That is to say, their loading makes the ether behave to optical waves as if--being a homogeneous medium without these discontinuous loads--it had a density μ² times that which it has in space outside matter. Calling the density outside 1, the extra density inside must be μ²-1, so as to make up the total to μ². The μ²-1 portion is that which we call "matter," and this portion is readily susceptible to locomotion, being subject to--that is, accelerated by--mechanical force. The free portion of normal density 1 is absolutely stationary as regards locomotion, whether it be inside or outside a region occupied by ordinary matter, for it is not amenable to either mechanical or electric forces. They are transmitted by it, but never terminate upon it; except, indeed, at the peculiar structure called a wave-front, which simulates some of the properties of matter. (If free or unmodified ether can ever be moved at all, it must be by means of a magnetic field; along the lines of which it has, in several theories, been supposed to circulate. Even this, however, is not real locomotion.) Fizeau tested that straightforward consequence of this theory which is known as Fresnel's Law, and ascertained by experiment that a beam of light was accelerated or retarded by a stream of water, according as it travelled with or against the stream. And he found the magnitude of the effect precisely in accordance with the ratio of the locomotive portion of the ether to the whole,--the fraction (μ²-1)/μ² of the speed of the water being added to or subtracted from the velocity of light, when a beam was sent down or up the stream. But even if another mode of expression be adopted, the result to be anticipated from this experiment would be the same. For instead of saying that a modified portion of the ether is moving with the full velocity of the body while the rest is stationary, it is permissible for some purposes to treat the whole internal ether as moving with a fraction of the velocity of the body. On this method of statement the ether outside a moving body is still absolutely stationary, but, as the body advances, ether may be thought of as continually condensing in front, and, as it were, evaporating behind; while, inside, it is streaming through the body in its condensed condition at a pace such that what is equivalent to the normal quantity of ether in space may remain absolutely stationary. To this end its speed backwards relative to the body must be u/μ² and accordingly its speed forward in space must be u(1-1/μ²).