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1. Introduction.
The numerous experimental and theoretical investigations that have followed Bradley's discovery of aberration have led finally to the enunciation of a principle of relativity. This principle which is being widely used as a working hypothesis is gradually attaining a position analogous to that held by the second law of thermodynamics, inasmuch as it gives a satisfactory account of the experimental results and is not in direct contradiction with any known facts.

The mathematical analysis on which the principle is based was introduced by Lorentz in 1892. It consists of a certain space time transformation of the coordinates in which the fundamental equations of the electrodynamical field are unaltered in form.

The principal features of the transformation are the introduction of a local time, and the hypothesis that a body moving through the aether with a constant velocity v suffers a contraction $$\sqrt{1-v^{2}/c^{2}}:1$$ in the direction of motion: c being the velocity of light. With the help of this hypothesis