616 Professor Georrje E. Hale [May 14, 



laboratory plays in the work of the Mount Wilson Solar Observatory. 

 I shall also show that in other ways the astronomer may advautage- 

 onsly follow the physicist, particularly in the choice of observational 

 methods, and in the design of instruments of research. 



Sun-spots were discovered as soon as Galileo and his contem- 

 poraries directed their little telescopes to the sun. In fact, ancient 

 Chinese records indicate that spots of exceptional size had been 

 detected by the naked eye many centuries before. Long after their 

 discovery, the most diverse views were held as to the nature of sun- 

 spots. Sir AVilliam Herschel mentioned the uncertainty which had 

 existed prior to his time, remarking that the spots had been variously 

 described as solid bodies revolving about the sun, very near its 

 surface ; the smoke of volcanoes : smoke floating on a liquid surface : 

 clouds in the solar atmosphere ; the summits of solar mount<iins, 

 uncovered from time to time by the ebb and flow of a fiery liquid, 

 etc. In Herschel's own view, the spots are to be considered as the 

 opaque body of the sun, seen through openings in the himinous 

 atmosphere which envelops it. Indeed, he considered that the sun 

 should be regarded as the primary planet of our system, and even 

 suggested the probability that it is inhabited. " Whatever fanciful 

 poets might say, in making the sun the abode of blessed spirits, or 

 angry moralists devise, in pointing it out as a fit place for the 

 punishment of the wicked, it does not appear that they had any 

 other foundation for their assertions than mere opinion and vague 

 surmise ; but now I think myself authorised, upon astronomical 

 principles, to propose the sun as an inhabitable world, and am 

 persuaded that the foregoing observations, with the conclusions I 

 have drawn from them, are fully sufficient to answer every objection 

 that may be made against it." * 



Sir John Herschel did not abandon the idea of an opaque solar 

 globe, but suggested that hurricanes or tornadoes might account for 

 the piercing of the two strata of luminous matter which ordinarily 

 conceal this globe. " Such processes cannot be unaccompanied by 

 vorticose, motions, which left to themselves, die away by degrees and 

 dissipate — with this peculiarity, that their lower portions come to 

 rest more speedily than their upper, by reason of the greater resis- 

 tance below, as well as the remoteness from the point of action, which 

 lies in a higher region, so that their centre (as seen in our water- 

 spouts, which are nothing but small tornadoes) appears to retreat 

 upwards. Now, this agrees perfectly with that which is observed 

 during the obliteration of the solar spots, which appear as if filled 

 in by the collapse of their sides, the penumbra closing in upon the 

 spot, and disappearing after it." 



We now know that sun-spots are brighter than the brightest arc 



* William Herschel, * On the Nature and Construction of the Sun and 

 Fixed Stars,' p. 20. 



