The testing of plane surfaces

There is only one way of originating a true plane, viz., to make it in triplicate and to work all three surfaces so as to obtain a "fit" between them. In engineering trade practice this is done when a standard plate is required, and any one of the three plates thus obtained can be used as being truly plane (within the limits of the "raddle" process) for the making of further surface-plates. In many modern exact apparatus,e. g. , a standard measuring machine or a Michelson's interferometer, accuracy of measurement directly depends on the truth of the plane "ways" of the bed along which the various parts of the apparatus slide. These ways are made either by scraping or lapping, and it is of importance to know what errors occur from place to place along the bed due to defective workmanship. The bed generally has three worked planes all inclined to one another, and the movable parts rest on these by, five contact points, thus allowing only one degree of freedom. These five feet bear on the surfaces below by small areas, and amongst other considerations it is important to know how far these feet rise and fall in and out of the scraping marks if the surfaces have been made by scraping.