THE SUN'S CORONA AND HIS SPOTS. 13 



when they are increasing. This point seems the more to 

 need investigation when we note that the evidence derived 

 from eclipses occurring near the time either of maximum or of 

 minimum solar disturbance has not been altogether satisfac- 

 tory. It may be that we may find an explanation of the 

 discrepancies we have recognised, in some distinction 

 between the state of the corona when spots are increasing 

 and when they are diminishing in number. 



It is noteworthy that several careful observers of the 

 corona in 1842 believed that they could recognise motion 

 in the coronal rays. Francis Baily compared the appear- 

 ance of the corona to the flickering light of a gas illumi- 

 nation. O. Struve also was much struck by the appearance 

 of violent agitation in the light of the ring. It seems probable 

 that the appearance was due to movements in that part of 

 our atmosphere through which the corona was observed. 

 The extent of the corona was variously esfimated by 

 different observers. Petit, at Montpelier, assigned to it 

 a breadth corresponding to a height of about 200,000 miles ; 

 Baily a height of about 500,000 miles ; and O. Struve a 

 height of more than 800,000 miles. The last-named 

 observer also recognised luminous expansions extending 

 fully four degrees (corresponding to nearly seven million 

 miles) from the sun. Picozzi, at Milan, noticed two jets of 

 light, which were seen also by observers in France. Rays 

 also were seen by iMauvais at Perpignan, and by Baily at 

 Paria. But Airy, observing the corona from the Superga, could 

 see no radiation ; he says * although a slight radiation might 

 have been perceptible, it was not sufficiently intense to affect 

 in a sensible degree the annular structure by which the 

 luminous appearance was plainly distinguished.' These 

 varieties in the aspect of the corona were doubtless due to 

 varieties in the condition of the atmosphere through which 

 the corona was seen. Now it cannot be questioned that, so 

 far as extension is concerned, the corona seen in 1842 was 

 one which, if the theory we are considering were sound, we 

 should expect to see near the time of maximum rather than 



