Already in his second communication Zeeman expressed the opinion that the accepted existence of strong magnetic fields on the surface of the sun could be verified, since these should alter spectral lines derived from the celestial body. (It is typical of Zeeman to extend physical concepts into the realm of celestial phenomena.) In a letter to him (1908) the astronomer G.E. Hale, Director of Mount Wilson Observatory, corroborated this opinion by means of photographs which indicated that in solar vortices the spectral lines indeed appeared to be affected by magnetic fields. Even the theoretical prediction concerning the probable interrelationship between the directions of polarization and those of the magnetic fields was subsequently confirmed by Hale.