July 1 8, 1872] 



NA TURE 



231 



to "science tliirly-five or forty years ago, form now a part of the 

 physical geography of these regions. \Mth these are associated 

 seme foreign names which, however, are not always so happily 

 applied, very eminent names being, in some instances, given to 

 very insignificant localities. We had twice a beautiful view of 

 Mount Burney ; first coming up through Mayne Channel, where 

 we had an opportunity of seeing the vast difference between its 

 aspect when covered with snow to the very base, as represented 

 by Dr. Cunningham, and as we saw it, with its upper part only 

 shrouded in perpetual snow and ice. It will be long before the 

 real level of perpetual snow is ascertained in these regions, as 

 any boisterous day may change the appearance of a mountain 

 range to an astonishing degree. Tiie mountains to the north of 

 Cape Providence, Mount Burney, the Cordillera of Sarmiento, 

 and the mountain ranges east and north of Snowy Glacier, form 

 part of one and the same chain, and are in reality the southern 

 termination of the Andes. L. Agassiz 



(To be coitliitucJ.) 



ON THE SPECTRUM OF THE GREAT 

 NEBULA IN ORION, AND ON THE MO- 

 TIONS OF SOME STARS TOWARDS OR 

 FROM THE EARTH* 

 T N my early observations of the spectrum presented by the 

 ■*■ gaseous nebula;, the spectroscope with which I determined 

 the coincidence of two of the bright lines respectively with a 

 line of nitrogen and a line of hydrogen, was of insuflicient dis- 

 persive power to show whether the brightest jnebular line was 

 double, as is the case with the corresponding line of nitrogen. 



Subsequently I took some piins to determine this important 

 point by using a spectroscope of greater dispersive power. I 

 found, however, that the light furnished by the telescope of eight 

 inches aperture, to which tlie spectroscope was attached, was too 

 feeble, even in the ca^e of the lorightest nebula:, to give the line 

 with sufficient distinctness when a narrow slit was used. The 

 results of this later examination are given in a paper I had the 

 honour of presenting to the Royal Society in 1S68. Itlure 

 say+ : — 



"I expected that I might discover a duplicity in the line in 

 the nebula corresponding to the two component lines of the line 

 of nitrogen, but I was not able, after long and careful scrutiny, 

 to see the line double. The line in the nebula was narrower 

 than the double line of nitrogen ; this latter line may have ap- 

 peared broader in consequence of irradiation, as it was much 

 brighter than the line in the nebula." When the spark was 

 placed before the object-glass of the telescope, the light was so 

 much weakened that one line only was visible in tlie spectro- 

 scope. "This line was the one wliich agrees in position 

 with the line in the nebula, so that under these circum- 

 stances the spectrum of nitrogen appeared precisely similar to 

 the spectra of those nebula;, of which the light is apparently 

 monochromatic. Tliis resemblance was made more complete 

 by the faintness of the line ; from which cause it appeared 

 narrower, and the separate existence of its two components 

 could no longer be detected. When the line was observed 

 simultaneously with that in the nebula, it was found to appear 

 but a very little broader than that line." I also remark : — " The 

 double line in the nitrogen-spectrum docs not consist of sharply 

 defined lines, but each component is nebulous, and remains of a 

 greater width than the image of the slit. J The breadth of these 

 lines appears to be connected with the conditions of tension and 

 temperature of the gas. I'liicker states that when an induction- 

 spark of great heating-power is employed, the lines expand so 

 as to unire and form r.n undivided band. Even when the dupli- 

 city exists, the eye ceases to have the power to distinguish the 

 component lines, if the intensity of the light be greatly 

 diminished." I state further : — "I incline to the belief that it 

 [the line in the nebula] is not double." 



One of the first investigations which I proposed to myself 

 when, by the kindness of the Royal Society, I had at my com- 

 mand a much more powerful telescope, was the determination of 

 the true character of the bright line in the spectrum of the 

 nebula, which is apparently coincident with that of nitrogen. 

 Prom various circumstances, chitfly connected with the altera- 

 tions and adjustments of new instruments, I was not able to 



♦ By Wii'.iam Huggirs, LL.D., D.C L., F.R S. Paper read before the 

 Royaf Society, Jure 13, 1872. 

 t Pliil. Trans 1868, pp. 542, 543. 

 t Phil. Trans. 1863, p. 13. _. 



accomplish this task satisfactorily until within the last few 

 months. 



