POPULAR SCIENCE 289 



made in recent years. In the year 1896 Prof. Zeeman had 

 demonstrated a new relation between light and magnetism, by 

 proving that the emission and absorption of luminous radia- 

 tions are modified, when exposed to the influence of magnetic 

 forces. He experimented with a flame containing sodium 

 vapour, which was placed between the poles of a strong electro- 

 magnet. The effect produced was that when the current was 

 passed through the coils of the magnet, the D lines of sodium, 

 a double yellow line in a spectroscope of sufficiently high 

 dispersive power, were seen to be widened. Subsequent 

 experiments showed that nearly every radiation observed in 

 a powerful spectroscope is split into three components when 

 the original radiations are observed in a direction at right 

 angles to the lines of force. When, however, the lines are 

 observed along the lines of force, the central line of the triplet 

 disappears, and the two terminal lines remain which are found 

 to be circularly polarised in opposite directions. With suitable 

 polarising apparatus one or other of the two components of 

 the double can be quenched, leaving the other unchanged. 

 Now, in sun-spots the spectrum when observed by means of a 

 spectroscope of moderate dispersive power, as compared to 

 modern instruments, appears to be full of broadened and thick- 

 ened lines. More than a quarter of a century ago at Stonyhurst 

 some lines appeared to be doubled in the spectrum of a sun-spot, 

 suggesting at the time rather the appearance of a bright reversal 

 in the widened dark line, than of two separate lines. But 

 Prof. Hale, armed with the powerful instruments of Mount 

 Wilson, argued that if a sun-spot is an electric vortex, and the 

 observer looks along the axis of the whirling vapour, which 

 coincides with the direction of the lines of force, he should find 

 the spectrum lines double, and by means of a polarising attach- 

 ment of a Fresnel rhomb and Nicol prism to a spectroscope, 

 he should be able to quench either component at will. This 

 test he applied triumphantly on Mount Wilson to two spots 

 in June 1908, and at once found all the characteristic features 

 of the Zeeman effect . Most of the lines of the sun-spot spectrum 

 are widened in the magnetic field, but others are split into two 

 components. 



When a current of electricity flows through a coil of wire 

 in the form of a helix, lines of magnetic force are produced 

 threading the helix. Putting the first finger of the right 



