62 THE CHEMISTRY OF THE SUN. [CHAP. 



would have to do to make the sound-waves passing along it 

 audible to the observer, would be locally done, so to speak, 

 upon the screen of fiddles ; the work done would decrease the 

 amplitude of the sound-waves, and the sound would be 

 weakened. 



Now apply this to light. Suppose we have at one end of 

 a room a vivid light-source giving us all possible waves of light 

 from red to violet. This we may represent as before by 



W m V & ':'-'>: ,<r 



Also suppose that we have in the middle of the room a screen 

 of molecules capable of emitting yellow light, 



'-'---. - ': 



What will happen ? Will the light cotne to our eyes exactly 

 as if the molecules were not there ? No ; it will not. There will 

 be a difference. What, then, will be the difference ? 



Experimentally we find that the molecules which give out 

 yellow light, have kept for their own purpose filched, so to 

 speak, from the light passing through them the particular 

 vibrations which they want to carry on their own motions, and 

 we shall have 



w a [g & /'.: 



as a result ; the light comes to us minus the vibrations which 

 have thus been utilised, as we may put it, by the screen of 

 vapour. 



We have, in fact, an apparently dark space which may be 

 represented in this way : 



In the spectroscope we see what would otherwise be a con- 

 tinuous spectrum, with a dark band across the yellow absolutely 



