[en] The opinion about the important role of the Michelson-Morley experiment in the establishment of the principle of the constancy of the light velocity is expressed. It is noted that Fitzgerald - Lorentz contraction introduced for the explanation of its negative result contradicts to the Galilei transformations (and consequently, to ether theory). The theorem is proved that if a length of a moving rod is given by the instantaneous length, the propagation velocity of light along the rod is an inconstant quantity. It is stressed that the using of the radar length for the treatment of Michelson - Morley experiment leads to elongation formula for a longitudinal arm of the interferometer. 19 refs