PENDING PROBLEMS OF ASTRONOMY. 61 



ceeded in photographing the corona in full sunshine, and so in estab- 

 lishing its objective reality as an immense solar appendage, sub-perma- 

 nent in form, and rotating with the globe to which it is attached. 

 One may call it " an atmosphere," if the word is not to be too rigidly 

 interpreted. I am bound to say that plates which he has obtained 

 do really show just such appearances as would be produced by such a 

 solar appendage, though they are very faint and ghost-like. I may 

 add further that, from a letter from Dr. Huggins, recently received, 

 I learn that he has been prevented from obtaining any similar plates 

 in England this summer by the atmospheric haze, but that Dr. Woods, 

 who has been provided with a similar apparatus, and sent to the Riffel- 

 berg in Switzerland, writes that he has an assured success. 



Our American astronomer, on the other hand, at the last eclipse (in 

 the Pacific Ocean), observed certain phenomena which seem to confirm 

 a theory he had formulated some time ago, and to indicate that the 

 lovely apparition is an apparition only, a purely optical effect due to 

 the diffraction (not refraction, nor reflection either) of light at the 

 edge of the moon — no more a solar appendage than a rainbow or a 

 mock-sun. There are mathematical considerations connected with the 

 theory which may prove decisive when the paper of its ingenious and 

 able proposer comes to be published in full. In the mean time it must 

 be frankly conceded that the observations made by him are very awk- 

 ward to explain on any other hypothesis. 



"Whatever may be the result, the investigation of the status and 

 possible extent of a nebulous envelope around a sun or a star is un- 

 questionably a problem of very great interest and importance. We 

 shall be compelled, I believe, as in the case of comets, to recognize 

 other forces than gravity, heat, and ordinary gaseous elasticity, as 

 concerned in the phenomena. As regards the actual existence of an 

 extensive gaseous envelope around the sun, it may be added that other 

 appearances than those seen at an eclipse seem to demonstrate it be- 

 yond question — phenomena such as the original formation of clouds of 

 incandescent hydrogen at high elevations, and the forms and motions 

 of the loftiest prominences. 



But, of all solar problems, the one which excites the deepest and 

 most general interest is that relating to the solar heat, its maintenance 

 and its duration. For my own part, I find no fault with the solution 

 proposed by Helmholtz, who accounts for it mainly by the slow con- 

 traction of the solar sphere. The only objection of much force is, that 

 it apparently limits the past duration of the solar system to a period 

 not exceeding some twenty millions of years ; and many of our geol- 

 ogical friends protest against so scanty an allowance. The same 

 theory would give us, perhaps, half as much time for our remaining 

 lifetime ; but this is no objection, since there is no reason to deny 

 the final cessation of the sun's activity, and the consequent death of 

 the system. But while this hypothesis seems fairly to meet the re- 



