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beneath the photosphere, similar to those which exist in the less normal phenomenon of sun-spots.
Mr. Lockyer’s observations and my own would seem to show that probably no considerable part of the light which emanates from the umbra of a spot is due to luminous gas. It does not appear to me that this negative evidence is of great weight as to the complete absence of light in the umbra from such a source. The luminous gas would almost certainly emit light of the same refrangibility as some of the dark lines of the solar spectrum; and if there existed above the spot the vapours of the same substances in a cooler state, the light might be wholly absorbed, and the feebler emanations of the still luminous but cooler vapours might not do more than render somewhat less intense the dark gaps produced by the vapours in the stronger light of all refrangibilities which is evidently present.
What may be the source of the light which gives the continuous spectrum of the umbra we know not. It is not impossible that the dense and intensely heated gases which probably form the inner substance of the sun, may in some cases emit lines so greatly expanded as to form, when numerous spectra are superposed, a sensibly continuous spectrum. In addition to this consideration, Dr. B. Stewart has suggested that, as gases possess a power of general absorption of light, a heated mass of gas if sufficiently dense to be opake, or partly so, would give a continuous spectrum as well as the spectrum of bright lines peculiar to it. It may be that, notwithstanding the high temperature, some substances may exist in the liquid state in consequence of the pressure produced by the sun’s mass.
§ VI. Observations of Comet II., 1868.
Received July 2, 1868.
On June 13 a comet was discovered by Dr. Winnecke, and also independently the same night by M. Becquet, Assistant Astronomer at the Observatory of Marseilles.
I was prevented by buildings existing near my observatory from making observations of this comet before June 22. On that evening the comet was much brighter than Brorsen’s comet, a description of the spectrum of which I recently presented to the Royal Society*, and it gave a spectrum sufficiently distinct for measurement and comparison with the spectra of terrestrial substances.
Telescopic appearance of the Comet.—A representation of the comet as it appeared on June 22 at 11 p.m. is given in fig. 1, Plate XXXIII. The comet consisted of a nearly circular coma, which became rather suddenly brighter towards the centre, where there was a nearly round spot of light. The diameter of the coma, including the exterior faint nebulosity, was about 6' 20". The tail, which was traced for more than a degree, was sharply defined on the following edge, but faded so gradually away on the opposite side that no limit could be perceived. No connexion was traced between the tail and
* Proceedings of the Eoyal Society, vol. xvi. p. 386.
the brighter central part of the coma. The circular form of the coma was uninterrupted on the side of the tail, which appeared as an extension of the faint nebulosity which formed the extreme margin of the coma.
The bright roundish spot of light in the centre, when examined with eyepieces magnifying from 200 to 600 diameters, presented merely a nebulous light without a defined form.
Spectrum of the Comet.—When a spectroscope furnished with two prisms of 60° was applied to the telescope, the light of the comet was resolved into three very broad bright bands, which are represented in the diagram.
In the two more refrangible of these bands the light was brightest at the less refran" gible end, and gradually diminished towards the other limit of the bands. This gradation of light was not uniform in the middle and brightest band, which continued of nearly equal brilliancy for about one-third of its breadth from the less refrangible end. This band appeared to be commenced at its brightest side by a bright line.
The least refrangible of the three bands did not exhibit a similar marked gradation of brightness. This band, though of nearly uniform brilliancy throughout, was perhaps brightest about the middle of its breadth.
These characters, which are peculiar to the light emitted by the cometary matter, must be distinguished from some appearances which the bands assumed in consequence of the mode of distribution of the light in the coma of the comet. The two more refrangible bands became narrower towards their most refrangible side, as well as diminished in brightness. This appearance was obviously not due to any dissimilarity of the light in the parts of the coma, but to the circumstance that as the light of the coma became brighter towards the centre, it was emitted by a smaller area of the cometary matter. The strong light of the central spot could be traced the whole breadth of the band; but the light surrounding this spot, in proportion as it became fainter and broader, was seen for a shorter distance, so that the light from the faintest parts near the margin of the coma was visible only at the brightest side of the band. Since in the least refrangible band a similar gradation of light did not take place, this band appeared of nearly the same width throughout.
The increasing brightness of the coma up to the brilliant spot in the centre showed itself in this band as a bright axial line fading off gradually in both directions.
On this evening I took repeated measures of the positions of these bands with the micrometer attached to the spectroscope. These measures give the following numbers for the commencement and termination of the three bands on the scale adopted in the diagram.
. (1094 ,(1298 (1589
First band<,, Second band<„ Third band^
(1196 (1440 (1700
I could not resolve the bands into lines. When the slit was made narrow the bands became smaller both in breadth and length, from the invisibility of the fainter portions. I suspected, however, the presence of two or three bright lines in the bright central part