22 ROUGH WAYS MADE SMOOTH. 



is the typical corona for years of small solar disturbance. 

 The generalisation is flatly contradicted by the evidence 

 which I have presented in the preceding pages. It may be 

 that such a corona as was seen in 1871 is common in years 

 of great disturbance, just as spots are then more common, 

 though not always present ; while such a corona as was seen 

 in July, 1878, is more common in years of small disturbance, 

 just as days when the sun is wholly without spots are then 

 more common, though from time to time several spots, and 

 sometimes very large spots, are seen in such years. On the 

 whole, I think the evidence I have collected favours rather 

 strongly the inference that an association of this sort really 

 exists between the corona and the sun-spots. It would, 

 however, be unsafe at present to generalise even to this 

 extent ; while certainly the wide generalisation telegraphed 

 to Europe from America as the great result of the eclipse 

 observations in July, 1878, must unhesitatingly be rejected. 

 It remains to be considered how science may hope to 

 obtain more trustworthy evidence than we yet have respect 

 ing the corona and its changes of form, extent, lustre, and 

 physical constitution. In the case of the prominences, we 

 have the means of making systematic observations on every 

 fine, clear day. It has been, indeed, through observations 

 thus effected by the spectroscopic method that an association 

 has been recognised between the number, size, and brilliancy 

 of the prominences on the one hand, and the number, size, 

 and activity of the sun-spots on the other. But in the case 

 of the corona, we are as yet unable to make any observations 

 except at the time of total solar eclipse. It seems almost 

 impossible to hope that any means can be devised for seeing 

 the corona at any other time. Of course, without the aid 

 of the spectroscope the corona, as ordinarily seen during 

 total eclipses, must be entirely invisible when the sun is 

 shining in full splendour. No one acquainted with even 

 the merest elements of optics could hope to see the corona 

 with an ordinary telescope at such a time. The spectro- 

 scope, again, would not help in the slightest degree to show 



