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 would cause none of the complications introduced by the second postulate, is possible. The velocity of light and other electromagnetic propagations might not be independent of the motion of the source, but their velocity and that of the source might be additive. This assumption would be very simple, would be no contradiction to the first postulate of relativity, and would directly explain all our failures to detect an ether drift. It is not difficult, for example, to see that this assumption does directly explain the Michelson-Morley experiment. If O is a source of light and A and B are mirrors placed a meter away from O, the Michelson-Morley experiment shows that the time taken for light to travel to A and back is the same as for the light to travel to B and back, in spite of the fact that the whole apparatus is moving through space in the direction of O—B, due to the earth's motion around the sun.



The above assumption, however, would require exactly this result, since it says that light travels out from O with a constant velocity in all directions with respect to O, and not with respect to some ether through which O is supposed to be moving. It is in fact obvious, in general, that this principal if true would lead to the simplest kind of relativity. For, if light or any electromagnetic disturbance which is being emitted from a source, partakes in the motion of that source in such a way that the velocity of the source is added to the velocity of emission, then a system consisting of the source and its surrounding disturbances acts as a whole and suffers no change in configuration when the velocity of the source is changed. The possibility of such an assumption has already been pointed out in various places. Nevertheless, in spite of the apparent