54 CO8M.OS. 



Light, from whatever source it comes, whether from the 

 sun, as solar light, or reflected from the planets ; from the 

 fixed stars ; from putrescent wood ; or as the product of the 

 vital activity of glow-worms, always exhibits the same con- 

 sun or moon is seen through a dry and thin cloud, when 

 those bodies likewise appear reddish." This passage has re- 

 cently been pronounced corrupt (see Kramer, in Strabonis Geogr. 

 1844, vol. i. p. 211), and dl vd\a>v (through glass spheres) sub- 

 stituted for K av\u>v (Schneider, Eclog. phys., vol. ii. p. 273). 

 The magnifying power of hollow glass spheres, filled with 

 wr.ter (Seneca, i. 6), was, indeed, as familiar to the ancients 

 as the action of burning glasses or crystals (Aristoph. Nub., 

 v. 765), and that of Nero's emerald (Plin., xxxvii. 5) ; but 

 these spheres most assuredly could not have been employed 

 as astronomical measuring instruments. (Compare Cosmos, 

 vol. i. p. 61 9, and note J.) Solar altitudes, taken through thin 

 light clouds, or through volcanic vapours, exhibit no trace 

 of the influence of refraction. (Humboldt, Recueil d Ob- 

 serv. astr., vol. i. p. 123.) Colonel Baeyer observed no 

 angular deviation in the heliotrope light on the passage of 

 streaks of mist, or even from artificially developed vapours, 

 and therefore fully confirms Arago's experiments. Peters, 

 at Pulkowa, in no case found a difference of 0"'017 on com- 

 paring groups of stellar altitudes, measured in a clear sky, and 

 through light clouds. See his Recherches sur la Parallaxe des 

 Etoiles, 1848, pp. 80, 140-143; also Struve's Etudes Stel- 

 laires, p. 98. On the application of tubes for astronomical 

 observation in Arabian instruments, see Jourdain, Sur I Ob- 

 wrvatoire de Meragha, p. 27 ; and A. Sedillot, Mem. sur les 

 Instruments astronomiques des Arabes, 1841, p. 198. Arabian 

 Astronomers have also the merit of having first employed 

 large gnomons with small circular apertures. In the colossal 

 sextant of Abu Mohammed al-Chokandi,,the limb, which was 

 divided into intervals of five minutes, received the image of the 

 sun. " A midi les rayons du soleil passaient par une ouver- 

 ture pratique dans la voute de 1'observatoire qui couvrait 1' in- 

 strument, suivant le tuyau. et formaient sur la concavite du 

 sextant une image cii 3ulaire, dont le centre donnait, sur 1'arc 

 gradue, le complemer t de la hauteur du soleil. Get instru- 



