THE SEMI-CENTENNIAL ANNIVERSARY 45 



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

 analogous to tornadoes or water-spouts in the earth's atmosphere 

 (Fig. 8). 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 in- 

 fluence of magnetism on light discovered by Zeeman in 1896. 



In Zeeman's experiment a flame containing sodium vapor was 

 placed between the poles of a powerful electro-magnet. The 

 two yellow sodium lines, observed with a spectroscope of high 

 dispersion, were seen to widen the instant a magnetic field was 

 produced by passing a current through the coils of the magnet. 

 It was subsequently found that most of the lines of the spectrum, 

 which are single under ordinary 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. When looking along the 

 lines of force, the central line of such a triplet disappears 

 (Fig. 9), and the light of the two side components is found to be 

 circularly polarized in opposite directions. With suitable polar- 

 izing 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 polarizing 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 

 characteristic 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. 10), 

 which can be cut off at will by the observer. Moreover, the 



