This is the mirror of the Hooker 100-inch telescope at Mount Wilson Observatory. It's been stripped of its reflective aluminum coating in preparation for recoating. The bubbles seen within the mirror were created when the mirror blank was poured in France in the early twentieth century. The French Glass Works Company of St. Gobain, France was the best in the world but they didn't have the facitilites to melt and pour 9000 pounds of glass at one time. Three pours were required and bubbles of air were trapped between the layers of each pour. The bubbles aren't in flat sheets, though. They show waves and swirls frozen in place, with some bubbles coming to within a fraction of an inch of the polished f/5 parabolic surface. The mirror is solid -- no honeycombing or other material removed -- and is green plate glass like what you'd find in a bottle containing a French red wine. The Cassegrain focus of this telescope is bent -- a third mirror reflects the light out of the tube above the primary mirror so that a hole wasn't needed through the center of the massive glass disk.
The waves of bubbles trapped between pours as seen from the side of the mirror:
A large wave of glass frozen in place as seen through the uncoated front surface of the mirror: