The Collaborative International Dictionary of English v.0.48:

Camera lucida \Cam"e*ra lu"ci*da\ [L. camera chamber + L.
   lucidus, lucida, lucid, light.] (Opt.)
   An instrument which by means of a prism of a peculiar form,
   or an arrangement of mirrors, causes an apparent image of an
   external object or objects to appear as if projected upon a
   plane surface, as of paper or canvas, so that the outlines
   may conveniently traced. It is generally used with the
   microscope.
   [1913 Webster]

WordNet (r) 3.0 (2006):

camera lucida
    n 1: an optical device consisting of an attachment that enables
         an observer to view simultaneously the image and a drawing
         surface for sketching it