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basin corresponds — the fork, string, or basin will be put in vibration and 

 will give back the sound. Suppose now that you have a box, open at 

 the two opposite sides, and full of tuning forks corresponding to the 

 same note, and suppose you sound that note : you put all the forks 

 very slightly in vibration. What will be the consequence to a person on 

 the other side of the box 1 This, that it will be difficult for him to 

 hear the sound which you make ; because the vibrations which you 

 produce are used up in passing through the box by setting the forks in 

 motion. Any other note the person on the other side would hear quite 

 well : the one note which he cannot hear, which is in fact absorbed, is 

 the one which the tuning forks would themselves give if made to 

 vibrate. 



Now bear in mind the analogy between sound and light, and you 

 will not have much difficulty in perceiving that a certain kind of light, 

 which belongs to a particular substance, may be in like manner absorbed 

 by havmg to pass through that very substance which would itself emit 

 the kind of light in question. 



If I were to go beyond this rough and most elementary, though highly 

 ingenious explanation, I fear that I should not be able to stop short of 

 a discussion which would be quite unsuitable to the purpose and limits 

 of this address. I shall therefore assume that by means of the scientific 

 apparatus of the spectroscope, it is possible to analyze the light which 

 comes from any luminous source ; the light coming from one source is 

 as clearly distinguished from that coming from another, as the sound of 

 a flute is different from that of a violin ; incandescent hydrogen gas, for 

 example, will have certain unmistakable characteristics, — certain bright 

 lines in its spectrum, as scientific men say— -and if other light is made to 

 pass through this incandescent gas, it will produce an effect upon that 

 light, according to what has been already said, by absorbing those very 

 portions of the light which are characteristic of itself when used as a 

 source of illumination. 



Examining the light which comes from distant bodies, by the method 

 of the nature of which I have endeavoured to give just the slightest 

 indication, we arrive at very remarkable results. The Spectrum 



