February 23, 1899] 



NATURE 



391 



A SIMPLE SPECTROSCOPE AND ITS 



TEACHINGS."- 



II. 



Series. 



MESSRS. RUNGE AND PASCHEN 2 first showed 

 in 1890 that the spectra of lithium, sodium, and 

 ipotassium were the summation of the spectra of various 



Fig. 7.— Parts of the spectra of (a) bariurji and (b) iron (from a photograph)- I 



"series." Later they have shown that lead and other ' 

 metals and the Cle\ eite yasc- follow suit. 



A " series " of spectral lines may be defined as a 

 sequence of lines the intensity of which decreases with 



1?[ 



Fig. o. — Fluilng of magnesium. 



the wave-length, and the number of vibrations of which 

 may be determined by the formula — 

 -A. + B/«- + C;«\ 



where n is given the integers from three - 



upwards, and the constants .A, B, and C are = — ^^-^^ifT 

 determined for each element separately. I 



The fact that lines must close up to one . . I! 



another, as the violet end of the spectrum 

 is reached, indicates that the character of 

 a "series" is best brought under notice in 

 the ultra-violet end of the spectrum. In 

 the visible part of the spectrum the lines 

 forming "series" are too far apart to be 

 recognised as belonging to series. HH 1 1 



As soon as it becomes apparent that a | 



set of lines in the violet seems to form a 

 series, computation will at once give the 

 hnes that belong to it in the visible part of 

 the spectrum. 



The accompanying diagram (Fig. 10) 

 shows how the apparently irregular lines 

 observed in the spectra of the Cleveite gases 

 can be arranged into the most exquisite 

 order when the six series of lines which build up the 

 spectra are shown separately. 



Some of these series are composed of triplets instead 

 of single lines. 



I wrote thus on this subject in 1879 ■ — 



" I am at present engaged in investigating this ques- 

 tion of rhythm, and 1 have already found that many of 

 the first order lines of iron may probably arise from the 

 superposition or integration of a number of rhythmical 

 triplets. All this goes to show how long the series of 

 simplifications is that we bring about in the case of the 

 so-called elementary bodies by the application of a 

 temperature that we cannot as yet define. 



" Indeed, the more one studies spectra in detail, and 

 especially under varying conditions of temperature which 

 enable us to observe the reversal now of this set of lines, 

 now of that, the more complex becomes the possible 

 origin. Some spectra are full of doublets ; others, again, 

 are full of triplets, the wider member being sometimes on 

 the more, sometimes on the less, refrangible side." ^ 



Mascart- had noted this recurrence of similar features 

 in spectra ten years earlier. 



Discontinuous Spectra ivitk Dark Lines. 

 It is time now to make still another experiment with 

 our needle and prism. 



If we study sunlight — taking care again to 

 shield the prism, by allowing a sunbeam to 

 illuminate the needle, we get a spectrum of 

 a kind differing from those we have seen 

 before, inasmuch as the continuous band of 

 colour is broken, it is full of dark lines ; 

 that is, some of the coloured rays are lack- 

 ing ; and hence images of the needle are 

 not forthcoming in places. The positions 

 of some of the chief dark lines lettered by 

 Fraunhofer are shown in Fig. 11. 

 We now know that this result is produced by what is 

 termed the adsorption of li<^ht. To understand it we 

 have only to look at a candle through glasses of different 

 colours : a blue glass absorbs or stops the blue light, and 

 only the red end of the spectrum remains ; a red glass 

 absorbs or stops the red, and only the blue end remains. 

 In these cases large regions of the spectrum are alter- 

 nately blotted out as differently coloured glasses are used, 

 but the absorption with which we have to do mostly is of 

 a more restricted character ; lines, that is single images 

 of the slit, are in question. 



One of the most important things that has been 

 gathered from the 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 



Fin. 10.— The : 



the Cle 



ntinued from p. ^7^. 



NO. 1530. VOL. 59] 



Al'h. k. Akad. Ulss., Berlin, 



1 Proc. Roy. Soc; vol. xxviii., March 1879. 



- Ill 1869, he wrote as follows : "II semble difficile que ]a reproduction 

 dun pareil phenomene soit un eflfet du hasard : n'est-il pas plus naturel 

 d'admettre que ces groupes de raies semblables sont des harmoniques qui 

 h, la constitution mol^culaire du gaz lumineux? II faudra sans 



