38 NATIONAL ACADEMY OF SCIENCES 



we need is a globe of great size, which has been rotating for 

 centuries at high velocity. The sun, with a diameter one hund- 

 dred times that of the earth (Fig. i*), may throw some light on 

 the problem. Its high temperature probably precludes the exist- 

 ence of permanent magnets : hence any magnetism it may exhibit 

 is presumably due to motion. Its great mass and rapid linear 

 velocity of rotation should produce a magnetic field much 

 stronger than that of the earth. Finally, the presence in its at- 

 mosphere of glowing gases, and the well-known effect of mag- 

 netism on light, should enable us to explore its magnetic field 

 even at the distance of the earth. The effects of ionization, prob- 

 ably small in the region of high pressure beneath the photosphere 

 and marked in the solar atmosphere, must be determined and 

 allowed for. But with this important limitation, the sun may 

 be used by the physicist for an experiment which can not be per- 

 formed in the best equipped laboratory. 



Schuster, in the lecture already cited, remarked: 



" The form of the corona suggests a further hypothesis which, extravagant as 

 it may appear at present, may yet prove to be true. Is the sun a magnet? " 



Summing up the situation in April, 1912, he repeated: 



" The evidence (whether the sun is a magnet) rests entirely on the form of 

 certain rays of the corona, which — assuming that they indicate the path of project- 

 ing particles — seem to be deflected as they would be in a magnetic field, but 

 this evidence is not at all decisive." 



There remained the possibility of an appeal to a conclusive 

 test of magnetism: the characteristic changes it produces in light 

 which originates in a magnetic field. 



Before describing how this test has been applied, let us rapidly 

 recapitulate some of the principal facts of terrestrial magnet- 

 ism. You see upon the screen the image of a steel sphere 

 (Fig. 2), which has been strongly magnetized. If iron filings 

 are sprinkled over the glass plate that supports it, each minute 

 particle becomes a magnet under the influence of the sphere. 



*The cliches used in illustration of Dr. Hale's address were courteously furnished by the 

 Editor of Popular Science Monthly in which journal the address has been published 

 (August, 1913). — The Home Secretary. 



