Rising to the rank of captain in the engineers, he became increasingly interested in physics and, in 1881, resigned his commission to join Alfred Cornu's laboratory at the École Polytechnique, working on spectroscopy.In 1868, Pierre Janssen's solar observations had led him to report to the Académie des Sciences that It is no longer geometry and mechanics which dominate [in astronomy] but physics and chemistry.In 1889, Le Verrier was succeeded by Amédée Mouchez who set to work to bring astrophysics into the mainstream by hiring Deslandres.In 1918, following the armistice, he resumed his office at Meudon until 1926 when its administration merged with that of the Paris Observatory, Mouchez assuming the role of director of both institutions until his retirement in 1929.Fellow astronomer Raymond Michard observed that In his bearing, his character and his style of life Deslandres always remained more akin to the soldier (and the officer) than to the scholar.