Didymium
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Didymium is a mixture of the elements praseodymium and neodymium. It is used in safety glasses for glass blowing, where it provides a narrow filter which blocks mainly the light emitted by the hot sodium in the glass, without having much effect on general vision, unlike dark welder's glasses. When used as a photographic filter it tends to strengthen red tones. It is also used in calibration materials.
Didymium (Gr. "twin element") was discovered by Carl Mosander in 1841 and was so-named because it is very similar to lanthanum, with which it was found. Mosander wrongly believed didymium to be an element, however in 1885 Carl Auer von Welsbach discovered that didymium was really a mixture of the elements praseodymium and neodymium. Mosander had concluded that "ceria" (sometimes called cerite) isolated by Jöns Jakob Berzelius in 1803 was really a mixture of cerium, lanthanum and didymium. His was right about lanthanum being an element, but not about didymium. During the period when it was believed to be an element, the symbol Di was used for it.