204 PROGRESS OF SCIENCE IN THE CENTURY. 



Fabricins and Galilei discovered sun-spots, which are 

 still of fascinating interest to astronomers. In early 

 days, some regarded them as due to the transits of 

 small planets across the sun^s disc, others thought of 

 them as clouds, others as masses of cindery slag in 

 process of being sloughed off, and so on. In 1774, 

 Prof. Alexander Wilson of Glasgow was able to give 

 geometrical definiteness to the suggestion, which had 

 been repeatedly made, that the spots were due to 

 great excavations in the sun's substance. He also 

 expounded the idea, which William Herschel elabo- 

 rated, that the sun was like an earth within, but sur- 

 rounded by an aurora of resplendent clouds. Some 

 estimate of the state of knowledge in regard to the 

 physical constitution of the sun may be got from 

 Sir William Herschers eloquent descriptions about 

 the beginning of the nineteenth century. It was to 

 him a sort of glorified earth, with hills and valleys, 

 luxuriant vegetation, and a population, protected by 

 a cloud-canopy from a radiant outer shell some thou- 

 sands of miles in thickness. This " was nothing less 

 than the definite introduction into astronomy of the 

 paradoxical conception of the central fire and hearth 

 of our system as a cold, dark, terrestrial mass, wrapt 

 in a mantle of innocuous radiance — an earth, so to 

 speak, within — a sun without." * Herschel's author- 

 ity gave vitality to this conception, whose main util- 

 ity was that it helped to definitise error — often the 

 first step to its demolition. But it would be histor- 

 ically unjust to ignore the fact that although Her- 

 schers main idea was quite erroneous, it was the peg 

 to which a number of accurate observations were tem- 

 porarily attached. 



* A. M. Gierke. History, 1885, p. TL 



