September 1, 1891.] 



KNOWLEDGE. 



169 



In the spectra of the stars of Seochi's first type the K line 

 is generally faint, and it is sometimes entirely absent. 



As early as 1863 Dr. Huggins attempted to photograph 

 the spectrum of Vega, and succeeded in getting an 

 impression of the spectrum, but without any of the lines. 

 In 1872 Dr. Henry Draper, at Dobbs's Ferry, on the 

 Hudson, succeeded in obtaining a photograph of the 

 spectrum of Vega, showing, for the first time, four of 

 its hydrogen lines.* The introduction of more sensitive 

 dry plates in 1870 induced Dr. Huggins to turn again 

 to the photography of Stellar spectra, and ho soon 

 succeeded in obtainmg pictures showing many lines. 



In a paper published in the PhUoxDjihind Tmnsiiction-s 

 for 1880 on " The Photographic Spectra of Stars," he called 

 attention to the arrangement of twelve strong lines in the 

 spectrum of Vega. Eemarking that the group possesses 

 a distinctly symmetrical character, as the refrangibility 

 increases the lines diminish in breadth, and the distance 

 between any two adjacent lines is less ; and he hazarded 

 the suggestion that the lines must be " intimately con- 

 nected with one another and present the spectrum of one 

 substance." 



Two of them may be seen in our reproduction, though 

 they are very faint, and only just visible. There is a spot 

 on the line t, corresponding to a flaw on the negative of 

 the Brothers Henry, which will enable the line t to be 

 easily identified, and the two faint lines will be seen 

 beyond it, making in all sixteen J lines which can be 

 counted on our reproduction. 



All the lines of the group are of the same character, 

 that is to say, they are broad and winged at the edges, 

 and they become less intense and better defined in the 

 order of their refrangibility. At the date of Dr. Huggins' 

 paper in 1880, the lines of the hydrogen spectrum had 

 only been photographed in the laboratory as far as S, and 

 their wave-lengths as given by Dr. H. W. Vogel corresponded 

 within the limits of probable error with the wave-lengths 

 of the. typical lines measured by Dr. Huggins. 



Vogel's numbers. Dr. Huggins' numbers. 



H A 3908 X 3968 



a. 3887 3887-5 



13 3831 3834 



7 3795 8795 



8 3769 3767-5 



a Lyrse. 

 Sirius. 



ilajoris. 

 " Virginis. 

 " Aquilse. 

 a C.vgni. 



Diagram sliowing tlie relative intensity of the lines of the Hydrogen Spectrum in six Stellar Speetra, photographed by Dr. Huggiiis, 

 copied from his'jiaper on the Photographic Spectra of Stars in the Phil. Ti-aiis. for ]SSO. 



In this paper Dr. Huggins designates the nine most 

 refrangible of the twelve lines photographed by him by 

 the Greek letters, a, yS, 7, &c, t being the last one seen on 

 his photographs. He adds : " A cu'cumstance of great 

 importance is the entire absence of any lines in the spec- 

 trum beyond i. The spectrum, which then becomes 

 continuous, is strong, and extends beyond S in the ultra 

 violet. In solar photographs, taken with the same 

 apparatus, the lines in this region are well defined for 

 some distance beyond S, and therefore this abrupt cessation 

 of lines cannot be referred to an instrumental cause." 



Dr. Huggins seems to have thought it strange that the 

 group of lines was not continued further, and that if the 

 lines belonged to a degrading series, and were due to a 

 common physical cause, the group ought not to end 

 abruptly •at i. His suspicion has been confirmed, for the 

 photograph of the ]5rothers Henry, from which the upper- 

 most spectrum on the plate has been copied, shows at 

 least t three more lines on the more refrangible side of t. 



* The photographic record of Dr. Henry Draper, and his father, 

 Dr. J. W. Drajjcr, is vei'y remarkable. The father, nearly half a 

 century a^o, olitiiined (he first |iho(ogra])h of a liunian being, as well 

 as tlic lirst photc)gra|ili of the Solar s|«-ctruni, and the son obtained 

 the first |ihotograph of a ncl>ula (tlie ( )ri.in nebula, which was photo- 

 graphed by hiui on (lie night of the TOth September, 1880). as well 

 as the fir.st photograj)!! of a Stellar spectrum showing lines. 



t Prof. Geo. K. Hale, with whom I have bad the advantage of 

 examiuingtlic|)botograiihsgiven mcbythe Hrolbers Henry. ])ointe(l out 

 four lines beyond Dr. Huggins' line sand tlu>u;;lit tliat he saw a 

 suspicion of a fifth line. 



On the strength of this evidence, Mr. Johnstone Stoney 

 wrote in a note published with Dr. Huggins' paper : — 



" There can remain very little doubt that your typical 

 lines are due to hydrogen. The evidence of their all being 

 members of one physical system is made very plain when 

 their positions are plotted down, for it then becomes con- 

 spicuous that they lie on, or very near a definite cm.-ve, 

 which could not happen by chance. 



" This (question of whether they lie aotuaUy on, or only 

 near a deQnite curve is, if I mistake not, of very great 

 significance in the theory. If they lie mi a curve obeying 

 any exact mathematical law their connection must, I think, 

 be attributed to their corresponding to the ro/isirutiiv partial 

 tones of some vibrating system (like those of an elastic rod 

 or bell, for example). If, on the other hand, they lie near 

 but not on the curve, this circumstance would support the 

 hypothesis that the visible lines are members of harmonic 

 series, most of the members of which are innsible, those 

 only being seen whose positions chance nearly to fulfil a 

 definite condition.'' 



Mr. Jolmstone Stoney converts Dr. Huggins' wave- 

 lengths into wave-frequencies in air, and comes to the con- 

 clusion that assuming that the irregularities in the second 

 differences cannot be referred to errors of observation, the 

 positions of the lines ilo not lie oh but lie near to a dejinite curve. 



t Thi.s includes the 

 spectrum, and the V 

 photograph. 



K line, which does not belong to the hydrogen 

 liiu'. Hbich was not shown in Dr. Huggins' 



