DISCOVICKY 



171) 



lower hydrogen clouds. This gave strong support to 

 the hypothesis of the vortical nature of sun-spots. 

 " We know now," said Hale in November 1908, " that 

 they are caused by vortices in the solar atmosphere, 

 and the various theories which do not recognise this 

 fact may be laid aside." At that time. Hale inclined 

 to the view that " in the case of sun-spots we see a 

 phenomenon somewhat analogous to a tornado or 

 waterspout on the earth. If the gases at high levels 



E.xperiments conducted b}- Rowland at Baltimore as 

 far back as 1S75 indicated that, if electrically-charged 

 bodies existed at all in the solar atmosphere, their 

 revolution in sun-spot vortices would produce a mag- 

 netic field ; and in 1S96 Zeeman, of Amsterdam, showed 

 that the light from a luminous vapour is altered in a 

 remarkable way when under the influence of a strong 

 magnetic field — the lines of the spectrum being widened 

 or broken up into several constituents (the Zeeman- 



PHOTO OF SU-NSPOTS T.\KEN' .4T GREENWICH OBSERV.VroRY, MAV 

 (By kind permission ol the Astronomer Royal.) 



whirl with sufficient velocity, they develop a tube-like 

 extension which reaches down through the compara- 

 tively undisturbed gases at lower levels. At the centre 

 of the storm the expansion of the gases due to their 

 rapid rotation cools them and thus produces a compara- 

 tively dark cloud which we see in the sun-spot." At 

 that time Hale believed the disturbances giving rise 

 to the spots to have their origin in the upper regions 

 of the solar atmosphere. This view he modified 

 later. 



effect). With the aid of the 30-foot spcctroheliograph 

 of the Mount Wilson Observatory, Hale, in 1908, 

 closely scrutinised the spectra of sun-spots for traces 

 of the Zeeman-effect, and his scrutiny was soon re- 

 warded by the discovery that the double and triple 

 lines in these spectra are exactly similar to the lines 

 produced by the presence of a magnetic field. In the 

 annual report of the observatory for 1909, Hale was 

 able to announce that the existence of magnetic fields 

 in sun-spots " has been placed beyond doubt through 



