The Collaborative International Dictionary of English v.0.48:

Finder \Find"er\, n.
   1. One who, or that which, finds; specifically (Astron.), a
      small telescope of low power and large field of view,
      attached to a larger telescope, for the purpose of finding
      an object more readily, called also a finder telescope
      or finder scope.
      [1913 Webster +PJC]

   2. (Micros.) A slide ruled in squares, so as to assist in
      locating particular points in the field of vision.
      [Webster 1913 Suppl.]