430 



NA TURE 



\_Aitgust 29, 1889 



As some importance attaches to the precise character of this 

 line, I wrote to Prof. H. C. Vogel for permission to quote thi 

 result of his experience, which has been nearly as long as my 

 own, of the character of this line. He says in his reply, dated 

 March 20, 1889: " Beeile ich mich Ihnen mitzutheilen, dass 

 tneine langjahrigen Beobachtungen iiber die Spectra der Gas- 

 Nebel voUkommen mit den Ihrigen darin iibereinstimmen, dass 

 die Nebellinie \ 5004 schmal, scharf und nicht verwachsen ist. 

 Auch D'Arrest hat in seiner Untersuchung iiber die Nebel- 

 Spectra (Kopenhagen, 1872) nicht erwahnt dass die hellste 

 Nebellinie unscharf sei." 



Dr. Copeland permits me to quote the following sentences of 

 a letter dated March 19, 1889 : — " Respecting the appearance 

 of the line A. 5034 in the spectrum of the Orion nebula, I may 

 say that I have always drawn and seen it quite sharp and well 

 defined on both edges. About nine years ago I made a special 

 •effort to divide it, if possiblp, with a large spectroscope in which 

 the viewing telescope was 3 inches in aperture. The lines were 

 then seen as sketched." (The diagram shows the nebular lines 

 with sharply ruled lines for edges.) "They were drawn by 

 holding the note-book 10 inches from the left eye, in such a 

 position that the image seen in the instrument with the right 

 eye was apparently projected on the paper. If I had noticed 

 any peculiarity about K 5004, it would certainly have been 

 noted." 1 



In an early observation of the dumb-bell nebula Prof. Vogel, 

 indeed (" Beobachtungen zu Bothkamp," p. 59, 1872), describes 

 this line as less defined towards the violet side. In a letter 

 (April 3, 1889) Prof. Vogel says this appearance of the line was 

 probably due to a slit not sufficiently narrow. He says that he 

 re-examined this line in his observations with the great Vienna 

 refractor, and that it did not then aj^pear otherwise than defined 

 and narrow. 



The other line in the spectrum of the nebulae upon which 

 Mr. Lockyer mainly relies for the presence of magnesium is the 

 line shown in my photographic spectrum of 1882 (Roy. Soc. 

 Proc, vol. xxxiii. p. 425), and to which I assigned the wave-length 

 of about 3730. Mr. Lockyer says of this line (Roy. Soc, 

 Proc, vol. xliii. 1887, p. 122): " In the bunsen as ordinarily 

 employed the fluting at 500 far eclipses the other parts of the 

 spectrum in brilliancy, and at this temperature, as already ob- 

 served by Messrs. Liveing and Dewar, the ultra violet line 

 visible is that at 373-" Passing by a minor point, which Live- 

 ing and Dewar have already pointed out (Roy. Soc. Proc, 

 vol. xliv., 1888, p. 244), namely, that their observation was made 

 at the higher temperature of burning magnesium, this statement 

 is insufficiently complete, for what occurs at this part of the 

 spectrum, and is characteristic of the magnesium-flame spectrum, 

 is a triplet, of which the line given by Liveing and Dewar at 

 about 3730 is the least refrangible member only. 



I have given a representation of this triplet at the wave- 

 lengths given by Liveing and Dewar, namely A. 3730, 3724, 

 and 3720. In the photograph of 1888, in which the strong 

 line can be seen distinct from the lines near it, the line is found 

 to be very near the middle line of the triplet. I have therefore 

 assigned to this line the position of about K 3724. This line 

 appears pretty strong, and therefore if it were really one of the 

 lines of the triplet, the other two members of the triplet should 

 have appeared on the plate. On one side of the s:ar-spectra 

 this hne is a little broader than on the other side, hut as a 

 similar appearance is presented by G, and the stronger of the 

 lines of the group, it may arise from some optical or photo- 

 graphic cause. The line at 3724 impresses me strongly as a 

 single line, and there is certainly no trace of the first line of the 

 triplet at 3730. The line appears to me stronger where it ig 

 upon the star-spectra. 



