Complex sentence: From 1884 to 1888 he studied mathematics at the University of Warsaw, and in 1889 he was selected to continue his mathematics studies on the strength of a paper on his observations made at the Warsaw Astronomical Observatory.
Split into simpler sentences: From 1884 to 1888 he studied mathematics at the University of Warsaw. In 1889 he was selected to continue his mathematics studies on the strength of a paper on observations that he had made at the Warsaw Astronomical Observatory.