anaclastic glass

noun2

1. •
   A glass or phial, shaped like an inverted funnel, with a very thin convex bottom that can be made concave or convex by sucking out or blowing in air, used to demonstrate the malleability of glass.

   • Anaclastic glasses are chiefly manufactured in Germany of a fine white glass, but any other glass, which is uniform in its substnace, and not very hard, will do equally well. — 1832
2. •
   Synonym of vexing glass.

   • One astronomer's anaclastic glass; if a friend looks through it and a small screw is turned, it will blow pepper and snuff into his eyes. — 1990