http://arxiv.org/ftp/arxiv/papers/1307/1307.2132.pdf Galina Weinstein: "In 1905 Planck was coeditor of the Annalen der Physik and soon a great supporter of Einstein's theory of relativity. Einstein used a seemingly conventional notion, "light complex", and he did not invoke his novel quanta of light heuristic with respect to the principle of relativity. He chose the language "light complex" for which no clear definition could be given in the relativity paper. But with hindsight, in 1905 Einstein made exactly the right choice not to mix concepts from his quantum paper with those from his relativity paper. He focused on the solution of his relativity problem, whose far-reaching perspectives Planck already sensed."
Why did Einstein have to use a muddled notion and hide "his novel quanta of light heuristic"? Because "his novel quanta of light heuristic" could have called the attention to the fact that Newton's emission theory of light naturally explains the null result of the Michelson-Morley experiment, "without recourse to contracting lengths, local time, or Lorentz transformations":
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."