“The nature of planetary nebulæ, which has hitherto been involved in much darkness, may now be explained with some degree of satisfaction, since the uniform and very considerable brightness of their apparent disc accords remarkably well with a much condensed, luminous fluid; whereas, to suppose them to consist of clustering stars will not so completely account for the milkiness or soft tint of their light, to produce which it would be required that the condensation of the stars should be carried to an almost inconceivable degree of accumulation. “How far the light that is perpetually emitted from millions of suns may be concerned in this shining fluid, it might be presumptuous to attempt to determine; but notwithstanding the inconceivable subtilty of the particles of light, when the number of the emitting bodies is almost infinitely great, and the time of the continual emission indefinitely long, the quantity of emitted particles may well become adequate to the constitution of a shining fluid or luminous matter, provided a cause can be found that may retain them from _flying off, or reunite them_.”--_Observations on Nebulous Stars_: Philosophical Transactions, vol. lxxxi. A.D. 1791. In addition, the following Memoirs on the same subject, by Sir William Herschel, have been published in the Philosophical Transactions:--_Catalogue of 1000 Nebulæ and Clusters of Stars_, vol. lxxvi.; _Catalogue of another 1000, with remarks on the Heavens_, vol. lxxix.; _Catalogue of 500 more, with remarks as above_, vol. xcii.; _Of such as have a cometary appearance_, vol. ci.; _Of planetary nebulæ_, ibid.; _Of stellar nebulæ_, ibid.; _On the sidereal part of the heavens, and its connection with the nebulous_, vol. civ.; _On the relative distances of clusters of nebulous stars_, vol. cviii. [15] Lord Rosse’s beautiful telescopes have been formed upon principles which appear to embrace the best possible conditions for obtaining a reflecting surface which should reflect the greatest quantity of light, and retain that property little diminished for a length of time. The alloy used for this purpose consists of tin and copper in atomic proportions, namely, one atom of tin to four atoms of copper, or by weight 58·9 to 126·4.--_On the Construction of large Reflecting Telescopes_: by Lord Rosse. Report of the Fourteenth Meeting of the British Association, 1844, p. 79.