We find therefore that for sound the velocity with regard to the medium is always unity, while the velocity with regard to the observer, and the interval elapsed, depend only on the motion of the observer himself, and are independent of the motion of the source. The frequency of the vibrations, on the other hand, depends only on the relative motion of the observer and the source, but is independent of their common motion in any direction. Further, it makes no difference whether the source and the observer are moving on a train, or whether they are stationary, and a uniform wind is blowing past them. In the case of light waves we shall find a very different state of affairs, although the rules for frequency are the same as they are for sound. In case A we have the normal conditions, where both the source and observers are stationary. In case B we have a representation of the Michelson-Morley experiment as supplemented by that of Majorana,2 where the source is stationary and the observer advances. Unlike the case of sound, the interval elapsed, as shown by the experiment, is now the same as in case A, and since the distance to the observer is less, the velocity of light with respect to the ether must also be less than unity. Since the observer is advancing against the light, this will permit the velocity of light with regard to the observer to remain unity, in conformity with the second principle of relativity. Compare with case b for sound. As Jeans expresses it, "The velocity of light in all directions is the same, whatever the motion of the observer."3 That is to say it appears to be the same to him, however he moves. Case C represents Einstein's statement, as confirmed by Majorana's experiment. It does not differ from case c for sound. Case D is more complex, but accepting the statement above that the velocity is constant with regard to the observer, we see that the velocity through the medium must be less, and that the interval elapsed will be constant, as in case B. Could we use the brighter stars and planets as sources of light, several of these cases could be further tested.