Johannes Franz Hartmann
Hartmann had presented his doctoral thesis in Leipzig in 1891 on the Earth's shadow during moon eclipses (Die Vergrösserung des Erdschattens bei Mondfinsternissen). He worked in Wien, Austria, with de Ball and again in Leipzig with Bruns.
In 1896 he moved to Potsdam were he worked with H.C. Vogel and was promoted to 'Observer' in 1898 and to 'Professor' in 1902. During these years he became one of the leading astrophysicists of his time. His main work was on defining standards for wavelengths as well as in instrumentation (microphotometer). During this time the big refractor with a diameter of 80 cm was installed and Hartmann found the photographic telescope to be useless: the lenses were not good enough. Then he developed a method of testing telescope lenses, which is today named after him. After refiguring the main lens according to his recommendations the telescope was in good condition and went to work.
Hartmann found clouds of Calcium with this instrument during his spectrographic work. He found that Ca II absorption lines in the spectrum of the binary star δ Orionis failed to take part in the periodic oscillations of the other lines. He eventually that this was due to interstellar gas, an original concept at the time. Although receiving support from V.M. Slipher in 1909, Hartmann’s interpretation was not accepted immediately. In 1926, Eddington was able to produce conclusive support for it.
In 1909 he went to Göttingen as Director of the Observatory and Professor at the University there. Since the observing conditions in Göttingen were not to his needs he went to La Plata in 1921, where he developed a theory on Novae and discovered that the minor planet Eros is not a spherical body.