4 The Outline of Science 



laver. Each constituent element in this outer envelope stops its 







.,wn kind of light, that is, the kind of light made by incandescent 

 atoms of the same element in the photosphere. The "stoppages" 

 register themselves in the solar spectrum as dark lines placed 

 exactly where the corresponding bright lines would have been. 

 The explanation once attained, dark lines became as significant 

 as bright lines. The secret of the sun's composition was out. We 

 have found practically every element in the sun that we know to 

 be in the earth. We have identified an element in the sun 

 before we were able to isolate it on the earth. We have been 

 able even to point to the coolest places on the sun, the 

 centres of sun-spots, where alone the temperature seems to 

 have fallen sufficiently low to allow chemical compounds to form. 

 It is thus we have been able to determine what the stars, 

 comets, or nebula? are made of. 



A Unique Discovery 



In 1868 Sir Xorman Lockyer detected a light coming from 

 the prominences of the sun which was not given by any substance 

 known on earth, and attributed this to an unknown gas which he 

 called helium, from the Greek helios, the sun. In 189-1 Sir Wil- 

 liam Ramsay discovered in certain minerals the same gas identi- 

 fied l)ij the spectroscope. We can say, therefore, that this gas was 

 discovered in the sun nearly thirty years before it was found on 

 earth; this discovery of the long-lost heir is as thrilling a chapter 

 in the detective story of science as any in the sensational stories 

 of the day, and makes us feel quite certain that our methods really 

 tell us of what elements sun and stars are built up. The light 

 from the corona of the sun, as we have mentioned indicates a gas 

 still unknown on earth, which has been christened Coronium. 



Measuring the Speed of Light 



Hut this is not all: soon a new use was found for the spectro- 

 cope. We found that we could measure with it the most difficult 



