where n is the refractive index of the liquid (assumed a nondispersive medium).
Fresnel derived this result from the assumption that light imparts elastic vibrations
to the aether it traverses. According to him, the presence of the factor 1 — \/n^2
(now known as Fresnel's drag coefficient) expresses the fact that light cannot
acquire the full additional velocity v since it is partially held back by the aether
in the tube. In 1851 Fizeau had sent light from a terrestrial source into a tube
filled with a moving fluid and had found reasonable experimental agreement with
Eq. 6.2 [F2].
Lorentz discussed both effects from the point of view of electromagnetic theory
and gave a dynamic derivation of the Fresnel drag in terms of the polarization
induced in a medium by incident electromagnetic waves.* Throughout this paper
of 1895, the Fresnel aether is postulated explicitly. In rejecting these explanations
of aberration and the Fizeau experiment, Einstein therefore chose to take leave of
a first-order terra firma which had been established by the practitioners, limited
in number but highly eminent and influential, of electromagnetic theory. I shall
leave for the next chapter a discussion of his reasons for doing so. Note, however,
that it was easy to take the Michelson-Morley experiments for granted (as Ein-
stein repeatedly said he did) once a new look at the first-order effects had led to
the new logic of the special theory of relativity. Note also that this experiment was
discussed at length in Lorentz's paper of 1895 and that Einstein was familiar with
this paper before 1905!
Finally, there is the Kyoto address. It was given in German and translated into
Japanese by Jun Ishiwara** [II]. Part of the Japanese text was retranslated into
English [Ol]. I quote a few lines from this English rendering:
As a student I got acquainted with the unaccountable result of the Michelson
experiment and then realized intuitively that it might be our incorrect thinking
*For a calculation along these lines, see the book by Panofsky and Phillips [P2].
**From 1912 to 1914, Ishiwara studied physics in Germany and in Switzerland. He knew Einstein
personally from those days. He also translated a number of Einstein's papers into Japanese.
Il8 RELATIVITY, THE SPECIAL THEORYThe concept of an aether at absolute rest, introduced in 1818 by Fresnel in his
celebrated letter to Dominique Francois Jean Arago [Fl], served the express pur-
pose of explaining this aberration effect (which would be zero if the aether moved
along with the earth). As to the Fizeau effect, Fresnel had predicted that if a liquid
is moving through a tube with a velocity v relative to the aether and if a light
beam traverses the tube in the same direction, then the net light velocity c' in the
laboratory is given by