which will permit us to identify the structure of the luminous wave, freed from all external action, in its free propagation (or transmission) when the velocity of translation of the source can be varied at will. But, apart from the fact that we must inevitably experiment under the eventual action of our earth[1], two serious and almost insurmountable difficulties oppose themselves to the realization of such a programme. In the first place, it is not easy artificially to endow a luminous source with rapid movement[2], especially if this source (as is necessary in some interference methods) has to be very rigorously monochromatic; moreover, I shall publish an account in a forthcoming paper of a disposition of this nature with which I am about to experiment. Secondly, in order to be able to examine the structure of the light emitted by a moving source, with whatever disposition, the light itself has to be subjected to reflexion's, refractions, &c., sometimes fairly numerous; that is to say, the luminous ray must encounter ponderable matter after leaving the source. It does not follow, therefore, that even if c in a vacuum varies with the particular velocity of the source, this quantity does not return to the same fixed value after the said phenomena of reflexion, refraction, &c. It will be well, therefore, to endeavour to eliminate as far as possible, in an experiment of this nature, all causes tending to complicate the phenomenon, and in every case to consider its results carefully.
Meanwhile, to begin with a relatively simple experiment, we may undertake the study of the wave-length of a ray of light reflected by a moving mirror[3]. This may correspond with the experiment already realized, some years ago, by Belopolski, and afterwards repeated by Galitzin & Wilip; but if the first of these authors employed prisms for the observation of the Doppler effect (and consequently the question of the eventual variation of λ remained unsolved), the other two made use of a diffraction-grating, by which the controversy spoken of above arises. It would be better
- ↑ I cannot succeed in imagining an interplanetary experiment of the nature of that proposed (in jest) by Rose-Innes; see Phil. Mag. xxvii. p. 150 (1914).
- ↑ I understand by this a velocity higher than some hundreds of metres per second; this value may perhaps be attained, but it is difficult to conceive a practical disposition for a higher velocity. Naturally I leave out of account the employment of canal rays, which do not give simple and well-known velocities.
- ↑ While this article was in the press, M. Michelson has called my attention to his paper on the same subject, published in the 'Astrophysical Journal,' April 1913, the conclusions of which agree with those that I am stating.
