XVI >'OTES. 



notre Mural qu'en ce qn'il etait garni d'un simple tuyau an lieu d'une lunette" 

 Sedillot, p. 37, 202, and 205. Pierced Sight-vanes (Diopters, Pinnule,) 

 were employed by the Greeks and Arabians for the determination of the 

 diameter of the Moon in such manner, that the circular opening in the 

 moveable object-diopter was larger than that of the eye-diopter, which did not 

 move ; and the former was moved until the disc of the Moon seen through the 

 eye-aperture filled up the object-aperture (Delambre, Hist, de 1'Astr. du moyen 

 Age, p. 201 ; Sedillot, p. 198). The sight-vanes, with round or longitudinal 

 openings of Archimedes, who made use of the direction of the shadows of two 

 small cylinders attached to the same alidade, appear to be an arrangement 

 first introduced by Hipparchus (Bailly, Hist, de 1'Astr. mod. 2de edit. 1785, 

 T. i. p. 480). Compare also Theon Alexandrin. Bas. 1538, p. 257 and 262 ; 

 Les Hypotyp. de Proclus Diadochus, ed. Halma, 1820, p. 107 and 110; and 

 Ptolem. Almag. ed. Halma, T. i. Par. 1813, p. Ivii. 



( M ) p. 45. According to Arago. See Moigno, Repert. d'Optique moderne, 

 1847, p. 153. 



(^ p. 46. Respecting the comportment of the dark streaks of the Sun's 

 image in the Daguerreotype, see the Comptes rendus des Seances de P Academic 

 des Sciences, T. xiv. 1842, p. 902904; and T. xvi. 1843, p. 402407. 



( 97 ) p. 47. Kosmos, Bd. ii. S. 370 (English edition, Vol. ii. p. 329). 



(S 8 ) p. 47. For the important distinction of proper and reflected light 

 I may adduce, as an example, Arago's investigation of the light of comets. 

 By the employment of chromatic polarisation, discovered by him in 1811, 

 the production of the complementary colours, red and green, showed that the 

 light of Halley's comet (1835) contained reflected solar light. I was myself 

 present at his earlier attempts to compare, by means of the equal or unequal 

 intensities of the images in the polariscope, the proper light of Capella with 

 the light of the bright comet which emerged suddenly from amidst the rays of 

 the Sun in the beginning of July 1819. Annuaire du Bureau des Long, 

 pour 1836, p. 232 ; Kosmos, Bd. i. S. Ill and 392 (English edition, p. 97 

 and p. xix. Note 51) ; and Bessel, in Schumacher's Jahrbuch fiir 1837, 

 S. 169. 



(99) p> 47._Lettre de M. Arago a M. Alexandre de Humboldt, 1840, 

 p. 37 : " A 1'aide d'un polariscope de mon invention, je reconnus (avant 

 1820), que la lumiere de tous les corps terrestres incandescents, solides ou 

 liquides, est de la lumiere naturelle, tant qu'elle emane du corps sous des 

 incidences perpendiculaires. La lumiere, an contraire, qui sort de la surface 

 incandescente sous un angle aigu, offre des marques manifestes de polarisation. 



