http://books.google.com/books?id=JokgnS1JtmMC
"Relativity and Its Roots" By Banesh Hoffmann, p.92: "There are various remarks to be made about this second principle. For instance, if it is so obvious, how could it turn out to be part of a revolution - especially when the first principle is also a natural one? Moreover, if light consists of particles, as Einstein had suggested in his paper submitted just thirteen weeks before this one, the second principle seems absurd: A stone thrown from a speeding train can do far more damage than one thrown from a train at rest; the speed of the particle is not independent of the motion of the object emitting it. And if we take light to consist of particles and assume that these particles obey Newton's laws, they will conform to Newtonian relativity and thus automatically account for the null result of the Michelson-Morley experiment without recourse to contracting lengths, local time, or Lorentz transformations. Yet, as we have seen, Einstein resisted the temptation to account for the null result in terms of particles of light and simple, familiar Newtonian ideas, and introduced as his second postulate something that was more or less obvious when thought of in terms of waves in an ether. If it was so obvious, though, why did he need to state it as a principle? Because, having taken from the idea of light waves in the ether the one aspect that he needed, he declared early in his paper, to quote his own words, that "the introduction of a 'luminiferous ether' will prove to be superfluous."

This means that the following two sets of postulates/hypotheses are able to explain the null result of the Michelson-Morley experiment:

SET I:

1. The principle of relativity is correct.

2. The speed of light varies with the speed of the emitter like the speed of any material projectile.

SET II:

1. The principle of relativity is correct.

2. The speed of light is independent of the speed of the emitter.

3. Lengths contract so that SET II's postulates 1 and 2 can become consistent with the null result of the Michelson-Morley experiment.

Initially Poincaré found SET II's postulate 3 unacceptable:

http://www.brera.unimi.it/sisfa/atti/1998/giannetto.pdf
Henri Poincaré: "...les termes du second ordre auraient dû devenir sensibles, et cependant le résultat [de l'expérience de Michelson-Morley] a encore été négatif, la théorie de Lorentz laissant prévoir un résultat positif. On a alors imaginé une hypothèse supplémentai tous les corps subiraient un raccourcissement dans le sens du mouvement de la Terre... cette étrange propriété semblerait un véritable coup de pouce donné par la nature pour éviter que le mouvement de la Terre puisse être révélé par des phénomènes optiques. Ceci ne saurait me satisfaire..."

In the end Poincaré did accept length contraction but only as an initial postulate/hypothesis whose truthfulness is by no means guaranteed (Einsteinians managed to convince the world that length contraction is a glorious consequence of SET II's postulates 1 and 2):

http://www.amazon.com/Understanding-.../dp/0520200292
Understanding Relativity: A Simplified Approach to Einstein's Theories, Leo Sartori, p.131: "The special force, which became known as "Poincaré stress" or "Poincaré pressure" is a red herring. As Einstein showed, the contraction is inherently a kinematic effect, a direct consequence of the properties of space and time expressed through the Lorentz transformation. Whatever forces are present in matter must transform in a manner consistent with the contraction; no special force is needed. As late as 1909, Poincaré still had not disabused himself of this fundamental misunderstanding. In a lecture at Göttingen, he asserted that the "new mechanics" is based on three hypotheses, of which the third is the longitudinal deformation of a body in translational motion. (The first two were Einstein's two postulates.)"

Pentcho Valev