at a university). During this time, Einstein described why the sky is blue in his paper on the phenomenon of critical opalescence, which shows the cumulative effect of scattering of light by individual molecules in the atmosphere. In 1911, Einstein became first associate professor at the University of Zurich, and shortly afterwards full professor at the German language-section of the Charles University of Prague. While at Prague, Einstein published a paper calling on astronomers to test two predictions of his developing theory of relativity: a bending of light in a gravitational field, measurable at a solar eclipse; and a redshift of solar spectral lines relative to spectral lines produced on Earth's surface. A young German astronomer, Erwin Freundlich, began collaborating with Einstein and alerted other astronomers around the world about Einstein's astronomical tests. In 1912, Einstein returned to Zurich in order to become full professor at the ETH Zurich. At that time, he worked closely with the mathematician Marcel Grossmann, who introduced him to Riemannian geometry. In 1912, Einstein started to refer to time as the fourth dimension (although H.G. Wells had done this earlier, in 1895 in The Time Machine). In 1914, just before the start of World War I, Einstein settled in Berlin as professor at the local university and became a member of the Prussian Academy of Sciences. He took Prussian citizenship. From 1914 to 1933, he served as director of the Kaiser Wilhelm Institute for Physics in Berlin. He also held the position of extraordinary professor