SPECTROSCOPIC OBSERVATIONS. 59 



a thin shell of similarly coloured material, since known as the 

 *' chromosphere," which covers the sun pretty uniformly every- 

 where to a depth of about 3000 miles, whilst the prominences 

 throw their weird fantastic branchings sometimes to heights of 

 100,000, or it might even be of 150,000 or 200,000 miles. 



But beside the prominences, these eclipses had caused the 

 recognition of a more astonishing object still : an irregular glow 

 of silvery light which spreads out from the sun like a vast star, 

 in all directions, and which has been known to us since as the 

 " corona." From that time forward the form and details of 

 the corona have been drawn and photographed upon every 

 possible occasion; though from 1851 up to the present time 

 the entire duration of all the available opportunities has not 

 amounted in the aggregate to as much as two hours. 



No great progress was made, or was indeed possible, until the 

 spectroscope was turned upon the prominences in the eclipse of 

 1868, August 18; like that of last year, visible in India. It 

 was at once seen that they gave a spectrum of bright lines the 

 unmistakable sign that they were composed of glowing gas and 

 these lines were so brilliant that M. Janssen found himself able 

 to observe them after the eclipse had passed away. The chief 

 lines were speedily seen to be those of hydrogen gas; others 

 remained unassociated with any known terrestrial element until 

 1895, when Prof. Kamsay discovered " helium," the source of 

 most of them, in the Norwegian mineral " cleveite." 



From the time that Janssen and Lockyer succeeded in observ- 

 ing the prominence lines without an eclipse, and so brought 

 them within the range of daily observation, the interest attach- 

 ing to prominences as parts of the phenomena of an eclipse 

 pa-sed away, and the chief questions which eclipses were looked 

 to to answer were connected with the corona. The answer of 

 the spectroscope as to its character came more slowly than in 

 the case of the prominences, for the corona was a more complex 

 structure and its spectrum showed a corresponding want of 

 simplicity. The coronal light apj>ears in effect to proceed from 

 three sources. Part of it is reflected sunlight, and this shows 

 itself in a feeble reflection of the solar spectrum-^a continuous 

 spectrum, that is to say, crossed by the dark lines, known by the 

 name of their discoverer Fraiinhofer. Part is due to the glow 

 of intensely heated particles of dust or solid matter, and shows 

 a continuous spectrum free from the Fraiinhofer lines. These two 

 sources are naturally not easy to discriminate the one from the 

 other. The third source is glowing gas, giving a sjiectrum of 

 bright lines ; and of these the first discovered and by far the 

 best known is the '* 1474 K " line, in the green, discovered by 

 Prof. Young in the eclipse of 1809, August 7, and so called 

 because he read its }>osit ion' erroneously, as it now appears as 

 1474 on the scale of Kirch hofFs spectroscope. \Ve know as yet 



