The article translated in this chapter is commonly known as the Palermo paper. On first consideration it seemed that the aberration of light and the optical and electrical phenomena associated with it were going to provide us a means for determining the absolute motion of the Earth or more accurately its motion, not with respect to the other stars, but with respect to the ether. Fresnel had already tried it, but he soon recognized that the motion of the Earth did not change the laws of refraction and reflection. Analogous experiments, like that of the water-filled telescope and all those where only first-order terms in the aberration were considered were to give only negative results; the explanation for this was soon found. But, Michelson, who had imagined an experiment sensitive to the terms depending on the square of the aberration, failed in turn.