The author gives a résumé of the principal theories of the cause of the aurora published since 1896,—;those of Birkeland, Arrhenius and Nordmann— and points out the flaws in each one. All agree in considering the auroral rays as a luminescence produced by the absorption of cathode rays in the atmosphere, but none seem capable of explaining all the phenomena. Birkeland's first theory rests on the hypothesis that cathode rays emitted by the Sun are attracted by the Earth's magnetic poles, producing the aurora by their absorption in the atmosphere. This seems incapable of explaining the diverse forms of the aurora, and the changes taking place in a single appearance of the phenomenon.