FOOTNOTES:
[1] Herschel, Outl. of Astr. Art. 893.
[2] Herschel, Outl. of Astr. Art. 874, and Plate 11, Fig. 3.
[3] Ibid. Art. 897.
[4] Hersch. 874.
[5] Ibid. 881-8.
[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.