Julius, Willem Henri

(b. Zutphen, Netherlands, August 1860; d. Utrecht, Netherlands, 15 April 1925)

solar physics.

The son of Willem Julius and Maria Margareta Dumont, Julius studied at the University of Utrecht. He became professor of physics at the University of Amsterdam in 1890 and at the University of Utrecht in 1896. A modest man, he lived well and in the traditional manner, showing full devotion to both science and the arts. Among his friends were Einstein, Ehrenfest, Zeeman, Eykman and Einthoven.

Julius studied the infrared radiation of flames with a radiometer he had constructed. In order to avoid tremors he mounted this instrument in such a way that its center of gravity was supported, a technique known as the Julius suspension. His observation of the solar eclipse of 1901 was the turning point in Julius’ activity; from then on he devoted all of his work to solar physics. August Schmidt had stressed the effects of refraction in the solar gaseous sphere; Julius modified this conception and gave more consideration to the refraction in irregular inhomogeneities and the anomalous refraction of rays having wave lengths quite near to the wave length of an absorption line. He explained solar prominences as regions with strong inhomogeneities, where the light of the sun is refracted toward us. The darkness of sunspots was explained by regular refraction. In these conceptions the importance of refraction was vastly exaggerated. Later, however, Julius extended his argument, stating that the rays of a Fraunhofer line would also show anomalous scattering, which would explain theIndividual works “Solar Phenomena, darkness inside the line. This view was later developed independently by Uns②d, and even today anomalous scattering is assumed to be the mechanism by which strong solar resonance lines are formed.

During the eclipses of 1905 and 1912 Julius applied a new method for determining the distribution of brightness over the sun’s disk by recording the variation of the total intensity during the partial phases.

BIBLIOGRAPHY

A survey of Julius’ theories and a complete bibliography of his works are in his De Natuurkunde van de Zon (Groningen, 1927).

Individual works inculde “Solar Phenomena, Considered in Connection With Anomalous Dispersion,” in Astrophysical Journal, 12 (1900), 185; “Hypothese over den oorsprung der zonneprotuberanties,” in proceedings of the Academy of Zesterdam, 5 (1902-1903), 162, and Physikalische Zeitschrift, 4 (1902-1903), 85 “A New Method for Determining the Rate of Decrease of the Radiating Power From the Center Toward the Limb of the Solar Disk,” in Astrophysical Journal, 23 (1906), 312; “Selective Absorption and Anomalous Scattering of Light in Extensive Masses of Gas,” in Proceedings of the Academy of Amsterdam, 13 (1910-1911), 881, and PhysikalischeZeitschrift, 12 (1911), 329; “The Total Solar Radiation During the Annular Eclipse of April 17, 1912,” in Astrophysical Journal, 37 (1913), 225; “Anomalous Dispersion and Fraunhofer Lines. Reply to Objections,” ibid., 43 (1916), 43; and “How to Utilize Actinometric Results Obtainable During Soalar Eclipses,” in Bulletin of the Astronomical Institute of the Netherlands, no. 33 (1923), p. 189.

M. G. J. Minnaert