July 31, 1919] 



NATURE 



429 



In many cases, as shown in Fig. 3, the small, 

 'lark hydrogen flocculi surrounding spots present 

 a well-defined vortex structure, and it is now well 

 known that the hypothesis which associates a 

 sun-spot with an electric vortex has been brilli- 

 antly confirmed by Prof. Hale in the discovery of 

 Zeeman effects in sun-spot spectra corresponding 

 with the magnetic field produced by the whirling 

 ions. A large percentage of sun-spots are double, 

 and the two members have been found to be of 



middle of Ho so as to bring out the structure of 

 the overlying atmosphere. The two photographs 

 were obtained simultaneously in this way. 



The polarities of sun-spots have been extensively 

 investigated by the Mount Wilson observers, and 

 arrangements have lately been made for the rapid 

 visual determination of the polarities and field- 

 strengths of all sun-spots as part of the daily 

 programme of observations. The 150-ft. Tower 

 telescope, giving a i6-in. image of the sun, and 



. — Hydrogen flocculi 



ding a group of .'un-spois. (a) 1915, August 3 ; (^) August 5 ; (c) August 7 ; (a) August 9, i3-ft. spectroheliograph. 



pposite polarity, as indicated by the polarisation 

 )hen.omena of the components of the complex lines 

 >;oduced in spot spectra by the magnetic field, and 

 IV the opposite direction of whirl over the two 



omponent spots. Such groups are called bi- 

 j)olar groups. In the case of Fig. 3 two isolating 

 ^lits, side by side, were employed ; one was set on. 

 a porticm of the continuous spectrum, and thus 

 shows practically an ordinary image of the spot 

 group, while the other was adjusted to the 

 NO. 2596, VOL. 103] 



the 75-ft. spectrograph, with a compound quarter- 

 wave-plate placed over the slit, are utilised in 

 these observations. The method of recording the 

 phenomena is shown in Fig. 4, representing a bi- 

 polar group observed on August 10, 1917. In this 

 diagram, R means that a given strip of the com- 

 pound quarter-wave-plate, used with a Nicol 

 prism, transmits the red n component of the Zee- 

 man triplets, of which \ 6173 is selected for 

 observation, and the appended figures indicate the 



