tion of the triple lines, increases with the diameter 

 of the spot. The field is strongest near the center 

 of the spot, where the lines of the triplet are most 

 widely separated, and decreases to very low in- 

 tensity at points just outside the edge of the 

 penumbra. The spectrograph, when equipped 

 with suitable polarizing apparatus, serves as an 

 extraordinarily delicate means of measuring these 

 fields, which can be observed in regions where 

 they are not much more intense than the magnetic 

 field of the earth. In this way it became possible. 





FIG. 24. Lines of Force about + and -- Poles of a Magnet. 



as described below (p. 43), to detect the compara- 

 tively weak magnetic field of the entire sun. 



It has long been known that sun-spots usually 

 occur in pairs, and our study of the Zeeman effect 

 indicates that the two principal spots in such a 

 group are almost invariably of opposite polarity. 

 The natural inference is that we are here dealing 

 with a semi-circular vortex (like half a smoke ring) 

 the two ends of which, coming to the sun's surface 



