CHAPTER I
INTRODUCTION
The scheme of this essay may be summarized as follows.
1. In the first section a historical account is given of the progress of experimental knowledge of the influence of the motion of matter through the aether on phenomena directly connected with that medium. For a long period this group of phenomena was purely optical; the principal member of the group was the astronomical aberration of light, and the main interest of the others was centred in their use as tests of theories constructed to account for or explain aberration. The recent development of the connexion between the theories of electricity and optics has added to this domain the class of electro-optic and magneto-optic phenomena, and also various phenomena purely electric. The progress of theory is here passed under review alongside of the progress of experimental knowledge, and some attempt is made to compare the strengths and relative bearings of the positions that have been from time to time taken up by successive investigators. In this review brevity and interconnexion have been chiefly aimed at, for the subject has been historically a rather tangled one owing to the number of writers who have treated it and the variety and isolation of their standpoints: it will be seen that the estimates here given of the relative merits of the various partial theories differ somewhat in character from those generally current. For