As therefore there is little doubt that the "remnant of the 

 fluting at 500, which far eclipses the other parts of the spectrum 

 in brilliancy," is not coincident with the brighte?t nebular line, 

 and the next most characteristic group of this spectrum, the 

 triplet at 3722, 3724, and 3730, according to Liveing and Dewar, 

 does not appear to be present in the photographs, we may con- 



' Mr. Taylor, late of the South Kensington Laboratories, observing at Sir 

 Heniy Tno.npson's observatory in November 1 388, says: — "The 5001 liie 

 is by far the brightest in the spectrum. It is never seen sharp, but w.th the 

 narrowest sHt always has a fluffy appearance, this b;ing much more marked 

 on the blue than on the red edge. This hne was most carefully exa nioed 

 for evidence of structure, but was always fjund to be s'ngle, and no de- 

 cided evidence of fluting structure could be made out. It may bi that 

 greater dispersion may show structure, but with the dispersion used here no 

 ttructure could be seen." — Mmthly Notices R. A. S., vol. .x!ix. p. 125. 



elude that the remarkable spectrum of the gaseous nebulae has 

 not been produced by burning magnesium.^ 



I should mention that Mr. Lockyer attributes one other line 

 occasionally seen in the gaseous nebulae to the flame spectrum 

 of magnesium — namely, a very faint line at about \ 4700. 

 Now, according to my experience, it is only in the spark and 

 arc that a line of magnesium appears at this place, a condition of 

 the spectrum when the lines at b are very conspicuous, and the 

 band at \ 5006 "5 is usually absent. When, however, the spark 

 is taken in magnesium chloride, the band is present under some 

 conditions, but ihe triplet at b is always bright. I therefore 

 consulted Prof. Liveing, who says : — " I have never seen the line 

 at A 4703 in the spectrum of the magnesium flame. As it is a 

 conspicuous line in the arc and spark, we looked for it in the 

 flame, but did not find it." 



With reference to the second nebular line at A 4957, Mr. 

 Lockyer says (Roy. Soc. Proc, vol. xliii. p. 133): "The lines 

 at 500 and 495 have been seen in the glow of the Dhurmsala 

 meteorite when heated, but the origin of 495 has not yet been 

 determined." And further (at p. 135) : "I should add that the 

 line at 495 makes its appearance much more rarely than the one 

 at 500 in meteorite glows." In the diagram on the same page 

 this line is represented as coincident with the nebular line. 



The circumstance of a line appearing at 495 can scarcely be 

 regarded, considering the very great number of spectral lines, 

 as amounting to a presumption that the material to which it is 

 due in the meteorite is the same as that present in the nebulae 

 which gives the line at 4957. If it should be shown that the 

 unknown substance in the me'.eorite gives rise to a line at the 

 position of the nebular line — namely, A 4957 — in that case the 

 observation would have sufficient importance to make it desir- 

 able to compare the spectrum of the meteorite directly with that 

 of the nebula. 



Lines Observed and Photographed i7i the Spectrum of the Nebula. 



Line measured by Dr. Copeland, probably ) 



D3 ... j 



Brightest line 



Second line 



Third line, H)3 



Fourth line, Hy 



Line measured by Dr. Copeland 

 Strong line in photographs 1882 and i 



about 



Line in photographs 1888, about 



%i,\ 



K 5874-0 

 5oo4"6 to 5004 "8 



495 7 'o 

 48607 



4340 "I 

 4476 "O 



3724-0 



Photograph 1888 



Line across star spectra, 1st 

 group 



2nd group 



• ... 37090 



M 3699*0 



(about 37520 



t „ 374i'o 



/ M 32850 



\ .. 32750 



,, 3060-0 



/ ,. 30530 



1 „ 30470 



{ ,, 4116 o 



„ 41230 



] ,y 4i3'^'"o 



\ ,, 41420 



1 n 4i54'o 



I M 41670 



/approximate 3998-0 



j „ 3988 o 



) „ 39750 



V ,. 3959 "o 



3893-0 



3887-0 



38780 



38700 



38590 



38540 



1 „ 38480 



3842-0 



3832-0 



\ „ 3825-0 



' On the narrower basis of the magnesium spectrum only, Profs. Liveing 

 and Dewar point out that "the appearance of a line in the p isition of the 

 first band without any trace of the second band, which is nearly a'* bright as 

 the first, and without any trace of the b group, is quite sufficient to create a 

 suspicion of mistaken identity when Mr. Lockyer ascribe^ the sharp green 

 line in the spectrum of nebulae to this band of magnesia. Th.s susoicion will 

 be strengthened when it is noticed that the line in question is usually in 

 nebula; associated with the F line of hydrogen, if it be borna in mind 



3rd group 



