A lens or prism should be well polished, and free from bubbles and other defects in the glass, scratches and other surface marks. Before dealing with the testing of polished surfaces, it is worth recording how a test of some sensitiveness can be applied to grey surfaces. Accurate plates, whether flat or curved, which are used for the purpose of testing surfaces which should fit them, are known as test plates or proof plates. They may be used either in comparing a surface with one of assured flatness, or for ascertaining whether a surface has a desired radius by the use of a proof plate of the same radius but opposite curvature. Collimator is embodied in an optical testing bench which has seen continuous service for forty-eight years in the optical testing room at Adam Hilger's.