This instrument, invented independently by Henri Deslandres (1853-1948) and an American, George Hale (1868-1938), enables one to obtain an image of the Sun in a very narrow range of wavelengths. To do this, the entrance slit of a spectrograph is moved over the solar image, while at the same time moving the exit slit, which selects the desired wavelength: the image of the exit slit is projected on a fixed photographic plate, thus obtaining a monochromatic image of the Sun. After his arrival at the Meudon Observatory in 1897, Deslandres built a large spectroheliograph, which is still there.

caption : Spectroheliogram of 10 September 1897 - credits : Observatoire de Paris

To know more : The Meudon spectroheliograph