VI
In optics Newton developed a powerful particle-
and-wave theory of light that, if misread, can seem
an extraordinary foreshadowing of the modern quan-
tum theory. His rejection of the pure wave theory
propounded by his contemporary Huygens and others,
and the superiority of his own theory in accounting
for the optical phenomena known at the time were
major reasons for the neglect of the wave theory during
the eighteenth century.
In the early nineteenth century, however, Young and
Fresnel brilliantly revived the wave theory and brought
it to victory over the prevalent particle theory. The
rise of the wave theory, with its ubiquitous aether as
the bearer of the waves, brought a threat to the New-
tonian principle of relativity, and one that Newton
would probably have welcomed. For aberration im-
plied an aether essentially undisturbed by the passage
of matter through it. The aether could thus be consid-
ered stationary, so that though mechanical experiments
were powerless, optical experiments had a chance to
succeed in detecting absolute rest and absolute velocity
—meaning now rest and velocity relative to the sta-
tionary, all-pervading aether.
The aether was not what one might reasonably con-
sider a credible concept. Because of the phenomenon
of polarization, light waves were taken to be trans-
verse, and the aether to be an elastic solid. Yet it had
to offer no perceptible impediment to the motions of
the planets, for these motions were in excellent accord
with Newton's system of mechanics. Nevertheless, since
the wave theory of light, developed in detail by
Fresnel, was as successful in encompassing the intricate
phenomena of optics as Newton's laws were in encom-
passing the intricate phenomena of celestial and ter-
restrial mechanics, the aether could hardly be ignored,
for all its conflicting properties.
If v is the speed of the earth through the aether
and c the speed of light, an experiment to detect the
quantity v/c is said to be of the first order, as distin-
guished from a second-order experiment designed to
detect v2/c2. A first-order experiment was soon per-
formed, but it failed to detect v/c. To account for the
failure, Fresnel proposed that matter carries aether
wholly entrapped within it yet allows aether to pass
freely through it. Moreover the amount of aether en-
trapped in, say, glass had to depend on the wavelength
of the light passing through it, so that if various wave-
lengths were present, as they certainly were, the
amount of entrapped aether was given by a self-
contradictory formula. Fresnel's extraordinary hypoth-
esis, which goes by the misleading name partial aether
drag, proved highly successful. Without hurting the
theory of aberration, it implied that every feasible
first-order experiment to detect the earth's motion
through the aether would fail. And since, over the
years, all such experiments did fail, Fresnel's hypothesis
had to be taken seriously. Indeed, it was confirmed by
difficult laboratory experiments on the speed of light
in streaming water.
