CELESTIAL CHEMISTRY. 369' 



one violet : place this vapour between the spectroscope and the 

 white-hot lime, and now you will see that there are two dark 

 lines across the spectrum, one in the blue region, and the other 

 in the violet part, and that they occupy identically the same 

 positions which the bright lines of the indium spectrum occupied 

 before. If the vapour which we place between the white-hot 

 lime and the spectroscope is neither pure sodium nor pure 

 indium, but a mixture of both, we shall find the continuous 

 spectrum crossed by three dark lines, one occupying the position 

 of the sodium line, and the other two the positions of the twO' 

 indium lines. 



It simply comes to this, that by the optical law known as the 

 law of exchanges, every incandescent substance is capable of 

 absorbing just those particular rays of light which it gives off 

 at that temperature. 



What we have just been considering gives us a direct clue 

 to the origin of the dark lines in the sun's spectrum. If the 

 sun were simply a glowing solid or liquid body it would present 

 to our view an absolutely continuous spectrum without any dark 

 lines. The dark lines we see in it are obviously due to vapours 

 interposed between the glowing solid body of the sun and the 

 spectroscope through which we are looking. Where are these 

 vapours ? Where else can they be than in the atmosphere of the 

 sun itself ? In the earth's atmosphere they certainly are not, for 

 the dark lines in the sun's spectrum correspond to the bright 

 lines of such metals as iron, for instance, not only in number, 

 but also in relative width and intensity : the connection is there- 

 fore quite clear. The heat needed to bring iron to the condition 

 of vapour is of course enormously greater than that of the 

 earth's atmosphere. Here then we have a means of analysing" 

 the sun's atmosphere and of ascertaining whereof it consists. 



For examining" the sun's spectrum in great detail a spectros- 

 cope with only one prism will not suffice. The thousands of 

 dark lines in the spectrum are so close together that we require 

 to separate them out more in order to study them more 

 accurately. So the solar beam is made to pass through a whole 

 series of prisms, one after another. This widens out the 

 spectrum very considerably, but has the great disadvantage of 

 causing great loss of light : as much as 99 per cent, of the light 

 is lost in a spectroscope of ten prisms. More recently, there- 

 fore, another means has been devised and has been so success- 

 ful that the scope of our knowledge has been vastly extended. 

 What is called a diffraction grating is used; it consists of a 

 concave reflecting mirror of polished speculum metal, on which 

 are engraved, by means of very delicate machinery, more than 

 40,000 parallel lines per inch. So greatly does this magnify 

 the sun's spectrum that the yellow line of Sodium as we are 

 accustomed to call it, is split up into two distinct lines with 

 about eight or nine fainter lines between them. The spectrum 

 thus produced has been photographed, with the result that still 

 greater detail has been obtained, and over 3,000 dark lines have 

 thus been counted in the spectrum, many of them in a part of 

 the spectrum, beyond the violet, which is wholly invisible to 

 mortal eye. 



