39^ 



NA TURE 



[February 23, 1899 



gases we have so far considered as producing bright 

 lines ; provided the light source is hotter than the gases 

 or vapours, the particular rays constituting the bright 

 line or discontinuous spectrum of each of the vapours as 

 gases will be cut out from the light of the continuous 

 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 also is re-echoed 

 from the piano. How is this ? 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 



any one of the open strings of the solitary fiddle. Why? 

 The reason is that the air-pulses set up by the open 

 strings of this fiddle, in unison with all iht others, would 

 set all the other open strings in vibration ; the air pulses 

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

 those strings vibrating and still pass on to one's ear at 

 the other end of the room as if nothing had happened 

 to them. 



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 ] [E © ^ © ^^ 



Also suppose that we have in the middle of the room 

 a screen of molecules, say a sodium flame, capable of 

 emitting yellow light, 



V 



What will happen ? Will the light come to our eyes 

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

 not. What, then, will be the difference ? The molecules 

 which vibrate at such a rate that they give out orange 



Fit;. II. — Copy of Fraunhofer's map of the solar spectrum. 



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 manner by means of two large tuning-forks 

 mounted on sounding-bo.\es and tuned in e.xact 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-bo.\es 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 pro- 

 ducing 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, .ill tuned like. the 

 solitary one, we can imagine that in that case "the 

 observer would scarcely hear the note produced upon 



doulc un Krantl nombre d'obscrvations analogues pour dccouvrir la loi qui 

 r^git ces liarmonit|Ues." 



NO. 1530, VOL. 59] 



light, keep for their own purpose— filch, 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 [13 ^ ©\k- 



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 ha\e, in fact, an apparently dark 

 space which may be represented thus ; 



^J ] [§ © Y © lf2 



In the spectroscope we see what would otherwise be 

 a continuous spectrum, with a dark band across the 

 yellow absolutely identical in position with the bright 

 band observed when the molecules of the vapour of 

 which the screen is composed radiatctl light in the first 

 instance. It is not. however, a case of absolute black- 

 ness, or absence of that particular ray, for the molecules 

 are set in vibration by the rays which they absorb, and 



