ON THE ORIGIN OF THE PLANETARY SYSTEM. 151 



can only be seen by the most perfect instruments. 

 The mass of Sirius is found to be 13-76, and that of 

 its satellite 6'71, times the mass of the sun; their 

 mutual distance is equal to thirty-seven times the 

 radius of the earth's orbit, and is therefore somewhat 

 larger than the distance of Neptune from the sun. 



Another fixed star, Procyon, is in the same case as 

 Sirius, but its satellite has not yet been discovered. 



You thus see that in gravitation we have dis- 

 covered a property common to all matter, which is not 

 confined to bodies in our system, but extends, as far in 

 the celestial space, as our means of observation have 

 hitherto been able to penetrate. 



But not merely is this universal property of all 

 mass shared by the most distant celestial bodies, as 

 well as by terrestrial ones ; but spectrum analysis has 

 taught us that a number of well-known terrestrial 

 elements are met with in the atmospheres of the 

 fixed stars, and even of the nebulae. 



You all know that a fine bright line of light, seen 

 through a glass prism, appears as a coloured band, red 

 and yellow at one edge, blue and violet at the other, 

 and green in the middle. Such a coloured image is 

 called a spectrum the rainbow is such a one, produced 

 by the refraction of light, though not exactly by a 

 prism ; and it exhibits therefore the series of colours 

 into which white sunlight can thus be decomposed. 



