Attempt at clarification of Einstein's postulate of constancy of light   velocity
  We have realized that under Lorentz transformations the tick number of a moving common clock remains unchanged, that is, the hand of the clock never runs slow, but the time interval between its two consecutive ticks contracts, so the relative time has to be recorded by using the tau-clocks required by the transformations, instead of unreal slowing clocks. Thus it is argued that using rest common clocks or the equivalent the measured velocity of light emitted by a moving source, which is quasi-velocity of foreign light, is dependent of the source velocity. Nevertheless, the velocity of foreign light that should be measured by using tau-clocks is independent of the source velocity. The velocity of native light emitted by a rest source obeys the postulate of relativity in accordance with both Maxwell equations and the result of Michelson-Morley experiment. On the other hand, the velocity of foreign light obeys both Ritz's emission theory except the Lorentz factor and the postulate of constancy of light velocity if measured by using tau-clocks. Thus the emission theory does not conflict with special relativity. The present argument leads to a logical consequence that the so-called positive conclusions from experiments testing constancy of the velocity of light emitted by moving sources if using common clocks or the equivalent, instead of tau-clocks, exactly contradicts Lorentz transformations. 
Wang Guowen