On the 20th August, 1864, Mr. Huggins turned his telescope on to this particular nebula. I am afraid I cannot give you any idea of the delicacy of such observations. Those, however, of my audience who have seen such a planetary nebula through a telescope will know that the light which those bodies give off is less than the light given off by perhaps even the smallest fixed star; and the difficulty of obtaining a spectrum and of examining the nature of this light is therefore exceedingly great.1 What, however, was Mr. Huggins' astonishment,

on bringing the image of this nebula on to the slit of his spectroscope, to observe that he no longer had to do with a class of bodies of the nature of stars!—that instead of having a band of light intersected by dark lines, indicating the physical constitution of the body to be that corresponding to the stars, he found the light from this nebula consisted simply of three isolated bright lines, of which we have here (Fig. 62, and in No. 7 of the Chromolith.) a very rough representation. If the spectrum of

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1 Mr. Lockyer, in his admirable little book on Astronomy, gives an idea of the extreme faintness of the more distant nebulæ. "The light of some of those visible in a moderately large instrument has been estimated to vary from 0 to zoooo of the light of a single sperm candle consuming 158 grains of material per hour, viewed at a distance of a quarter of a mile; that is, such a candle a quarter of a mile off is 20,000 times more brilliant than the nebula !”

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this nebula had been continuous, it would have been very difficult to see it. It was only because the light given off consisted of three bright lines that he was enabled to examine this spectrum at all. You will have already anticipated me in the conclusion that these most curious bodies do not consist of a white-hot nucleus, enveloped in an atmosphere in passing through which the light is absorbed, giving us dark lines, but, on the contrary, that these nebulæ are in the condition of luminous gases, and that it really is nebulous matter with which we have here to do.

The history of these nebulæ is one into which I cannot

enter.

FIG. 63.

You all know that the names of Herschel and of Rosse are associated with the most accurate and careful examination of these particular bodies, and that it is especially to the late Lord Rosse that we are indebted for the very careful examination, by means of his magnificent telescope, of these most singular bodies. It now became a matter of the very greatest interest to examine the character of the light given off by the other nebulæ. I will indicate to you the appearance of some of these nebulæ, though very roughly, by means of the drawings. The nebula in Aquarius is seen in Fig. 63.

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drawing of this nebula gives you but a faint notion of its appearance in the telescope. I may also show you another nebula (Fig 64), having a spiral form, and whose spectrum exhibits a fourth bright line. Mr. Huggins then found, on examining the character of the lines which these nebulæ give off, that the spectrum was likewise distinguished by the same three distinct bright lines. The questions will occur to every one, Do all the nebulæ give similar spectra? and especially, Do those

FIG. 65.

which the telescope had certainly resolved into a close aggregation of bright points give gaseous spectra?

Mr. Huggins has examined the spectra of about seventy nebulæ, and he finds that these can be divided into two great groups. One group (about one-third of the whole number) consists of the nebulæ giving spectra of three bright lines similar to those which I have shown you, or else containing only one or two of these bright lines. "Of these seventy nebulæ, about one-third belong to the class of gaseous bodies: the light of the remaining nebulæ and clusters becomes spread out by the prism

into a spectrum which is apparently continuous." To the class of nebulæ giving continuous spectra the wellknown nebula in Andromeda belongs. This singularly shaped body is visible to the naked eye (Fig. 65), and is not unfrequently mistaken for a comet. It was observed as early as the year 1612, by Simon Marius. The spectrum of this nebula, though apparently continuous, possesses some curious characteristics, the whole of the red and a portion of the orange being wanting, besides the brighter parts exhibiting an unequal and mottled appearance.

It next becomes a most important point to ascertain the chemical nature of the three bright lines in the spectra of the gaseous nebulæ. Mr. Huggins finds that the brightest of the lines of the nebula coincides with the strongest of the lines which are peculiar to nitrogen, whilst the faintest of the lines was found to coincide with the green line (F) of hydrogen. The middle line of the three does not coincide with a line of any known element.

The upper part of this drawing is intended to repre

sent a portion of the solar spectrum. Here you see the dark line F, due to hydrogen, and the lines formed by magnesium, corresponding with the letter b. Below are the lines corresponding with some of the bright lines of hydrogen, barium, nitrogen, and magnesium, whilst between them are the three lines observed in these nebulæ (Fig. 66). Now it may be asked, "How is it, if one of these three lines is due to hydrogen, and another to nitrogen, that the other well-known lines of these elements are not present in the spectra of the nebulæ ? Can we come to the conclusion that nitrogen and hydrogen are contained in the nebula, when we only see two out of the many characteristic lines? Why do not the others appear?" With regard to this point, Mr. Huggins has quite recently shown-and I have to thank him for his kindness in allowing me to see the paper before it was published—that if the intensity of the light coming from glowing nitrogen be diminished to a certain point, only one line is seen, and if you diminish the intensity of the hydrogen spectrum, this one blue line (F) alone becomes visible. We may therefore safely follow in Mr. Huggins' steps, and take all his conclusions as being the result, not only of careful experimentation, but of philosophic caution, for in all these new and difficult subjects that is an absolute necessity. I think we may be well satisfied to adopt his decision, that in fact nitrogen and hydrogen do exist in the nebulæ, and that the cause of the non-appearance of the other lines is simply to be ascribed to the fact which I have already endeavoured to point out to you, that the light coming from these nebula is of such excessively slight intensity. I am almost afraid to take up your time in exhibiting 1 This fact has also been observed by Padre Secchi.

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