' It appears in the telescope as a nearly round nebulosity, in which the light increases rapidly towards the centre, where, on some occasions, I detected, I believe, a small stellar nucleus. Generally, this minute nucleus was not to be distinguished in the bright central part of the comet. The spectrum consists for the most part of three bright bands. The length of the bands in the instrument shows that they are not due alone to the stellar nucleus, but are produced by the light of the brighter portions of the coma. I took some pains to learn the precise character of these luminous bands. When the slit was wide they resembled the expanded lines seen in some gases. As the slit was made narrow the two fainter bands appeared to fade out without becoming more defined. I was unable to resolve them into lines. The middle band, which is so much brighter than the others that it may be considered to represent probably three-fourths, or nearly so; of the whole of the light which we receive from the comet, appears to possess similar characters. In this band, however, I detected occasionally two bright lines which appear to be shorter than the band, and may be due to the nucleus itself. .... Besides these bright bands there was a very faint continuous spectrum.'

Interpreting these observations according to the principles which have been already stated, we deduce the following interesting results.

The nucleus of Brorsen's comet consists of luminous gas. The coma is also gaseous in the neighbourhood of the nucleus, but its outer portions are of a different character and shine by reflecting the solar light. This part of the coma may be either liquid or solid. There is nothing opposed to the supposition that it is of the nature of cloud-that is, that it is produced by the condensation of true vapour into minute liquid globules.

Returning to the consideration of the gaseous part of the comet the question will at once suggest itself what the gases may be which constitute the substance of the nucleus and coma. Here our information is not quite so satisfactory as could be desired.

The brightest band is in the green part of the spectrum, and agrees very nearly with the brightest line in the spectrum of nitrogen. The want of exact agreement prevents us from assuming that nitrogen really exists (in any form) in the substance of the comet. The other lines of the spectrum of nitrogen are not present in the spectrum of the comet: but this peculiarity is not so perplexing as the other, for it is well known that certain lines will disappear from the spectra of hydrogen, nitrogen, and other gases, under particular circumstances of illumination, temperature, and so on.

Nor is the circumstance that there are bands of light instead of well marked lines a peculiarity which need cause perplexity. For under certain circumstances of temperature and pressure, the lines of the spectra of various gases become expanded or diffused until they appear as bands of light.

The two fainter bands are yellow and blue, respectively. They cannot be identified with the lines seen in the spectra of any known terrestrial gases.

Of whatever gases the nucleus is composed it appears that conditions wholly different from any with which we are familiar on earth prevail in this, and doubtless in all other comets. The gases which form the nucleus, though self-luminous, are probably not incandescent. Remembering that comets are luminous when situated far out in space beyond the orbit of our own earth, we are prevented from assuming the existence of an intensity of heat (due to solar action) sufficient to account for their inherent light. And if the light of a comet were due to a state of incandescence in the component gases, there would be a rapid consumption of the substance of the comet, and we should be quite unable to account for the fact that Halley's comet has continued to shine, with no appreciable loss of brilliancy, for upwards of three hundred years. We seem forced therefore to surmise that the gases which form the substance of comets owe their light to a species of phosphorescence which is independent of the comet's temperature, or else to some electrical properties the nature of which it would not be easy to divine.

Our perplexity is increased when we see the gases which form the nuclei assuming either the liquid or the solid form in the outer part of the coma. The change from gaseity to liquidity or solidity is an evidence of loss of heat, whereas one would expect the outer part of the coma, which is exposed to the full intensity of the sun's action, to be the most heated portion of a comet's volume.

None of the comets which have been examined have had a tail, so that we are unable as yet to form any certain opinion respecting the nature of this portion of a comet's volume. It seems almost certain, however, that the tail shines by reflected light, because in every known instance the tail has appeared as an extension from the outer part of the coma, and may therefore be expected to resemble that portion of the comet in its general characteristics.

One of the comets which has been examined with the spectroscope, though it has not a visible tail, has been shown to have an appendage of a very remarkable character, respecting which, also, we have been able to learn several interesting particulars.

In the year 1866 a telescopic comet was discovered by M. Tempel. This was the first comet examined by Dr. Huggins. Its orbit was carefully calculated by the German astronomer Oppolzer, and found to pass very near the orbit of our own earth. Soon after this, Professor Adams calculated the orbit of the November shooting stars; and to the surprise of the astronomical world it was found that these minute bodies travel along the very path in space which had been already assigned to Tempel's comet. We need not here discuss the circumstances of this discovery. Let it suffice to state that all astronomers who are competent to form an opinion on the subject are agreed that the November shooting-stars are certainly due to the existence of a long-extended flight of cosmical bodies travelling in the track of Tempel's comet..