We cannot do better than quote Eddington’s description of this famous experiment. “The principle of the experiment may be illustrated by considering a swimmer in a river. It is easily realized that it takes longer to swim to a point 50 yards up-stream and back than to a point 50 yards across-stream and back. If the earth is moving through the ether there is a river of ether flowing through the laboratory, and a wave of light may be compared to a swimmer travelling with constant velocity relative to the current. If, then, we divide a beam of light into two parts, and send one-half swimming up the stream for a certain distance and then (by a mirror) back to the starting point, and send the other half an equal distance across stream and back, the across-stream beam should arrive back first. ——>_u_ O A—————........ | _x_ | |B Let the ether be flowing relative to the apparatus with velocity _u_ in the direction O_x_, and let OA, OB, be the two arms of the apparatus of equal length _l_, OA being placed up-stream. Let _c_ be the velocity of light. The time for the double journey along OA and back is l l 2lc 2l t₁ = ------ + ------ = -------- = --- β² c - u c + u c² - u² c where $$ \beta = (1 - \frac {u^2}{c^2})^{-\frac {1}{2}} $$ a factor greater than unity. For the transverse journey the light must have a component velocity _n_ up-stream (relative to the ether) in order to avoid being carried below OB: and since its total velocity is _c_, its component across-stream must be √(_c²_ - _u²_), the time for the double journey OB is accordingly $$ t_2 = \frac {2a}{\sqrt {c^2 - u^2}} = \frac {2a}{c} \beta $$ so that _t₁_ > _t₂_. But when the experiment was tried, it was found that both parts of the beam took the same time, as tested by the interference bands produced.” After a most careful series of observations, Michelson and Morley failed to detect the slightest trace of any effect due to earth’s motion through ether. The Michelson-Morley experiment seems to show that there is no relative motion of ether and matter. Fresnel’s stagnant ether requires a relative velocity of—_u_. Thus Michelson and Morley themselves thought at first that their experiment confirmed Stokes’ viscous ether, in which no relative motion can ensue on account of the absence of slipping of ether at the surface of separation. But even on Stokes’ theory this viscous flow of ether would fall off at a very rapid rate as we recede from the surface of separation. Michelson and Morley repeated their experiment at different heights from the surface of the earth, but invariably obtained the same negative results, thus failing to confirm Stokes’ theory of viscous flow.