34 6 NINETEENTH CENTURY. FT. HI. 



that it is very difficult to analyse their light, have been 

 made to reveal their secrets. So far as is yet known, 

 they are composed chiefly of gas made luminous by 

 electrical disturbance, though some comets at least have 

 probably a metallic core, similar to that of meteorites. 



Travelling Stars studied by the Spectroscope. 

 Nor has the spectroscope merely enabled the chemist to 

 analyse the materials in distant suns, and ascertain their 

 physical and chemical composition. It has also enabled 

 them to measure movements otherwise inappreciable, and 

 actually to calculate the rate at which some stars are 

 travelling towards, and others away from, our earth. It 

 is an old conjecture that a star has a red appearance when 

 it is moving from the earth, or blue if it is moving towards 

 it, but it has been shown that unless this movement is very 

 great, the change of colour would not be perceptible to 

 ordinary observation. The German physicist, Dopier, in 

 1841, was the first to point out that the rate of movement 

 of a star might be measured by its change of colour, 

 because this change depends upon the succession of light- 

 waves upon the eye being more rapid when a star is 

 approaching, and less rapid when it is receding. Such a 

 change, however, can only be rendered perceptible by the 

 spectroscope, and in this way Dr. Huggins measured the 

 movement of Sirius or the dog-star. It must be remem- 

 bered that the position of the lines which form the spectrum 

 of any star depends upon the rapidity of the vibrations of 

 the rays which produce them. Now, if a star be receding 

 from us, it will send fewer vibrations in a second, conse- 

 quently its colour will change, and its lines move towards 

 the red end of the spectrum. It is necessary, therefore, 

 first to identify some of the lines of a star-spectrum with 

 the lines of some substance such as hydrogen or sodium, 



