Two changes improved the apparatus over the one used at Potsdam [in 1886]. Morley suggested floating the heavy sand stone slab bearing the optical parts on mer cury. The stone, about 5 feet square, was mounted on a doughnut-shaped wooden block, which floated in a cast-iron trough containing the mercury. This method was economical and efficient, for the mercury-bearing removed practically all stresses and allowed the interferometer to glide smoothly around all points of the compass. Vibrations from outside disturbances, so troublesome in Berlin and even in Potsdam, were virtually eliminated.
With the new "interferential refractometer" Michelson and Morley were able to determine effects of the second order with an accuracy hitherto unobtainable. The second improvement lay in placing the mirrors so that the light was reflected over a path about 10 times longer than that of the earlier experiment.
While the stone, floating on its channel of mercury, turned ...
