A universal interferometer

Abstract

In a communication to the Royal Society in the year 1909 the author described, by the kind authorisation of the Board of Trade, an Interferential Comparator for Standards of Length, which had been provided, under his direction and at the request of the Board, for the Standards Department. The essential novelty presented by the comparator was that each of the two travelling microscopes, employed to observe and read the positions of the two fiducial marks forming the limits of the standard bar, the imperial yard for instance, was given so exceedingly fine and steady a movement that the truly plane and polished surface of a black glass disc carried by the microscope could form one of the two reflecting surfaces producing the interference bands. The movement of the microscope parallel to itself and to the length of the bar, over a very true V-and-plane guiding bed, was thus accompanied by movement of the interference bands parallel to themselves and to the vertical pair of spider-lines of the autocollimating telescope, which was used, along with a constant deviation prism and a hydrogen, cadmium, or neon Geissler vacuum tube, for the production and observation of the bands in fight of a single specific wave-length. The success of this apparatus, owing to the steadiness and perfect control of the movement of the magnificent field of interference bands afforded—in red hydrogen or cadmium, or yellow neon fight—suggested the suitability of employing such a microscope, together with the optical train and an adequate length of the V-and-plane bed, as an interferometer of the most perfect character and of completely general application, for the measurement of small amounts of motion, slight deformations, short distances, or minute objects, of any nature whatsoever.