No archaeologist, whether Young or Champollion deciphering the Rosetta Stone, or Rawlinson copying the cuneiform inscription on the cliff of Behistun, was ever faced by a more fascinating problem than that which confronts the solar physicist engaged in the interpretation of the hieroglyphic lines of sun-spot spectra. The colossal whirling storms that constitute sun-spots, so vast that the earth would make but a moment's scant mouthful for them, differ materially from the general light of the sun when examined with the spectroscope. Observing them visually many years ago, the late Professor Young, of Princeton, found among their complex features a number of double lines which he naturally attributed, in harmony with the physical knowledge of the time, to the effect of "reversal" by superposed layers of vapors of different density and temperature. What he actually saw, however, as was proved at the Mount Wilson Observatory in 1908, was the effect of a powerful magnetic field on radiation, now known as the Zeeman effect. [Illustration: Fig. 31. The 150-foot tower telescope of the Mount Wilson Observatory. An image of the sun about 16 inches in diameter is formed in the laboratory at the base of the tower. Below this, in a well extending 80 feet into the earth, is the powerful spectroscope with which the magnetic fields in sun-spots and the general magnetic field of the sun are studied.] Faraday was the first to detect the influence of magnetism on light. Between the poles of a large electromagnet, powerful for those days (1845), he placed a block of very dense glass. The plane of polarization of a beam of light, which passed unaffected through the glass before the switch was closed, was seen to rotate when the magnetic field was produced by the flow of the current. A similar rotation is now familiar in the well-known tests of sugars--laevulose and dextrose--which rotate plane-polarized light to left and right, respectively. But in this first discovery of a relationship between light and magnetism Faraday had not taken the more important step that he coveted--to determine whether the vibration period of a light-emitting particle is subject to change in a magnetic field. He attempted this in 1862--the last experiment of his life. A sodium flame was placed between the poles of a magnet, and the yellow lines were watched in a spectroscope when the magnet was excited. No change could be detected, and none was found by subsequent investigators until Zeeman, of Leiden, with more powerful instruments made his famous discovery, the twenty-fifth anniversary of which has recently been celebrated. [Illustration: Fig. 32. Pasadena Laboratory of the Mount Wilson Observatory.