Now the earth is moving in its orbit about the sun with a velocity of 18 miles per second. If the earth moves through the ether and a light-beam passes from one mirror to another and back again, the time taken for this journey ought to be longer when the light-path is in the direction of the earth's motion than when it is at right angles to this direction. For we can consider the light as a swimmer having a speed of 186,000 miles per second and travelling in a stream whose current is 18 miles per second. When Michelson and Morley tried the experiment they could not observe any difference in the velocity of light in the two directions. The experiment has since been repeated under various conditions, but always with negative results. Einstein's contribution to science consists in interpreting this result as being in accord with Newton's ideas on mechanical relativity in that it demonstrates the impossibility of measuring absolute motion, not only by mechanical, but also by optical or electrical experiments. Consequently the velocity of light must be regarded as constant and independent of the motion of either source or observer. THE RELATIVITY OF UNIFORM MOTION Let us consider some of the consequences which follow from this principle. An observer travelling with say one-half the velocity of light in the same direction as a ray of light would find that the latter has the usual velocity of 186,000 miles per second. Similarly an observer travelling in the opposite direction to that of the light-ray, with one-half the velocity of light, would obtain the same result. Einstein has shown that these conclusions can be valid only if the units of time and space used by the two observers depend upon their relative motions. A careful calculation shows that the unit of length used by either observer appears to the other observer contracted when placed in the direction of their relative motion (but not, when placed at right angles to this direction), and the unit of time used by either observer appears to the other too great. Moreover, the ratio of the units of length or of time varies with the square of the relative speed of the two observers, according to a relation which is similar to that mentioned above for the swimmer in the current. This relation shows that as the relative speed approaches that of light the discrepancy between the units increases.