PBISMA.TIC SPECTRA. 55 



ditions of refraction. 6 But the prismatic spectra yielded by 

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

 exhibit a difference in the position of the dark lines (rates du 

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

 tion of which was twelve years afterwards so accurately deter- 

 mined by Fraunhofer. While the latter observer counted 600 

 dark lines (breaks or interruptions in the coloured spectrum), 

 Sir David Brewster, by his admirable experiments with nitric 

 oxide, succeeded, in 1833, in counting more than 2000 lines. 

 It had been remarked that certain lines failed in the spec- 

 trum at some seasons of the year ; but Sir David Brewster 



ment 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 observa- 

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

 the concavity of the sextant a circular image, the centre 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, pp. 37, 202, 205. Dioptric rulers (pin- 

 nules) 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 disc, seen through the ocular aperture, completely 

 filled the object aperture. Delambre, Hist, de VAstron. du 

 moyen age, p. 201; and Sedillot, p. 198. The adjustment of 

 the dioptric rulers of Archimedes, with round apertures or slits, 

 in which the direction of the shadows of two small cylinders 

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

 originally introduced by Hipparchus. (Baily,.foW. de VAstron. 

 mod., 2nd ed. 1785, torn. i. p. 480.) Compare also, Theon 

 Alexandria, Bas., 1538, pp. 257, 262 ; Les Hypotyp. de Proclus 

 Diadochus ed. Halma, 1820, pp. 107, 110; and Ptolem. 

 Almag., ed. Halma, torn. i. Par. 1813, p. Ivii. 



6 According to Arago ; see Moigno, Repert. d' Optique mo- 

 derne, 1847, p. 153. 



