XXIV NOTES. 



20, and 7 English feet (though I use French measures everywhere else), not 

 only as more convenient, but also because the great labours of the father and 

 son in England and at Feldhausen, at the Cape of Good Hope, have given to 

 the names of these instruments an historical interest. [The French measures 

 are converted into English throughout this translation ; retaining, hovrever, 

 the original measures in addition wherever precision seems important, and 

 there could be room for doubt. But in the cases of the focal length by 

 which the telescopes severally referred to in pp. 59 and 60 of the text are 

 designated, the lengths specified by M. de Humboldt are left unchanged, for 

 reasons similar to those adduced by himself in the case of the Herschelian 

 telescopes. ED.] 



( 121 ) p. 62. Schumacher's Astr. Nachr., No. 371 and 611. Canchoix 

 and Lerebours have also sent out object-glasses of more than 12| (12'61 Eng.) 

 Paris inches, and 23 (24 Eng.) feet focal length. 



( 122 ) p. 63. Strove, Stellarum dupliciumet multiplicium Mensurse micro- 

 metricse, p. 241. 



( 123 ) p. 64. Mr. Airy has recently given a comparative description of the 

 methods of construction of these telescopes, the casting of the mirrors and 

 mixing of the metal, the polishing and the mounting (Abstr. of the Astr. Soc. 

 Vol. ix. No. 5, March 1849). Of the effect of the 6-foot metallic mirror of the 

 Earl of Rosse,it is there said (p. 120) : "The Astronomer-Royal (Mr. Air)') 

 alluded to the impression made by the enormous light of the telescope : partly 

 by the modifications produced in the appearances of nebulse already figured, 

 partly by the great number of stars seen even at a distance from the Milky 

 Way, and partly from the prodigious brilliancy of Saturn. The account given 

 by another astronomer of the appearance of Jupiter was, that it resembled a 

 coach-lamp in the telescope; and this well expresses the blaze of light which 

 is seen in the instrument." Compare also Sir John Herschel, Outlines of 

 Astronomy, 870 : "The sublimity of the spectacle afforded by the magni- 

 ficent reflecting telescope constructed by Lord Rosse of some of the larger 

 globular and other clusters, is declared by all who have witnessed it to be such 

 as no words can express. This telescope has resolved or rendered resolvable 

 multitudes of nebulse which had resisted all inferior powers." 



( 124 ) p. 64. Delambre, Hist, de 1'Astr. moderne, T. ii. p. 255. 



( 125 ) p. 65. Struve, Mens. microm. p. xliv. 



O 25 ) p. 65. Schumacher's Jabrbuch fur 1839, S. 100. 

 ( 127 ) p. 65. "La lumiere atmospherique diffuse ne peut s'expliquer par le 

 reflet des rayons solaires sur la surface de separation des couches de differeute 



