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tram of the umbra. The line nearly central between the two lines may be due, in part at least, also to sodium*. These lines appeared slightly broader, as if by the addition of a faint and narrowed nebulosity at both edges.
The group of lines at B are stronger, also b and E. Many of the lines marked in Kirchhoff’s map as coincident with iron appeared much stronger in the spectrum of the spot.
The absence of sensible increase in F was marked in comparison with a line or lines which were very strong, situated at a little distance on the less refrangible side of F— probably those of 2066-2 and 2067'1 of Kirchhoff’s map.
It may be well to consider some of the conditions of the solar surface by which the phenomena observed may have been produced. A cooler state of the heated vapours by which the lines of absorption are produced, would diminish the radiation from the gas itself, and'so leave more completely uncompensated the absorption by the gas of the light from behind it. This cause would produce increased blackness of the lines, but would not account for more than a slight apparent increase of breadth. The greater breadth of the lines seems to point rather to a condition of the gases in which their power of absorption embraces for each line an increased range of wave-length. That the power of absorption of gases varies in this respect is shown by the increase of breadth which some of the bright lines of some gases assume under altered conditions of tension and temperature. It will be sufficient to refer to the expansion of the lines of hydrogen as the tension increases. A similar increase in the range of its power of absorption on light passing through it should take place under similar conditions of density and temperature.
The phenomena may point to an increase of density in the vapours existing within the umbra. Such a state of things would necessarily exist at a point somewhat nearer the sun’s centre; but we do not know through how great a depth of gas below the photosphere we receive the light which comes from the umbra. Our views on this point will be connected with the interpretation we give to Mr. Dawes’s discovery of the existence within the umbrae of spots of a still darker part almost wholly devoid of light. Does this nucleus represent a more complete unveiling of the inner part of the sun ? or does it show a still cooler and less luminous part of the down-rushing solar atmosphere'? The latter suggestion, which is in accordance with the explanation of sun-spots proposed by Dr. Balfour Stewart, would seem to connect a lower temperature with the broader lines of absorption.
Some information might be gained if we could view the spectra of the dark pores of the solar surface, an observation which is perhaps not impossible, since these pores are of varying degrees of darkness, and are probably due to conditions of the solar substance
* The spectrum of the sodium at a high temperature is much more complex than was supposed. In addition to the three double lines besides D and a nebulous band, described in my paper “ On the Chemical Elements,” Philosophical Transactions, 1864, p. 147, there is a line nearly central between the lines D. It is possible there may be also other lines in the interval between the strong lines Dx and D2.
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.