[.] PRINCIPLES AND METHODS. 13 



study of these absorption effects is that if we look at a light source 

 competent to give us a continuous spectrum through any of the 

 vapours or gases we have so far considered as producing bright lines, 

 provided the .light source is hotter than the gases or vapours, the par- 

 ticular rays constituting the bright line or discontinuous spectrum of 

 each of the vapours as gases will be cut out from the light of the con- 

 tinuous spectrum. 



Explanation of Absorption. 



While in the giving out of light we are dealing with molecular 

 vibration taking place so energetically as to give rise to luminous 

 radiation ; absorption phenomena afford us evidence of this motion of 

 the molecules when their vibrations are far less violent. The molecules 

 can only vibrate each in its own period, and they will even take up 

 vibrations from light which is passing among them, provided always 

 that the light thus passing among them contains the proper vibrations. 



An illustration from what happens in the case of sound will help to 

 make this clear. If we go into a quiet room where there is a piano, 

 and sing a note and stop suddenly, we find that note echoed back from 

 the piano. If we sing another note, we find that it is also re-echoed 

 from the piano. How is this 1 When we have sung a particular note, 

 we have thrown the air into a particular state of vibration. One wire 

 in the piano was competent to vibrate in harmony with it. It did so, 

 and, vibrating after we had finished, kept on the note. 



This principle may be illustrated in another and very striking 

 mariner by means of two large tuning-forks mounted on sounding-boxes 

 and tuned in exact unison. One of the forks is set in active vibration 

 by means of a fiddle-bow, and then brought near to the other one, the 

 open mouths of the two sounding-boxes being presented to each other 

 to make the effect as great as possible. After a few moments, if the 

 fork originally sounded is damped to stop its sound, it will be found 

 that the other fork has taken up the vibration and is sounding, not so 

 loudly as the original fork was, but still distinctly. If the two forks 

 are not in perfect unison, no amount of bowing of the one will have 

 the slightest effect in producing sound from the other. Again, suppose 

 we have a long room, and a fiddle at one end of it, and that between it 

 and an observer at the other end of the room there is a screen of fiddles, 

 all tuned like the solitary one, we can imagine that in that case the 

 observer would scarcely hear the note produced upon any one of the 

 open strings of the solitary fiddle. Why 1 The reason is that the air- 

 pulses set up by the open string of this fiddle, in tune with all the 

 others, would set all the other similar strings in vibration ; the air pulses 

 set in motion by the vibration of the fiddle cannot set all those strings 



