In Michelson and Morley’s experiment, the distance travelled by the beam along OA (that is, in a direction parallel to the motion of the platform) is 2_l_β², while the distance travelled by the beam along OB, perpendicular to the direction of motion of the platform, is 2_l_β. Yet the most careful experiments showed, as Eddington says, “that both parts of the beam took the same time as tested by the interference bands produced. It would seem that OA and OB could not really have been of the same length; and if OB was of length _l_, OA must have been of length _l_/β. The apparatus was now rotated through 90°, so that OB became the up-stream. The time for the two journeys was again the same, so that 0B must now be the shorter length. The plain meaning of the experiment is that both arms have a length _l_ when placed along O_y_ (perpendicular to the direction of motion), and automatically contract to a length _l_/β, when placed along O_x_ (parallel to the direction of motion). This explanation was first given by Fitz-Gerald.” This Fitz-Gerald contraction, startling enough in itself, does not suffice. Assuming this contraction to be a real one, the distance travelled with respect to the ether is 2_l_β and the time taken for this journey is 2_l_β/_c_. But the distance travelled with respect to the platform is always 2_l_. Hence the velocity of light with respect to the platform is $$ \frac {2l}{\frac {2l\beta}{c}} = \frac {c}{\beta} $$ a variable quantity depending on the “absolute” velocity of the platform. But no trace of such an effect has ever been found. The velocity of light is always found to be quite independent of the velocity of the platform. The present difficulty cannot be solved by any further alteration in the measure of space. The only recourse left open is to alter the measure of time as well, that is, to adopt the concept of “local time.” If a moving clock goes slower so that one ‘real’ second becomes 1/β second as measured in the moving system, the velocity of light relative to the platform will always remain _c_. We must adopt two very startling hypotheses, namely, the Fitz-Gerald contraction and the concept of “local time,” in order to give a satisfactory explanation of the Michelson-Morley experiment.