camera lucida

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An optical device that projects an image of an object onto a plane surface, especially for tracing.

from the GNU version of the Collaborative International Dictionary of English.

• (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An instrument that uses a shaped prism or mirrors to cause an apparent image of external objects to appear as if projected upon a plane surface, so that the outlines may be conveniently traced.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an optical device consisting of an attachment that enables an observer to view simultaneously the image and a drawing surface for sketching it