On Molecular and Microscopic Science, Volume 1 (of 2)

by Mary Somerville

The annular nebula in Lyra, which is probably nearest to the earth, and the dumb-bell nebula, gave a spectrum indicating matter in a gaseous form. The annular nebula appears to be a hollow elliptical ring of nebulous matter of enormous magnitude. The interior opening of the ring is not entirely dark, but filled with a faint hazy light, like fine gauze stretched over a hoop. The dumb-bell nebula in the constellation Vulpecula is like an hour-glass of bright matter surrounded by a thin hazy atmosphere, which gives the whole the form of an oblate spheroid. Both of these nebulæ when viewed with a very high telescopic power seem to consist of minute clustering stars, but the spectra of these two nebulæ have one bright line, the structure of both being of the same gaseous constitution.

The great nebula on the sword handle of Orion was then examined. The spectrum of the light from the brightest parts of this nebula, near the trapezium, was crossed by three bright lines, in all respects similar to those on the spectra of the planetary and other nebulæ. Other portions of the great nebula were then brought successively under examination, but the spectra of the whole of those portions which still were sufficiently bright for this method of observation remained unchanged, and exhibited the three bright lines only. The whole of the great nebula, as far as it lay within the power of Mr. Huggins’ instrument, emits light which is identical in its characters; the light from one part differs from the light from another part in intensity alone. The brighter portions of this nebula have been to a certain extent resolved into stars, by the powerful telescopes of Lord Rosse and Professor Bond, of the United States of America; the whole, or the greater part, of the light from that portion of the nebula must therefore be regarded as the united radiation of numerous stellar points. The spectrum of this radiation being crossed by the three bright lines reveals its gaseous source; Mr. Huggins therefore infers that at least some of these stellar points are merely denser parts of a gaseous matter, and that the nebulæ which he examined are enormous gaseous systems.

The spectrum of the great nebula in Orion was subsequently examined by Padre Secchi. He describes the light of the spectrum as of a uniform green, crossed by three bright lines; one tolerably wide and perfectly sharp, a very slender one close to it, and the third at a little distance from the latter. This spectrum afforded a striking contrast to the spectra of the small stars in the brighter parts of the nebula. As soon as the light from one of these stars entered the slit of the instrument, its continuous spectrum was seen to flash across the field of vision in a long coloured band. This shows that the mass of matter in this immense nebula is in a different state from that of the stars themselves, as Mr. Huggins had already observed. Padre Secchi does not draw any inference from his observations as to the structure of nebulæ in general, probably thinking it premature, but he expresses astonishment at their results.

Since the preceding lines were written, Mr. Huggins and Professor W. A. Miller have continued their researches on the constitution of the celestial bodies by a method of direct simultaneous comparison of the lines in their spectra with the lines in the spectra of many of the terrestrial elements. The spectra for comparison were obtained from the spark of the induction coil taken between points of various metals; and sometimes a platinum wire was used, surrounded with cotton, moistened with a solution of the substance required. The telescope of the instrument was mounted equatorially, and followed the star by clockwork. By this arrangement the spectrum of the star, and the spectrum of the metal compared with it, are seen in juxtaposition; and the coincidence or relative position of a dark line in the stellar spectrum with a bright line in the metallic spectrum can be determined with great precision.

It was found that Jupiter’s atmosphere has a much greater absorptive power than the terrestrial atmosphere; that they have some gases or vapours in common, but that they are not identical.

Some of the lines seen in the atmosphere of Saturn appear to be identical with those seen in the spectrum of Jupiter.

‘The lines characterizing the atmospheres of Jupiter and Saturn are not present in the spectrum of Mars. Groups of lines appear in the blue portion of the spectrum; and these, by causing the predominance of the red rays, may be the cause of the red colour which distinguishes the light of this planet.’[^[25]]

All the stronger lines of the solar spectrum were seen in the brilliant light of Venus; but no additional lines indicating an absorptive action of the planet’s atmosphere.

The authors are of the opinion that in most of the planets the light is probably reflected from clouds floating at some distance from the surface, so that it is not subject to the strong absorptive action of the lower and denser strata of the planet’s atmosphere, which, like our own, are most effective in producing atmospheric lines.

The results of the observations on the fixed stars are exceedingly interesting, for they show that their elementary constituents are similar, but not identical; and that although they contain many of the sixty-five terrestrial elements, there are probably new unknown substances also.