D O C . 131 O B I T A R Y F O R W. . J L I S 231 W. H. JULIUS, 18 6 0 -19 25 m By A. EINSTEIN With the passing away of W. H. Julius, one of the most original exponents of solar physics has left us. These few lines will be de- voted to the work of this old friend of mine. They are written with [2] the hope that his views on taking refraction into account in explain- ing solar phenomena may not be temporarily forgotten by oversight. Julius began with his studies in mathematics and physics at the University of Utrecht in 1879. He directed his interest to experi- mental physics, working chiefly on emission and absorption in gases, until the age of thirty-one. Then, in 1891, a work of A. Schmidt, Die Strahlenbrechung auf der Sonne, ein geometrischer Beitrag zur Sonnenphysik, turned his attention to the field of solar physics, to [3] which he thereafter devoted his entire life. Julius did not advocate Schmidt’s conception that the sharp edge of the sun was a phenomenon caused by refraction due to a radial density-gradient for he recognized that Scattering and ab- sorption in the outer strata of the sun would necessarily prevent the formation of rays as long as those required by the Schmidt theory. He became convinced, however, that deviations from rectilinear propagation of light explained solar phenomena which would be diffi- cult to comprehend if we ascribed to emission, absorption, and mo- tion only, the distribution of light we see on the sun, and the velocity with which this distribution changes. To do justice to the viewpoint of W. H. Julius, we must next ask what velocities are attained by matter in the outer strata of the sun. Observation gives no direct answer to this question. A priori, it is doubtful that motion of material is responsible for every shift of center of intensity of a spectral line, and for every motion of a singu- larity of intensity on the sun’s disk. According to Julius, the only phenomena that give a direct measure of velocities of matter at the surface of the sun are sun-spots. That they are vortices is shown by the Zeeman effect found by Hale. Since familiar theorems of h y dro- [4] dynamics show that the material of a vortex moves along with it, 196