the copy below taken from the site posted by Jennyann,explains how molten glass was spun into a thin disc of glass (like a record)up to 2-3 foot in diameter which when cooled would be cut into panes of glass,the center section left would then be the bullseye
or crown.
I was shown examples of early glass which just looked like a old 78 record with the lines still in it,modern bullseye glass is still produced the same way by centrifugal casting
CROWN GLASS. Molten glass is gathered on a blowpipe, and a balloon shape is blown. The blowpipe is removed, a solid "punty" rod is attached and the glass is spun rapidly until a disc is formed. The outer portion beyond the central knob is then cut into panes. By the 18th Century quality was often very good with an almost unmarked fire-finished surface. Crown was the preferred choice for window glass, together with some imported Cylinder glass until the mid 19th Century.
Colin