Eight-Year Cycle in Relation to Physical Cause 125 



been made in understanding the phenomena. A corol- 

 lary of Maxwell's electro-magnetic theory of light was 

 that light must exert pressure. The idea was seized 

 upon by Arrhenius as a clue to the explanation of 

 comets' tails. He assmned that negatively charged 

 small bodies are driven off from the Sun under the 

 influence of light-pressure, and are electrically dis- 

 charged in the gases surrounding the comet thereby 

 causing the characteristic shape and glow of the 

 comet's tail.^ 



Birkeland, in his theory of comets' tails, substituted 

 cathode rays or streams of electrons for Arrhenius' 

 negatively charged small bodies.- Deslandres, Bernard, 

 and B osier rendered still more probable the hypothesis 

 of the cathode rays by their spectroscopic studies of 

 Morehouse's comet in 1908, in which they showed the 

 resemblance of the comet's radiation to that of cathode 

 rays in a Crookes' tube.^ 



1 "Eine andere Schwierigkeit bieteii die Gase der Kometen insofern, 

 als sie leuchten, wenn sie in einer Entfernung von der Sonne sind, wo 

 die Temperatur nicht wohl hoher sein kann als auf dem heissesten 

 Punkte des Mondes. Dies zeigt, dass wahrscheinlichervveise elek- 

 trische Vorgange hier vor sich gehen. Dies kann durch die Theorie 

 erkliirt werden, wenn man anninimt, dass von der Sonne nach alien 

 Seiten negativ geladene kleine Korper ausgestrahlt werden. Diese 

 treffen den Kometen und bewirken da Entladungen, welche die Gase 

 zum Gliihen bringen. Arrhenius, Lehrbuch der Kosmischen Physik, 

 1903, p. 204. 



2 "Arrhenius has also, as we know, maintained a similar theory, 

 only that instead of electric corpuscle-rays of the kind here considered, 

 he imagines rays of electrically-charged atoms, moving under the in- 

 fluence of light-pressure." Birkeland, The Norwegian Aurora Polaris 

 Expedition, 1902-1903, p. 631. 



3 "Dans un ordre d'idees bien different, mais egalement en faveur 

 de la theorie cathodique, on pent encore relever un fait interessant: 

 le spectre de la queue de certaines cometes, et plus particulierement 

 de la comete Morehouse (1908 c ), s'est montre identique a celui que 



