SIX LECTURES ON LIGHT. 



43 



O 



numerable dark lines. Four of them were 

 lirst seen by Dr. \Vollaston, but they were 

 afterwards multiplied and me.*sured by Fraun-. 

 hofer with such masterly skill that they are 

 now universally known as Fraunhofer's lines. 

 To give an explanation of 

 these lines was. as I have said, 

 a problem which long chal- 

 lenged the attention of philos- 

 ophers. (The principal lines 

 are lettered according to Fraun- 

 hofer in the annexed sketch of 

 the solar spectrum. A, it may 

 be stated stands near the ex- 

 treme red, and J near the 

 extreme'violet.) 



Now, Kirchhoff had made 

 thoroughly clear to his mind 

 the priric pies which linked to- 

 gether the emission of light 

 and the absorption of light ; 

 he had proved their insepara- 

 bility for each particular kind 

 of light and heat. He had 

 proved, for every specific ray 

 of the spectrum, the doctrine 

 that the body emitting any ray 

 absorbed with special energy a 

 ray of the same refrangibility. 

 Consider, then, the elfect of 

 knouhdge, such as you now 

 possess, upon a mind prepared 

 like that of Kirchhoff. We 

 have seen the incandescent va- 

 pors of metals emitting defi- 

 nite groups of rays ; accord- 

 ing to Kirchhoff 's principle, 

 those vapors, if crossed by solar 

 light, ought to absorb : ays of 

 the same refrangibility as those 

 which they emit. He proved 

 this to be the case ; he was 

 able, by the interposition of a 

 vapor, to cut out of the solar 

 spectrum the band correspond- 

 ing in color to that vapor. 

 Now. the sun possesses a pho- 

 tosphere, or vaporous enve- 

 lope doubtless mixed with vi- 

 olently agitated clouds and 

 Kirchhoff saw that the power- 

 ful rays coming from the solid, or the 

 molten nucleus of the sun, must be inter- 

 cepted by this vapor. One dark band of j 

 Fraunhof r, for example, occurs in the 

 yellow of the spectrum. Sodium vapor h 

 demonstrably competent to produce that dark 

 band ; hence Kirchhoff inferred the exist- 

 ence of sodium-vapor in the atmosphere of 

 ,the sun. In the case of metals, which emit 

 a large number of bands, the absolute coi - 

 c dence of every bright band of the metal 

 with a dark Fraunhofer line, raises to the 

 highest degree of certainty the inference that 

 the metal is present in the atmosphere of the 

 sun. In this way solar- chemistry was found- 

 ei on spectrum analysis. 



FIG. 27. 



But let me not skim, so lightly over this 

 great subject. I have spoken of emis- 

 sion and absorption, and of the link that 

 binds them. Let me endeavor to make plain 

 to you, through the analogy of sound, their 

 physical meaning. I draw a fiddle-bow across 

 this tuning-fork, and it immediately fills the 

 room with a musical sound; this may be re- 

 garded as the radiation or emission of sound 

 from the fork. A few days ago, on sounding 

 this fork, I noticed that, when its vibrations 

 were quenched, the sound seemed to be con- 

 tinued, though more feebly; The sound ap- 

 peared to come from under a distant table, 

 where stood a number of tuning-forks of dif- 

 ferent sizes and rates of vibration. One of 

 these, and one only, had been started by the 

 fork, and it was one whose rate of vibration 

 was the same as that of the fork which 

 started it. This is an instance of the ab- 

 sorption of sound of one fork by -another. 

 Placing two. forks near each other, sweeping 

 the bow over one of them, and then quench- 

 ing the agitated fork, the other continues to 

 sound. Placing a cent-piece on each prong 

 of one of the forks, we destroy its perfect 

 synchronism \\ith the other, and then no com- 

 munication of sound from the one to the other 

 is possible. 



1 will now do with //^vfc/what has been here 

 done with sound. Placing a tin spoon con- 

 taining sodium in a Bunsen's flame, we ob- 

 tain this intensely yellow light, which corn-- 

 sponds in refrangibility with the yellow band 

 of the spectrum. Like our tuning-fork, it 

 emits waves at a special period. I will send 

 the white light from our lamp through that 

 flame, and prove before you that the yellow 

 flame intercepts the yellow of the spectrum 

 S S, Fig. 28; in other words, absorbs wrves 

 of the same period as its own, thus produc- 

 ing, to all intents and purposes, a da:k 

 Fraunhofer's band in the place of the yellow. 

 (A Bunsen's flame contained within the chim- 

 ney C is placed in front of the lamp L. Tha 

 tin spoon with its pellet of sodium is plunged 

 into the flame. Vivid combustion soon seU 

 in, and, when it does, the yellow of the speo 

 trum, at D, is furrowed by a dark band. 

 Withdrawing and introducing the sodium. 

 flame in rapid succession, the sudden disap- 

 pearance a id reappearance of the strip of 

 darkness arc observed). 



Mentally, as well as physically, every age 

 of the world is the outgrowth and offsprin^ 

 of all preceding ages. Science proves itself 

 to be a genuine product of Nature by grow- 

 ing according to this law. We have no so- 

 lution of continuity here. Every great dis- 

 covery has been duly prepared for in two 

 ways: first, by other discoveries which form 

 its prelude; and, secondly, through the 

 sharpening, by exercise, of the intellectual 

 instrument itself. Thus Ptolemy grew ouij 

 of Hipparchus, Copernicus out of both, Kep- 

 ler out of all three, and Newton out of all 

 the four. Newton did not rise suddenly Jrora 