Description of Apparatics 



It seems to me desirable to give a description of the spectro- 

 scopic apparatus with which the observations in this paper 

 were made. In the former paper, to which I have already re- 

 ferred, I gave some reasons* to show that the ordinary method 

 of comparison, by reflecting light into the spectroscope by means 

 of a small prism placed before one half of the slit, is not satis- 

 factory for very delicate observations unless certain precautions 

 are taken. I then describe an arrangement for this purpose, 

 which, with one or two modifications, is adopted in the collimator 

 constructed for use with the Royal Society's telescope. I give 

 the description from that paper-f : — 



"The following arrangement for admitting the light from the 

 spark appealed to me to be free from the objections which have 

 been referred to, and to be in all respects adapted to meet the 

 requirements of the case. In place of the small prism, two 

 pieces of silvered glass were securely fixed before the slit at an 

 angle of 45". In a direction at right angles to that of the slit, 

 an opening of about iV inch was left between the pieces of glass 

 for the passage of the pencils from the object-glass. By means 

 of this arrangement, the spectrum of a star is seen accompanied 

 by two spectra of comparison, one appearing above, and the 

 other below it. As the reflecting surfaces are about 05 inch 

 from the slit, and the rays from the spark are divergent, the 

 light reflected from the pieces of glass will have encroached 

 upon the pencils from the object-glass by the time they reach the 

 slit, and the upper and lower spectra of comparison will appear 

 to overlap to a small extent the spectrum formed by the light 

 from the object-glass. This condition of things is of great 

 assistance to the eye in forming a judgment as to the absolute 

 coincidence or otherwise of lines. For the purpose of avoiding 

 some inconveniences which would arise from glass of the 

 ordinary thickness, pieces of the thin glass used for the covers of 

 microscopic objects were carefully selected, and these were 

 silvered by floating them upon the surface of a silvering solution. 

 In order to ensure that the induction-spark should always pre- 

 serve the same position relatively to the mirror, a piece of sheet 

 gutta-percha was fixed above the silvered glass ; in the plate of 

 gutta-percha, at the proper place, a small hole was made of 

 about -nV inch in diameter. The ebonite clamp containing the 

 electrodes is so fixed as to permit the point of separation of 

 these to be adjusted exactly over the small hole in the gutta- 

 percha. The adjustment of the parts of the apparatus was 

 made by closing the end of the adapting-tube, by which the 

 apparatus is attached to the telescope, with a diaphragm with a 

 small central hole, before which a spirit-lamp was placed. When 

 the lines from the induction-spark, in the two spectra of com- 

 parison, were seen to overlap exactly for a short distance the 

 lines of sodium from the light of the lamp, the adjustment was con- 

 sidered perfect. The accuracy of adjustment has been confirmed 

 by the exact coincidence of the three lines of magnesium with 

 the component lines of b in the spectrum of the moon." 



The modifications of this plan consist in the substitution of a 

 thin silver plate polished on both surfaces for the pieces of 

 silvered glass. The opposite side of the silver plate to that from 

 which tlie terrestrial light is reflected to the slit reflects the 

 images formed by the object-glass to the side of the tube where 

 a suitable eye-piece is fixed. I'his arrangement forms a very con- 

 venient finder, for it is easy to cause the image of the star to dis- 

 appear in tlie hole in the silver plate. When this is the case tlie 

 line of light formed by the star falls on the slit, and its spectrum 

 is visible in the spectroscope. This collimator is so constructed 

 that, by means of a coupling screw, any one of three spectro- 

 scopes can be conveniently attached to it. 



This apparatus performs admirably ; but it seemed to me de- 

 sirable, for observations of great delicacy, to be able to dispense 

 with reflection, and to place the source of the light for compari- 

 son directly before the slit. Formerly I accoinplislied this object 

 by placing the spark or vacuum-tube before the object-glass of 

 the telescope. The great length of the present telescope renders 

 this method inconvenient ; but a more important objection arises 

 from the great diminution of tlie light when the spark is re- 

 moved to a distance of 15ft. from the slit. I therefore resolved 

 to place the spark, or vacuum-tube, within the telescope at a 

 moderate distance from the slit. For this purpose holes were 

 drilled in the tube opposite to each other, at a distance of 2ft. 6in. 



» Phil Trans. 1868, pp. 537, 53S. 

 t Phil. Trans. 1868, p. 538. 



