A wave-length comparator for standards of length

Two and a-half years ago the author was requested by the Standards Department of the Board of Trade to devise and superintend the construction of a new comparator, for comparing standards of length ─the Imperial Standard Yard, for instance, with official or other copies ─in terms of wave-lengths of light. The instrument now described is the result. Besides performing its functions as a wave-length comparator, and being the first instrument specifically constructed as such, it is also the most perfect instrument yet devised for measurement in wave-length in general. It is described to the Royal Society by permission of the President of the Board of Trade. The principle of the instrument id that of the author's interferometer, described to the Society in 1898 in connection with an interference dilatometer, and again as improved in 1904 in connection with the author's elasmometer or interference elasticity apparatus. The interferometer, which is totally different from that of Michelson or that of Fabry and Perot, is adapted as regards details in a special manner for the specific object in view, but with the exception that a Hilger constant deviation prism is employed instead of a train of two spectroscope prisms, its principle is preserved intact.

The thermal expansion of pure nickel and cobalt

The author has carried out a series of re-determinations of the co­ efficients of thermal expansion of these two metals with the aid of the interference dilatometer described in a former communication to the Society.