HENRI POINCARE AND THE SPEED OF LIGHT

A paper which idolizes Einstein and bad-mouths Poincaré but still
clarifies Poincaré's approach:

http://arxiv.org/PS_cache/arxiv/pdf/...112.3175v1.pdf
"In summary, Poincaré's use of the Lorentz transformations differed
from Einstein's in two key ways. First, for Poincaré, the
transformations are not used to give relations between any two
inertial frames of reference - they are defined only relative to the
ether rest frame, so that the velocity v appearing in the formula must
always refer to a velocity with respect to the ether rest frame, i.e.
an absolute velocity. Moreover, even if we restrict ourselves to
transformations involving the ether rest frame, Poincaré's usage does
not coincide with Einstein's, since for Einstein the transformations
express a relationship between coordinate systems, whereas for
Poincaré they are merely a means of predicting the physical changes
that a system undergoes when set in motion relative to the ether."

Now it is easier to understand why the Lorentz transformations did not
force Poincaré (but did force Einstein) to conclude that the speed of
light (relative to the observer) does not vary with the speed of the
observer:

http://www.jstor.org/stable/3653092
The Mystery of the Einstein-Poincaré Connection
Olivier Darrigol: "It is clear from the context that Poincaré meant
here to apply the postulate [of constancy of the speed of light] only
in an ether-bound frame, in which case he could indeed state that it
had been "accepted by everybody." In 1900 and in later writings he
defined the apparent time of a moving observer in such a way that the
velocity of light measured by this observer would be the same as if he
were at rest (with respect to the ether). This does not mean, however,
that he meant the postulate to apply in any inertial frame. From his
point of view, the true velocity of light in a moving frame was not a
constant but was given by the Galilean law of addition of velocities."

Pentcho Valev