Henri Deslandres was one of the first French astrophysicists. Aged 28, he left a promising military career for research on molecular spectra and later on the atmosphere of the Sun. He developed a considerable instrumentation in both domains. In particular, he conceived and built several generations of spectroheliographs at Paris, then Meudon Observatories, and initiated a systematic patrol of solar activity and solar–terrestrial relations that has survived to this day. He directed Meudon Observatory from 1908 to its merging with Paris Observatory in 1926, and was also Director of the merged institutions until his retirement in 1928. In this paper, I concentrate on his work on the Sun, in which he showed considerable qualities as an instrument builder, an organizer and an observer, although he was not so successful in interpreting observations. I leave to others a study of his considerable, life-long work on molecular spectra.