perspective glass

noun2

1. •
   A kind of early telescope.

   • The instrument was at first called Galileo's tube; the double eye-glass; the perspective; the trunk; the cylinder. — 1896
2. •
   (historical) An instrument for viewing an enlarged image of engraved prints or maps https://web.archive.org/web/20150424011800/http://www.monticello.org/site/research-and-collections/perspective-glass.