burning glass

noun2

1. •
   A large convex lens or concave mirror that can concentrate the sun's rays onto a small area, heating up the area and thus resulting in ignition of the exposed surface.
2. •
   (old-fashioned) A magnifying lens, as used to focus sunlight on to an object.

   • The more he sees her, the worse he is, uritque videndo; as in a burning-glass the sunbeams are re-collected to a centre, the rays of love are projected from her eyes. — 1624