DuringthelastyearsoftheNineteenthCentury, thedevelopmentofnewte- niques and the re?nement of measuring apparatuses provided an abundance of new data, whose interpretation implied deep changes in the formulation of physical laws and in the development of new phenomenology. Severalexperimentalresultsleadtothebirthofthenewphysics.Abrieflist of the most important experiments must containthose performedby H. Hertz about the photoelectric e?ect, the measurement of the distribution in f- quency of the radiation emitted by an ideal oven (the so-called black body - diation), themeasurementofspeci?cheatsatlowtemperatures, whichshowed violations of the Dulong-Petit law and contradicted the general applicability of the equipartition of energy. Furthermore we have to mention the disc- ery of the electron by J. J. Thomson in 1897, A. Michelson and E. Morley's experiments in 1887, showing that the speed of light is independent of the reference frame, and the detection of line spectra in atomic radiation. From a theoretical point of view, one of the main themes pushing for new physics was the failure in identifying the ether, i.e. the medium propagating electromagnetic waves, and the consequent Einstein-Lorentz interpretation of the Galilean relativity principle, which states the equivalence among all reference frames having a linear uniform motion with respect to ?xed stars.