This could be accounted for only on the theory that light is affected by magnetism, since the beam was not rotated by the glass alone—in itself a very important discovery. But the experiment did not yield Faraday an answer to the question uppermost in his mind: namely, can a magnetic field change the rate of vibration of a light-emitting particle? That is to say, in effect, can a magnetic field cause a ray of light to shift its normal place in the spectrum? It was not until 1862, seventeen years after the experiment just described, that Faraday attempted to solve this important theoretical problem. He now placed a sodium flame in front of the slit of the spectroscope, which normally yields two characteristic yellow lines (the D lines of the spectrum), and observed them with the best spectroscope at his command, under the most powerful electromagnetic field which he could produce. No change from the normal could be detected. Other observers tried the same experiment, but with negative results. We know that his theory was well founded, and that only the lack of a better spectroscope and a more powerful magnet prevented his discovery of what is now known as the Zeeman effect—a discovery which has already thrown a flood of light on a number of difficult physical problems.[23] Working with much more powerful apparatus, but following the same method of procedure employed by the immortal Faraday, Dr. Pieter Zeeman, of Leyden, succeeded, in 1896, in experimentally demonstrating the close relationship between light and magnetism. Dr. H. A. Lorentz, then Professor of Physics in the University of Leyden, now mathematical physicist at the Norman Bridge Laboratory of Physics, Pasadena, California, had predicted the nature of the change in the spectral lines to be expected, and this knowledge was used by Dr. Zeeman as a check on his results. Using a Rowland grating, instead of a less efficient prism spectroscope, Dr. Zeeman found that when a relatively weak electric current was applied, the two sodium lines were merely widened. In a still more powerful magnetic field, each of the lines was decomposed into two or three components, when the lines of force were parallel to the line of sight.[24] Moreover, the rays of the components of each line “were not those of natural light,” but were “polarized in a characteristic way,” i. e., were circularly polarized in opposite directions—“the direction of the vibration depending in a simple manner on the direction of the magnetic lines of force.”[25]