34 



Soon after the discovery of the telescope, Fabricius noticed by- 

 its aid a black sjjot on the sun, which he took at first to be a 

 terrestial cloud. " My father and I, " he says, " passed the rest 

 of the day and the whole of the night in great impatience, trying 

 to think what this spot might be. But the next day it was still 

 there. One disappeared, but ten days after it reappeared — it had 

 gone round the sun. " Galileo also directed his " optick tube " to 

 the sun, and observing carefully the time when a spot disappeared 

 and when it reappeared, determined thereby the great fact that 

 the sun revolves on its axis. The period of rotation he de- 

 termined to be 26 days. Upon the screen is now a beautiful 

 photograph of the sun's disc taken on September 20, 1870, by Mr. 

 Warren de la Kue. 



To the early astronomers the nature of a sun spot was a 

 problem which their small telescopes and poor appliances could 

 not solve. They were thought to be planets passing over the sun's 

 disc ; vast masses of scoriae emitted by the solar volcanoes, &c. 

 It was not, however, until 1774 that Dr. Wilson, of Glasgow, 

 demonstrated that the spots were cavities in a luminous envelope, 

 surrounding the sun, wliich according to him was a dark globe 

 The appearance which a sun spot presents as it passes round the 

 limb of the sun, as seen through a large telescope, is now 

 represented on the screen. First, then, we have determined that 

 they are cavities in the luminous matter of which the surface of 

 the sun is mainly composed. Cavities ! yes, but of what size ? 

 In 1769 one was visible to the naked eye. Consider Avhat the 

 size of this must liave been when we learn that the smallest spot 

 observable by a large telescope must be at least as big as Scotland. 

 In August, 1859, a spot was measured by Xewall, which had a 

 diameter of 58,000 miles, that is, seven times the diameter of the 

 earth. In June, 1843, a spot was visible nearly 75,000 miles in 

 diameter. In March, 1858, during an eclii)se, the moon passed 

 over one 107,000 miles across, and in the same year the largest 

 spot of any on record was visible. It had a breadth of no less than 

 143,000 miles ! Across it 18 globes as large as our earth might 



