sunglass

from WordNet (r) 3.0 (2006)

sunglass
    n 1: a convex lens that focuses the rays of the sun; used to
         start a fire

from The Collaborative International Dictionary of English v.0.48

Sunglass \Sun"glass`\, n.; pl. {Sunglasses}.
   A convex lens of glass for producing heat by converging the
   sun's rays into a focus. "Lighting a cigar with a sunglass."
   --Hawthorne.
   [1913 Webster]