[6] At the recent meeting of the British Association (Sept. 1853), drawings were exhibited of the same nebulæ, as seen through Lord Rosse's large telescope, and through a telescope of three feet aperture. With the smaller telescopic power, all the characteristic features were lost. The spiral structure (see next Article but one) has been almost entirely brought to light by the large telescope. [7] See monthly Notices of the Royal Astronomical Society, Dec. 13, 1850. [8] The frontispiece to this volume represents two of these Spiral Nebulæ; those denominated 51 Messier, and 99 Messier, as given by Lord Rosse in the _Phil. Trans. for 1850_. The former of these two has a lateral focus, besides the principal focus or pole. [9] I am aware that some astronomers do not consider it as proved that cometary matter is entirely self-luminous. Arago found that the light of a Comet contained a portion of polarized light, thus proving that it had been reflected (_Cosmos_, I. p. 111, and III. p. 566). But I think the opinion that the greater part of the light is self-luminous, like the nebulæ, generally prevails. Any other supposition is scarcely consistent with the rapid changes of brightness which occur in a comet during its motion to and from the Sun. [10] We assume here that the number of revolutions to the centre is greater in proportion as the relative density of the resisting medium is less; which is by no means mechanically true; but the calculation may serve, as we have said, to show the scale of the numbers involved. [11] Humboldt, whom nothing relative to the history of science escapes, quotes from Seneca a passage in which mention is made of a Comet which divided into two parts; and from the Chinese Annals, a notice of three "coupled Comets," which in the year 896 appeared, and described their paths together. _Cosmos_, III. p. 570, and the notes. [12] Laplace has proved that the masses of comets are very small. He reckons their mean mass as very much less than 1-100000th of the Earth's mass. And hence, considering their great size, we see how rare they must be. See _Expos. du Syst. du Monde_. [13] Humboldt repeatedly expresses his conviction that our Solar System contains a greater variety of forms than other systems. (_Cosmos_, III. 373 and 587.) CHAPTER VIII. THE FIXED STARS.