282 PHYSICAL SCIENCE 



graphs taken by this method consists in the 

 well-marked differences in the distribution of 

 hydrogen and calcium. The faculae and promi- 

 nences, which stud the solar disc, contain floating 

 clouds of hydrogen, and other clouds of calcium, 

 but these clouds are often separate from each 

 other, and possess distinctive forms which are 

 well shown in Figs. 38 and 39, and can at once 

 be recognised by an accustomed observer as due 

 to hydrogen or calcium respectively. Prominent 

 objects on the sun, such as spots, often show 

 clearly only in one of these two kinds of light, 

 when they are faintly seen or are quite invisible 

 by the other elemental ray. Vast clouds of 

 calcium seem to arise from the neio'hbourhood 

 of sun spots, obscuring the calcium light coming 

 from the regions below, while at the same time 

 the hydrogen light from those regions is able to 

 make good its escape. 



Most of the dark lines of the solar spectrum 

 are probably due to elements known on the earth, 

 some imperfect coincidences being attributed 

 to the difference in physical conditions, which, 

 as we now know, affect the character of the 

 spectral lines. The bright lines of the outer 

 luminous layer or chromosphere, and of its 

 attendant prominences, were first detected during 

 eclipses, though with modern instruments they ' 

 can always be seen at the edge of the sun's disc. 

 A brilliant unknown line in the yellow was in 

 1868 referred by Sir Norman Lockyer to a 

 new element, to which was given the name of 

 helium. In 1895 Sir William Ramsay detected 

 the same spectrum by passing an electric spark 



