212 Dr Schuster. Some results of the [Nov. 18, 



only a few of the observers, while the great majority missed them 

 entirely, seeing only a continuous spectrum The same con- 

 stituents appear in the corona as hitherto only in altered propor- 

 tions, as might have been and was expected by students of Solar 

 Physics. In 1869, 1870 and 1871 the gaseous elements of the 



corona, the hydrogen and 1474 stuff, whatever that may be 



were in such quantity and condition and rose so high above the 

 solar surface, that their lines were conspicuous in the coronal 

 spectrum and attracted the attention of observers far more forcibly 

 than the feeble continuous spectrum of the light emitted from and 

 reflected by the minute solid and liquid particles which also form 

 an essential element of the corona ; at present the condition is 

 reversed. The gases are either too small in quantity or too cool to 

 be conspicuous. The lesson, and it is an important one, is simply, 

 as has been said, that to a certain extent the corona sympathises 

 with the sun-spots." 



About the connexion of sun-spots and corona I shall have to 

 say something further on. At present it is important to mention 

 that the almost uniform testimony of every observer goes to cou- 

 hrm Prof. Young's statement as to the extreme faintness of the line 

 spectrum. 



Another interesting fact came out through the photographs 

 taken in Siam. The most intense image of the prominences did 

 liot correspond to any of the known hydrogen lines ; but was due 

 to a line near H, the position of which, owing to the small dis- 

 persion, could not be exactly determined. Prof. Young suggested 

 that the line might be H itself, that is, due to calcium. He had 

 himself observed the calcium lines reversed in the solar spectrum 

 near sun-spots, and indication that calcium reached high up into 

 the chromosphere. During the late eclipse again Prof. Young 

 succeeded in seeing the calcium lines reversed in the chromospheric 

 layer which appeared at the end of the eclipse. The Siamese 

 photographs explain an observation made by Mr Warren de la Rue 

 in the eclipse of 1860. Mr de la Pue obtained the photograph 

 of prominence which was not observed with the naked eye ; a 

 result easily explained by the fact that the strongest line of a 

 prominence is at the edge of the visible part of the spectrum. 



We are however at present concerned with the sun's corona, and 

 I pass on to the discussion of polariscopic observations. 



2. ■Polari'scojiic observations. 



The question as to the polarisation of the sun's corona was first 

 started, as far as I know, by Arago. Various attempts to settle 

 the question were made at different times, but the question was 

 only definitely decided when, during the eclipse of 1858, the plane 

 of polarisation was shewn to pass through the centre of the sun. 



