EAKTH AND SUK AS MAGKETS HALE. 151 



The idea that sun spots may be solar tornadoes, which was strongly 

 suggested by such photographs, soon received striking confirmation. 

 A great cloud of hydrogen, which had hung for several days on the 

 edge of one of these vortex structures, was suddenly swept into the 

 spot at a velocity of about 60 miles per second. More recently 

 Slocum has photographed at the Yerkes Observatory a prominence at 

 the edge of the sun, flowing into a spot with a somewhat lower 

 A^elocity. 



Thus we were led to the hypothesis that sun spots are closely 

 analogous to tornadoes or waterspouts in the earth's atmosphere 

 (fig. 7). If this were true, electrons caught and whirled in the spot 

 vortex should produce a magnetic field. Fortunately, this could be 

 put to a conclusive test through the well-known influence of mag- 

 netism on light discovered by Zeeman in 1896. 



In Zeeman's experiment a flame containing sodium vapor was 

 placed between the poles of a powerful electromagnet. The two 

 yellow sodium lines, observed with a spectroscope of high dispersion, 

 were seen to widen the instant a magnetic field was produced by pass- 

 ing a current through the coils of the magnet. It was subsequently 

 found that most of the lines of the spectrum, which are single 

 under ordinar})" conditions, are split into three components when the 

 radiating source is in a sufficiently intense magnetic field. This 

 is the case when the observation is made at right angles to the lines 

 of force. Wlien looking along the lines of force, the central line 

 of such a triplet disappears (fig. 8) , and the light of the two side com- 

 ponents is found to be circularly polarized in opposite directions. 

 With suitable polarizing apparatus, either component of such a line 

 can be cut off at will, leaving the other unchanged. Furthermore, a 

 double line having these characteristic properties can be produced 

 only by a magnetic field. Thus it becomes a simple matter to detect 

 a magnetic field at any distance by observing its effect on light 

 emitted within the field. If a sun spot is an electric vortex, and the 

 observer is supposed to look along the axis of the whirling vapor, 

 which would correspond with the direction of the lines of force, he 

 should find the spectrum lines double, and be able to cut off either 

 component with the palarizing attachment of his spectroscope. 



I applied this test to sun spots on Mount Wilson in June, 1908, 

 with the 60-foot tower telescope, and at once found all of the char- 

 acteristic features of the Zeeman effect. Most of the lines of the 

 sun-spot spectrum are merely widened by the magnetic field, but 

 others are split into separate components (fig. 9), which can be 

 cut off at will by the observer. Moreover, the opportune formation 

 of two large spots, which appeared on the spectroheliograph plates 

 to be rotating in opposite directions (fig. 10), permitted a still more 

 exacting experiment to be tried. In the laboratory, where the polar- 



