  The so-called principle of relativity is able to fix a general coordinate
transformation which differs from the standard Lorentzian form only by an
unknown speed which cannot in principle be identified with the light speed.
Based on a reanalysis of the Michelson-Morley experiment using this extended
transformation we show that such unknown speed is analytically determined
regardless of the Maxwell equations and conceptual issues related to
synchronization procedures, time and causality definitions. Such a result
demonstrates in a pedagogical manner that the constancy of the speed of light
does not need to be assumed as a basic postulate of the special relativity
theory since it can be directly deduced from an optical experiment in
combination with the principle of relativity. The approach presented here
provides a simple and insightful derivation of the Lorentz transformations
appropriated for an introductory special relativity theory course.
