it's own weight while floating on top of a liquid surface, so that there are minimal distortions.