1 1 4 D O C . 3 1 I D E A S A N D M E T H O D S
necessary because light propagates with a well-defined velocity independent of
the wavelength through (empty) space that is free of matter in the ordinary sense.
2. Light Ether and the Movement of Matter
Soon, one pondered the question of what kind of substance this ether should be per-
ceived of. Is it akin to a fluid or to a solid? The fact of polarizability of light led to
the conception that the oscillations of light are transversal oscillations, that is, os-
cillations as they occur only in solid but not in fluid bodies. This led to the concep-
tion of the ether as a kind of solid body, i.e., a substance that resists changes of
shape or relative motions of its parts by virtue of a lively elastic resistance. It
seemed to behave like a quasi-rigid body that penetrates all matter.
The fundamentally important Fizeau experiment
which tried to an-
swer the question if moving matter carries the light ether contained in the same vol-
ume with itself, also led to the same conception. The consideration was the follow-
ing one. A fluid at rest propagates light with velocity V ( , where is the
index of refraction). If the fluid flows with velocity v from left to right through a
pipe, and the fluid carries its light ether with it, then the light ray sent through the
fluid from left to right will also have propagation velocity V relative to the flowing
fluid. Its propagation velocity relative to the pipe will then, due to the addition the-
orem of velocities, be larger by v than it is relative to the fluid, therefore
. (1)
But the Fizeau experiment did not confirm this result of the consideration. Empir-
ically, one found the correctness of the Fresnel
Thus it is shown that the propagation velocity of light is less strongly influenced by
the movement of matter than the previous consideration made us expect. For fluids
that do not refract light , formula (2) even yields
, (2a)
-- - = n
[p. 2]
V′ V v + =
V′ V 1

+ =
n 1) = (
V′ V =
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