NOTEb. XIX 



Per intervalla ignium, non per ipsos, vides" (vii. 26). Tlie addition is su- 

 perfluous, for we certainly can see through flame if it has not too great a thick- 

 ness ; as shewn by Galileo in the Saggiatore (Lettera a Monsiguor Cesarini, 

 1619). 



( 50 ) p. 96. Bessel, Astr. Nachr. 1836, N. 301, S. 204206. Stru\e, 

 Recueil des Mem. de 1'Acad. de St.-Petersbourg, 1836, p. 140143 ; and 

 Astr. Nachr. 1836, N. 303, S. 238. At Dorpat the star was in conjunction 

 only 2. "2 from the brightest point in the comet. The star remained constantly 

 visible, and its light was not perceptibly weakened, whereas the nucleus of the 

 comet seemed to fade even to extinction before the light of this small star of 

 the ninth or tenth magnitude 



( 51 ) p. 97. Arago's first attempt to analyse the light of comets by polarisa- 

 tion was on the 3d of July, 1819, on the evening of the sudden appearance of 

 the great comet. I was present at the Observatory, and satisfied myself, as did 

 Mathieu and the since deceased astronomer Bouvard, of the dissimilarity in 

 brightness of the images in the polariscope when the instrument received the 

 cometary light. "When it received the light from Capella, which was near 

 the comet and at the same altitude, the images were equal in intensity. "When 

 Halley's comet appeared in 1835, the apparatus was altered, so as to give, ac- 

 cording to Arago's " chromatic polarisation," two images of complementary 

 colours (green and red). Annales de Chimie, vol. xiii. p. 198. An- 

 nuaire, 1832, 216. "On doit conclure," says Arago, "de 1' ensemble de 

 ces observations, que la lumiere de la comete n'etait pas en totalite composee 

 de rayons doue's des proprietes de la lumiere directe, propre ou assimilee : il 

 s'y trouvoit de la lumiere renechie speculairement et polarisee, c'est a dire 

 venant du soleil. On ne peut decider par cette methode, d'une maTiiere 

 absolue, que les cometes brillent seulement d'un eclat d'emprunt. En effet en 

 devenant lumineux par eux-memes, les corps ne perdent pas pour cela la 

 faculte de reflechir des lumieres etrangeres." 



( 52 ) p. 98. Arago, in the Annuaire for 1832, p. 217220. Sir John 

 Herschel's Astronomy, 488. 



( 53 J p. 99. Encke, in the Astr. Nachr. 1843, N. 489, S. 130132. 



( 54 ) p. 100. Laplace, Exp. du Syst. du Monde, p. 216 and 237. 



( 55 ) p. 100. Littrow, Beschreibende Astr. 1833, S. 274. Respecting 

 the comet of short period recently discovered by Faye, of the Paris Ob- 

 servatory, of which the eccentricity is 0'551, its solar distance at its perihelion 

 1'690 and at its aphelion 5*832, see Schuni. Astr. Nachr. 1844, N. 495; 

 see also N. 239 of the Astr. Nachr. 1833, on the supposed identity of the 



