'SUBTLE is THE LORD' 119to take account of the motion of the earth relative to the aether, if we recognized
the experimental result as a fact. In effect, this is the first route that led me to
what is now called the special principles of relativity. ... I had just a chance to
read Lorentz's 1895 monograph, in which he had succeeded in giving a com-
prehensive solution to problems of electrodynamics within the first approxi-
mation, in other words, as far as the quantities of higher order than the square
of the velocity of a moving body to that of light were neglected. In this connec-
tion I took into consideration Fizeau's experiment....
In his first paper on relativity, Einstein mentions 'the failed attempts to detect
a motion of the earth relative to the "light-medium" ' without specifying what
attempts he had in mind.* Neither Michelson nor Fizeau is mentioned, though he
knew of both. Einstein's discontent with earlier explanations of first-order effects
may have made the mystery of Michelson and Morley's second-order null effect
less central to him. Yet this 'unaccountable result' did affect his thinking and thus
a new question arises: Why, on the whole, was Einstein so reticent to acknowledge
the influence of Michelson on him? I shall return to this question in Chapter 8.6b. The Precursors- What Einstein Knew. Historical accounts of electromagnetism in the late
nineteenth century almost invariably cite a single phrase written by that excellent
experimental and theoretical physicist, Heinrich Rudolf Hertz: 'Maxwell's theory
is Maxwell's system of equations.'** By itself, this is a witty, eminently quotable,
and meaningless comment on the best that the physics of that period had to offer.
The post-Maxwell, pre-Einstein attitude which eventually became preponderant
was that electrodynamics is Maxwell's equations plus a specification of the charge
and current densities contained in these equations plus a conjecture on the nature
of the aether.
Maxwell's own theory placed the field concept in a central position. It did not
abolish the aether, but it did greatly simplify it. No longer was 'space filled three
or four times over with ethers,' as Maxwell had complained [Ml]. Rather, 'many
workers and many thinkers have helped to build up the nineteenth century school
of plenum, one ether for light, heat, electricity, magnetism', as Kelvin wrote in
1893 [K3]. However, there still were many nineteenth century candidates for this
one aether, some but not all predating Maxwell's theory. There were the aethers
of Fresnel, Gauchy, Stokes, Neumann, MacCulIagh, Kelvin, Planck, and proba-
bly others, distinguished by such properties as degree of homogeneity and com-
*In a thoughtful article on Einstein and the Michelson-Morley experiment, Holton [H2] raised the
possibility that Einstein might have had in mind other null effects known by then, such as the absence
of double refraction [B2, Rl] and the Trouton-Noble experiment [Tl].
**See the second volume of Hertz's collected works [H3], which is also available in English trans-
lation [H4],