For testing the accuracy of a surface which must be exactly plane, one of the surface plates, shown in

Fig. 214, is used. These plates are sold in pairs so that one may be a test of accuracy of the other. When a plane surface is to be tested, one of the plates is smeared with a thin coat of oil colored slightly with red lead. This coating is spread evenly by rubbing both plates together and then the surface to be tried is rubbed over the oiled surface of one of the plates. High spots will be revealed by the oil coating, and these are scraped down.

The making of surface plates requires that each of three plates shall be scraped to fit the other two to insure plane surfaces.