I. ALTHOUGH the undulatory theory of light may now be considered as completely established, still any confirming test of a physical theory is in itself interesting as a fresh illustration of a natural truth. Considering how at one time crucial tests of this theory were sought after, it would appear perhaps rather an anomaly that attention should apparently not have been given to the effect attendant on the motion of a luminous source as a test between the two rival (undulatory and corpuscular) theories, and that more especially as the test would appear to be in principle a simple and decisive one. I should have considerable diffidence in directing attention to this point, but no record apparently exists of experiments proposed or attempted with this view. It might be said that the possible existence of practical difficulties in the way of carrying out the test may account for this; but then practical difficulties are seldom allowed to stand in the way, if a theoretic principle be correct; and, unless a thing were seriously proposed and discussed, no attempt would be made to surmount the difficulties that might exist in the way of carrying it out. Sir John Herschel, as far as he touches on this point, would appear to have had the idea that such a test between the two theories could not, in principle, be applicable; for he says (“Outlines of Astronomy,”p. 214), speaking of the effect attendant on the motion of a luminous body, “The effect in question, which is independent of any theoretical views regarding the nature of light, .... &c.”It is true he mentions afterwards, in a foot-note, a difference in the effect in the case of the two theories as regards the velocity of light, in the case of a luminous source moving directly towards or from the observer; but the following fundamental difference in the case of the two theories appears not to have been taken into account.