HISTORICAL INTRODUCTION Einstein's first paper on the restricted 'Theory of Relativity', originally published in the 'Annalen der Physik' in l905. Translated from the original German Papers by Dr. Meghnad Saha Lord Kelvin writing-in 1893, in his prefaceto the English edition of Hertz's Researches on Electric Waves, says" many workers and many thinkers have helped to build up the nineteenth century school of plenum, one ether for light, heat, electricity, magnetism; and the German and English volumes containing Hertz's electrical papers, given to the world in the last decade of the century, will be a permanent monument of the splendid consummation now realised." Ten years later, in 1905, we find Einstein declaring that " the ether will be proved to be superflous." At first sight the revolution in scientific thought brought about in the course of a single decade appears to be almost too violent. A more careful even though a rapid review of the subject will, however, show how the Theory of Relativity gradually became a historical necessity. Towards the beginning of the nineteenth century, the luminiferous ether came into prominence as a result of the brilliant successes of the wave theory in the hands of Young and Fresnel. In its stationary aspect the elastic solid ether was the outcome of the search for a medium in which the lightwaves may "undulate." This stationary ether, as shown by Young, also afforded a satisfactory explanation of astronomical aberration. But its very success gave rise to a host of new questionsall bearing on the central problem of relative motion of ether and matter. On the other hand, the negativeevidences in favour of the convection-coefficient had also multiplied. Mascart, Hoek, Maxwell and others sought for definite changesin different opticaleffects induced by the motion of the earth relative to the stationaryether. But all such attempts failed to reveal the slightesttrace of any optical disturbance due to the "absolute" velocityof the earthy, thus proving conclusivelythat all tne different optical effects shared in the general compensation arising out of the Fresnelian convection of the excess ether. It must be carefullynoted that the Fresnelian convection -coefficient implicitlasysumes the existence of a fixed ether (Fresnelo)r at least a wholly stagnant medium at sufficiently distant regions (Stokes), with reference to which alone a convection velocitycan have any significance.Thus the convection coefficient implying some type of a stationary or viscous, yet nevertheless "absolute" ether, succeeded in explaining satisfactorily known optical facts down to 1880.