give a visual picture of sounds, in order to make clear the formation of sounds to his pupils; this led him to construct an electrical apparatus, which finally led to the discovery of the telephone. The truest and sharpest contrast with the _experimentum crucis_ is furnished by experiment when it shows the exact opposite of what the explorer was expecting. But since an absolute No entails a very decisive Yes--namely, in this case, the affirmation of a relationship that was previously held to be impossible--a negative experiment of this kind, when it occurs, will be followed by momentous consequences; these will be the more important in proportion as the question, the affirmation of which was expected by the physicist, is of a fundamental character. The experiments of Michelson and Morley, directed at proving the existence of the ether, are to be regarded as the true classical instances of these experiments answering with an overwhelming negative. Their first effect was to produce a sense of helplessness, a check to thought, a void in the chamber of ideas. And to fill this void there arose new views of the world in which we nowadays recognize the true thought-pictures of the universe. The great names--Lorentz, Minkowski, Albert Einstein--shone out! As there are forerunners for almost every Important event, so also in the case of the _experimentum crucis_ of Michelson and Morley. Henri Poincaré, the famous mathematician, whilst still a student of the École Polytechnique, had initiated experiments with his fellow-student Favé, which followed the same object. The Michelson-Morley experiment was at least a hundred times more accurate. In each case the conclusion was that the laws of optics are not disturbed by a motion of translation, such as that of the earth through space this is, however, contrary to what the old physical ideas lead us to expect. If we assume the existence of a space-filling ether, the earth, owing to its own velocity of nineteen miles per second, would have to pass through a hurricane just as in the case of travellers sitting in an open train rushing along at very great speed. If we send out light rays in all directions simultaneously from any point on the earth's surface, some will travel in the teeth of the ether-storm, others will experience only a part of the storm's power; so that of two light-rays travelling in exactly opposite directions the retardation of the one should be equal to the acceleration of the other; and yet they are not quite equal, for a simple calculation shows that in every case the retardation is slightly more than the acceleration. This may be made clear by means of a model of easy construction, or, better still, by considering a ship that is subject to a constant current and, simultaneously, to a pressure of the wind. The time taken by the boat in making a trip up and down stream can never be the same for the cases when the wind is in the direction of the current, and vice versa.