On the Diffraction Spectrum and Wave Lengths

Abstract

THE letter which I wrote about the diffraction spectrum has called forth several inquiries. I have been asked how it is that in that spectrum the position of a line depends only on its wave-length, and I may take the opportunity of answering these questions through your columns. First, however, let me state that the numbers given in the table in my letter of February 9 are copied, including the obvious errors, verbatim from the translation of Mossotti's memoir in vol. v. of Taylor's “Scientific Memoirs.” I attach no importance to such analogies. Any analogy between these lines and interdependent notes of music must, I expect, be entirely accidental. The latest experiments made by Mr. Lockyer have shown that the lines given out by a gas vary according to temperature and pressure, and if these be caused by vibrations of the particles of the gas, whether atomic or molecular, the periods of these vibrations must be dependent on the temperature and pressure. A distinct numerical statement of this interdependence is a great desideratum in this part of science with a view of affording the materials for making and testing a mechanical theory of that interdependence. These are the great problems which demand our attention, as I take it, with respect to the fixed lines, and our ability to correlate light with other modes of force.