Pierre Jules Cesar Janssen (1824-1907), French astronomer, most commonly known as Jules Janssen. Janssen, along with Lockyer, discovered helium in 1868, by making spectrographic observations of the Sun. He also discovered the gaseous nature of solar prominences, and made numerous eclipse observations. He worked to establish an observatory on the summit of Mount Blanc, realising that the height would decrease atmospheric distortions. He was awarded the Rumford Medal of the Royal Society in 1876. Craters on the Moon and Mars are named in his honour.