Plücker, Julius (yōˈlyŏs plüˈkər) [key], 1801–68, German mathematician and physicist. He became professor of mathematics (1836) and of physics (1847) at the Univ. of Bonn. He is known for his work in analytical geometry, in magnetic and electrical properties (he discovered the fluorescence of cathode rays), and in spectroscopy (he first suggested that the lines in the spectrum of an element were peculiar to that element). Plücker originated line geometry, which substitutes the straight line for the point as the unit in space.