44 COSMOS. 



Light, from whatever source it comes whether from th 

 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- 

 ditions of refraction.* But the prismatic spectra yielded by 

 different sources of light (as the sun and the fixed stars) ex- 

 hibit a difference in the position of the dark lines (raies du 

 spectre) which Wollaston first discovered in 1808, and the po- 

 sition of which was twelve years afterward so accurately de- 

 termined by Fraunhofer. While the latter observer counted 

 600 dark lines (breaks or interruptions in the colored spec- 

 trum), Sir David Brewster, by his admirable experiments with 

 nitric oxyd, succeeded, in 1833, in counting more than 2000 

 lines. It had been remarked that certain lines failed in the 

 spectrum at some seasons of the year ; but Sir David Brew- 

 ster has shown that this phenomenon is owing to different al- 

 titudes of the sun, and to the different absorption of the rays 

 of light in their passage through the atmosphere. In the spec- 

 dam, Sur V Obseroatoire de Meragha, p. 27 ; and A. Sedillot, Mem. sur 

 les Instruments Astronomiques des Arabes, 1841, p. 198. Arabian astron- 

 omers 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 ouverture pratique dans la voflte de 1'observatoire qui couvrait 

 {'instrument, suivaut le tuyau, et formaient sur la concavite du sextant 

 une image circulaire, dont le centre donnait, sur 1'arc gradue, le com 

 plement de la hauteur du soleil. Cet instrument differe de notre mural, 

 qu'en ce qu'il etait garni d'un simple tuyau au lieu d'une lunette." " At 

 noon, the rays of the sun passed through an opening in the dome of the 

 observatory, above the instrument, and, following the tube, formed in 

 the concavity of the sextant a circular image, the center of which marked 

 the sun's altitude on the graduated limb. This instrument in no way 

 differed from our mural circle, excepting that it was furnished with a 

 mere tube instead of a telescope." Sedillot, p. 37, 202, 205. Dioptric 

 rulers (pinnulce) were used by the Greeks and Arabs in determining 

 the moon's diameter, and were constructed in such a manner that the 

 circular aperture in the moving object diopter was larger than that 

 of the fixed ocular diopter, and was drawn out until the lunar disk, seen 

 through the ocular aperture, completely filled the object aperture. 

 Delambre, Hist, de VAstron. du Moyen Age, p. 201 ; and S6dillot, p. 198. 

 The adjustment of the dioptric rulers of Archimedes, with round aper- 

 tures or slits, in which the direction of the shadows of two small cylin- 

 ders attached to the same index bar was noted, seems to have been orig- 

 inally introduced by Hipparchus. (Baily, Hist, de VAstron. Mod., 2d 

 ed., 1785, torn, i., p. 480.) Compare also Theon Alexandria, Bas., 1538, 

 p. 257, 262; Les Hypotyp. de Prochis Diadockus, ed. Halma, 1820, p 

 107, 110 ; and Ptolem. Almag., ed. Halma, torn, i., Par., 1813, p. Ivii. 

 * According to Arago. See Moigno, Rtpert. d'Optique Moderne, 1847 

 p. 153. 



