THE SUN'S COR OKA AND JUS SPOTS. 



551 



other words, in the physical structure of the 

 corona. The bright lines or bright images of the 

 inner corona (according as it was examined 

 through a slit or without one) were not seen last 

 July, showing that no part, or at least no appre- 

 ciable part, of its light comes from glowing gas- 

 eous matter. But also the dark lines seen by 

 Janssen in 1871 were wanting on this occasion, 

 showing that the corona did not shine appre- 

 ciably by reflecting sunlight. The spectrum was, 

 in fine, a continuous rainbow-tinted streak, such 

 as that given by glowing solid or liquid matter. 



The inference clearly is : 1. That last July 

 the gaseous matter which had been present in 

 the corona in 1871 was either entirely absent or 

 greatly reduced in quantity ; 2. The particles of 

 solid or liquid (but probably solid) matter which, 

 by reflecting sunlight, produced a considerable 

 portion of the corona's light in 1871, were glow- 

 ing with intense heat last July, and shone in the 

 main with inherent light ; and 3. The entire coro- 

 na was greatly reduced in size last July, a3 com- 

 pared with that which formed the " starlike dec- 

 oration " around the black body of the moon in 

 December, 1871. 



We cannot, however, accept the theory that 

 such a corona as was seen in 1871 invariably 

 surrounds the sun in years of great disturbance, 

 while the corona of last month is the typical 

 corona for years of small solar disturbance. The 

 generalization is flatly contradicted by the evi- 

 dence which I have presented in the preceding 

 pages. It may be that such a corona as was seen 

 in 1871 is more common in years of great dis- 

 turbance, just as spots are then more common, 

 though not always present ; while such a corona 

 as was seen last July 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 favors rather strongly the infer- 

 ence that an association of this sort really exists 

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

 however, be unsafe at present to generalize even 

 to this extent ; while certainly the wide generali- 

 zation telegraphed to Europe from America as 

 the great result of the eclipse observations last 

 July must unhesitatingly be rejected. 



It remains to be considered how science may 

 hope to obtain more trustworthy evidence than 

 we yet have respecting 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 spec- 

 troscopic method, that an association has been 

 recognized between the number, size, and brill- 

 iancy 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 ex- 

 cept at the time of total solar eclipse. It seems 

 almost impossible to Ifope 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 shin- 

 ing in full splendor. 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 spectroscope, again, would not help 

 in the slightest degree to show such a corona as 

 was shining last July. For the powei of the 

 spectroscope to show objects which under ordi- 

 nary conditions are invisible depends on the 

 separation of rays of certain tints from the rays 

 of all the colors of the rainbow which make up 

 solar light ; and as the corona last July shone 

 with all the colors of the rainbow, and not with 

 certain special tints, the power of the spectro- 

 scope would be thrown away on a corona of that 

 kind. All that we can ever hope to do is to dis- 

 cern the gaseous corona when, as in 1871, it is 

 well developed, by spectroscopic appliances more 

 effective for that purpose than any which have 

 hitherto been adopted ; for all which have as yet 

 been adopted have failed. 



Now, the difficulty of the problem will be rec- 

 ognized when we remember that the strongest 

 tint of the corona's light — the green tint classi- 

 fied as 1474 Kirchhoff — has been specially but 

 ineffectually searched for in the sun's neighbor- 

 hood with the most powerful spectroscopic ap- 

 pliances yet employed in the study of the colored 

 prominences. In other words, when the light of 

 our own air over the region occupied by the co- 

 rona has been diluted as far as possible by spec- 

 troscopic contrivances, the strongest of the spe- 

 cial coronal tints has yet failed to show through 

 the diluted spectrum of the sky. Again, we- 

 have even stronger evidence of the difficulty of' 

 the task in the spectroscopic observations made- 

 by Respighi during the eclipse of 1871. The 

 instrument, or I should rather, perhaps, say the- 

 arrangement, which during mid-totality showed! 

 the green image of the corona to a height of' 

 about 280,000 miles, did not show any green, 



