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of the middle band near its less refrangible limit. This part would consist chiefly of light from the bright central spot.
As has been stated, the middle band commences probably with a bright line; for the limit of the band is here abrupt and distinct. On the contrary the exact point of commencement and termination of the other bands could not be observed with certainty.
I could perceive no other bands, nor light of any kind beyond the three bands, in the parts of the spectrum towards the red and the violet.
When the marginal portions of the coma were brought upon the slit, the three bands of light could still be traced. When, however, the spectrum became very faint, it appeared to me to become continuous; but the light was then so very feeble that it could not be traced beyond the three bands towards the violet or the red.
On this evening I observed the spectrum of the comet in a larger spectroscope, which gives a dispersion equal to about five prisms. In this instrument the middle band was well seen. It retained its nebulous, unresolved character, and the abrupt commencement, as if by a bright line, already mentioned, was distinctly seen.
For convenience of comparison, the spectrum of Brorsen’s comet, and that of the gaseous nebulae, have been added to the diagram, fig. 2, Plate XXXIII. The spectrum of Brorsen’s comet consisted of three bright bands and a faint continuous spectrum. These bands appeared, as represented in the diagram, narrower than those of the comet now under examination. It is not possible to say to what extent this circumstance may be due to the much feebler light of this comet. Though the bands of Brorsen’s comet fall within the limits of position occupied by the broad bands of Comet II., they do not correspond to the brightest parts of these bands. In the middle band I suspected two bright lines, which appeared shorter than the band, and may be due to the nucleus. Brorsen’s comet differed from the two small comets which I had previously examined* in the much smaller relative proportion of the light which forms a continuous spectrum. In Brorsen’s comet the bright middle part of the coma seemed to emit light similar to that of the nucleus, in the other comets the coma appeared to give a continuous spectrum. The three comets resembled each other in the circumstance that the light of the central part was emitted by the cometary matter, while the surrounding nebulosity reflected solar light.
It will be seen in the diagram that the bands of Brorsen’s comet, and those of Comet II., occupy positions in the spectrum widely removed from those in which the lines of the nebulae occur. The spectra of the gaseous nebulae consist of true lines, which become narrow as the slit is made narrower.
The following day I carefully considered these observations of the comet with the hope of a possible identification of its spectrum with that of some terrestrial substance. The spectrum of the comet appeared to me to resemble some of the forms of the spectrum of carbon which I had observed and carefully measured in 1864. On comparing the
* Comet I., 1866. Proceedings, vol. xv. p. 5, and Comet 1867, Monthly Notices of Eoyal Astronomical Society, vol. xxvii. p. 288.
spectrum of the comet with the diagrams of these spectra of carbon, I was much interested to perceive that the positions of the bands in the spectrum, as well as their general characters and relative brightness, agreed exactly with the spectrum of carbon when the spark is taken in olefiant gas.
These observations on the spectrum of carbon were undertaken in continuation of my researches “On the Spectra of the Chemical Elements”*. I have not presented them to the Royal Society, as they are not so complete as I hope to make them.
Though the essential features of the spectrum of carbon remained unchanged in all the experiments, certain modifications were observed when the spectrum was obtained under different conditions. One of these modifications, which was referred to in my paper “On the Spectra of the Chemical Elements ”f, maybe mentioned here. One of the strongest of the lines of carbon is a line in the red a little less refrangible than the hydrogen line, which corresponds to Fraunhofer’s C. Now this line is not seen when the carbon is subjected to the induction-spark in the presence of hydrogen. Two of the other modifications of the spectrum of carbon are given in fig. 2. The first spectrum represents the appearance of the spectrum of carbon when the induction-spark, with Leyden jars intercalated, was taken between the points of wires of platinum sealed in glass tubes, and placed almost in contact in olive-oil. In this spectrum are seen the principal strong lines which distinguish carbon. The shading of fine lines which accompanies the strong lines cannot be accurately represented on account of the small size of the diagram. A spectrum essentially the same is produced when the spark is taken in a current of cyanogen. It may be mentioned that when the heating-power of the spark was reduced below a certain limit, though the decomposition of the oil still took place, the carbon was not volatilized, and the spectrum was continuous.
The third spectrum in the diagram represents the modification of this typical spectrum when the induction-spark is taken in a current of olefiant gas. The highly heated vapour of carbon emits light of the same refrangibiHties as in the case of the oil; but the separate strong lines, with a similar power of spark, were no longer to be distinguished. The shading, when the carbon was obtained from olefiant gas, was not composed of numerous fine lines, but appeared as an unresolved nebulous light.
Of course in all these experiments the lines of the other elements present were also seen, but they were known, and could therefore be disregarded.
In the case of the spark in olefiant gas, the three bands in the diagram constitute the whole spectrum, with the exception of a faint band in the more refrangible part of the spectrum.
It was with the spectrum of carbon, as thus obtained, that the spectrum of the comet appeared to agree. It seemed, therefore, to be of much importance that the spectrum of the spark in olefiant gas should be compared directly in the spectroscope with the spectrum of the comet. The comparison of the gas with the comet was made the same evening, June 23.
* Philosophical Transactions, 1864, p. 139. + Ibid. p. 145.