The Bridge Theory

       At the beginning of the twentieth century, physics was in full swing. After the invention of the spectroscope made by Fraunhofer (1814) and the discovery of absorption lines, dark lines, visible both in the spectra of sunlight and stellar light and in the light reflected from the surface of the Moon and some other brighter planets, the studies by the Kirchhoffs and Bunsen quickly led to the discovery and study of emission lines in the light spectra of chemicals (Fig. 1).   Fig. 1   The correspondence of some of the visible emission lines with the absorption ones, as in the case of the fundamental H-alpha line of hydrogen (red line in Fig. 5), has led to a qualitative leap in the study of the cosmos, opening the way to analyze the composition of stellar atmospheres: it was now clear that nature, so jealous in keeping the secrets of matter, could hide them for a while longer. The discovery and study of natural radioactivity (1896) by H. Becquerel and Pierre and Marie Cu