622 Professor George E. Hale [May 14, 



Let me pass rapidly in review a series of phenomena with which 

 you are all familiar. Sir William Crookes showed in this lecture- 

 room as long ago as 1879 that the negative pole of a vacuum tube 

 sends out a stream of particles capable of setting a light wind- mill in 

 rotation, and deviated from their straight path when under the 

 influence of a magnetic field. He has kindly consented to show the 

 same tube again to-night ; you now see the effect upon the screen. 

 The recent work of Sir Joseph Thomson and others has proved that 

 these are negatively charged particles, called " corpuscles " or 

 " electrons," and that their mass is about T-7V0 of the mass of an atom 

 of hydrogen. Moreover, Thomson has shown that at low pressures 

 these corpuscles are given off from a hot wire or from the carbon 

 filament of an incandescent lamp. He has also demonstrated that 

 this property of emitting corpuscles at high temperature is common to 

 carbon and to metals, whether in the solid or in the vaporous condition. 

 Thus we have warrant for the belief that the sun, composed of just 

 such elements as constitute the earth, must emit great numbers of 

 these corpuscles. As Thomson has estimated that the rate of emis- 

 sion of a carbon filament at its highest point of incandescence may 

 amount to a current equal to several amperes per sq. centimeter of 

 ^surface, we can hardly be mistaken in assuming the existence of still 

 more powerful currents in the sun. The emission of negatively 

 charged particles implies the emission of positively charged particles, 

 but in laboratory experiments, because of unequal rates of diffusion 

 or other causes, charges of one sign are always found to be in excess. 

 We thus have reason to believe that powerful magnetic fields may 

 result from the revolution of these particles in the solar vortices. 



In seeking a means of detecting such fields, let us first recall 

 Faraday's discovery of the effect of magnetism on hght, made at the 

 Royal Institution in 1846. This discovery relates to the rotation of 

 the plane of polarisation of light when passed through a plate of 

 dense glass in a strong magnetic field. Although Faraday, in what 

 was said to be his last experiment, endeavoured to detect the effect of 

 magnetism on the lines of the spectrum, he failed because the appa- 

 ratus then available was not sufficiently powerful. In 1896, Professor 

 Zeeman examined with a large spectroscope the two yellow lines 

 emitted by sodium vapour in a flame between the poles of a powerful 

 magnet. Observing in the direction of the lines of force, he saw that 

 the sodium lines widened when the magnet was excited. Subsequently 

 with more powerful apparatus, he found that a single line, when 

 observed under the above conditions, is split into tw^o components by 

 a magnetic field. The distance between the two components is a 

 measure of the strength of the field. But the most characteristic 

 quality of these double lines, which distinguishes them from double 

 lines produced by any other known means, is the fact that the light 

 of the two components is circularly polarised in opposite directions. 

 If, then, we encounter a double line in the spectmm of any substance, 



