A HISTORY OF SCIENCE 



ress; relative brightness becomes absolute lustre, as 

 compared with the sun ; and in the case of the double 

 stars the absolute mass of the components may be com- 

 puted from the laws of gravitation. It is found that 

 stars differ enormously among themselves in all these 

 regards. As to speed, some, like our sun, barely creep 

 through space compassing ten or twenty miles a sec- 

 ond, it is true, yet even at that rate only passing 

 through the equivalent of their own diameter in a day. 

 At the other extreme, among measured stars, is one 

 that moves two hundred miles a second ; yet even this 

 "flying star," as seen from the earth, seems to change 

 its place by only about three and a half lunar diameters 

 in a thousand years. In brightness, some stars yield to 

 the sun, while others surpass him as the arc -light sur- 

 passes a candle. Arcturus, the brightest measured star, 

 shines like two hundred suns; and even this giant orb 

 is dim beside those other stars which are so distant that 

 their parallax cannot be measured, yet which greet our 

 eyes at first magnitude. As to actual bulk, of which 

 apparent lustre furnishes no adequate test, some stars 

 are smaller than the sun, while others exceed him hun- 

 dreds or perhaps thousands of times. Yet one and all, 

 so distant are they, remain mere disklike points of light 

 before the utmost powers of the modern telescope. 



Revelations of the Spectroscope 



All this seems wonderful enough, but even greater 

 things were in store. In 1859 the spectroscope came 

 upon the scene, perfected by Kirchhoff and Bunsen, 

 along lines pointed out by Fraunhofer almost half a 

 century before. That marvellous instrument, by re- 



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