Page:Philosophical Transactions of the Royal Society A - Volume 184.djvu/830

whence

$x=1-{\frac {1}{n^{2}}}$.

Now whatever may be the inner meaning of this statement concerning the velocity of the internal ether, it certainly agrees with, and is at once suggested by, the fact, thoroughly established by both negative and positive experiments, that light travels down a running stream of matter at a pace

${\frac {\mathrm {V} }{n}}+v\left(1-{\frac {1}{n^{2}}}\right)$.

The negative experiments supporting this are such as the achromatic prism experiment suggested and tried by Arago, repeated more elaborately by Maxwell and by Mascart; the water-telescope observations suggested by Boscovich, tried by Airy and by Hoek; interference experiments of Babinet and of Hoek; and several other experiments by Mascart. The positive experiment establishing it is the very beautiful and well-known one of Fizeau, now repeated and confirmed beyond the reach of any but quantitative cavil by Michelson.^[1]

Whether any ether is moved by moving matter may still be an open question, but that the speed of light is affected in a fairly ascertained way by the motion of transparent matter through which it is passing, is certain.

8. But the specific motion of the internal ether is not the whole of Fresnel's hypothesis; there is the fixity of the external ether to be verified too. And that has not yet been done. In fact, one important experiment, to be discussed later on, throws grave doubt upon it, at least for large moving bodies like the earth.

But unless the fixity of external ether be granted, our argument from density concerning the value of the internal velocity breaks down. Consider again two planes moving with a slab of matter, one inside and one outside the mass, and let the space motion of the outside ether at the position of outside plane be affected by the motion of the slab to the extent $yv$, then all we can say is that

$v-yv=n^{2}\left(v-xv\right)$,

or,

$x=1-{\frac {1-y}{n^{2}}}$,

wherefore it is possible for $x$ and $y$ to be unity together.

We may take it, however, that the quantitative accuracy of the Fizeau experiment renders anything of this sort very unlikely, and that we are bound to suppose the ether immediately outside moving matter to be stationary, i.e., to be completely unaffected by its motion, unless we are directly forced by facts to admit the contrary.

1. 'Amer. J. Sci.,' vol. 31, p. 377.