wholly tumultuous train of disturbance which constitutes ordinary white light into an orderly series of trains of simple harmonic waves. It is held (with Lord Rayleigh) that the train of impulses or vibrations which constitutes the Röntgen radiation would be similarly resolved into simple wave-trains of very high frequencies if we had fine enough apparatus to bring to bear upon it; though the molecular structure of ordinary matter is probably too coarse to be sensibly effective for this purpose. Reasons are given for the view, opposed to what is now sometimes perhaps too generally stated, that counting the number of the succession of interference bands, that can be produced with the light from the whole of a sharp bright line in the spectrum of a gas, enables us to form an estimate of the degree of regularity of the vibrations of the individual molecules which emit the radiation: a result which is of importance for both optical and molecular theory.
