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44 COSMOS.Light, from whatever source it — conies whether from th«sun, as solar light, or reflected from the planets ;from thefixed stars ;from putrescent wood ;or as the product of thevital activity glow-worms— of always exhibits the same conditionsof refraction. =^ But the prismatic spectra yielded bydilTerent som'ces of Hght (as the sun and the lixed stars) ex-1 libit a difierence in the position of the dark lines (raies ditspectre) wliich Wollaston first discovered in 1808, and the positionof wliich was twelve years afterward so accurately determinedby Fraunhofer. Wliile the latter observer counted600 dark lines (breaks or interruptions in the colored spectrum),Sir David Brewster, by his admirable experiments withnitric oxyd, succeeded, in 1833, in counting more than 2000lines. It had been remarked that certain lines failed in thespectrum at some seasons of the year but Sir David Brewsterhas shown that this phenomenon is owing to difterent al-;titudes of the sun, and to the diflerent absorption of the raysof light in their passage through the atmosphere. In the specdaiu,Sur V Ohservatoire de Meragha, p. 27 ; and A. Sedillot,"il/^m. surles Instruments Astronomiqucs des Arahes, 1841, p. 198. Arabian astronomershave also the merit of having firstemployed large gnomons withsmall circular apertures. In the colossal sextant of Abu Mohammedal-Cliokandi, the limb, which was divided into intervals of five minutes,received the image of the sun. "A midi les rayons du soleil passaientpar una ouvertm-e pratique dans la vo(ite de I'observatoire qui couvrait['instrument, suivant le tuyau, et formaient sur la concavite du sextantune image circulaire, dont le centre donnait, sur I'arc gradue, le compleraent de la hauteur du soleil. Cat instrument diffare de notre mural,qu'en ca qu'il etait garni d'un simple tuyau au lieu d'une lunette." "Atnoon, the rays of the sun passed through an opening in the dome of theobservatory, above the instrument, and, folloviring the tube, formed inthe concavity of the sextant a circular image, the center of which markedthe sun's altitude on the graduated limb. This instrument in no waydiffered from our mural circle, excepting that it was furnished with amere tube instead of a telescope."— Sedillot, p. 37, 202, 205. Dioptricrulers (pinnnlce) were used by the Greeks and Arabs in determiningthe moon's diameter, and were constructed in such a manner that thecircular aperture in the moving object diopter was larger than thatof the fixed ocular diopter, and was drawn out until the lunar disk, seenthrough the ocular aperture, completely filled the object apertui'e.—Delambre, Hist, de V Astron. du Moyen Age, p. 201 ;and Sedillot, p. 198.The adjustment of the dioptric rulers of Archimedes, with round apertures3r slits, in which the direction of the shadows of two small cylindersattached to the same index bar was noted, seems to have been originallyintroduced by Hipparchus. (Baily, Hist, de V Astron. Mod., 2ded., 1785, torn, i., p. 480.) Compare also Theon Alexandrin., Bas., 1538,p. 257, 2G2; Les Hypoxxp. de Proclns Diadochus, ed. Halma, 1820, p107, 110 ;and Ptolem. Almag., ed. Halma, torn, i., Par., 1813, p.* Ivii.According to Arago. Sea Moigno, Hvpert. d^Optiquc, Moderne, 1 847o. 153.