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XIII. Intelligence and Miscellaneous Articles. 



ON THE SPECTRUM OF BRORSEN ; S COMET. 

 BY FATHER SECCHI. 



HP AKING advantage of a few tolerably bright evenings, I have ex- 

 ■*■ amined the prismatic spectrum of Brorsen's comet, which is 

 now tolerably visible. 



The spectrum is not visible enough with compound spectroscopes 

 with a slit, the light being too feeble ; I therefore used a simple 

 spectroscope with direct vision. To fix the position of the lines, I 

 first used the direct image of the comet seen with its spectrum ; but 

 as the light, was still too faint, I determined the relative positions of 

 the lines by comparing them with those of Venus, placing the comet 

 and the planet successively in the same position on the finder. 

 Both methods gave identical results, but the latter gave most light. 



The spectrum of the comet is discontinuous ; it consists at first of 

 feeble light, filling the field of view, in which three zones stand out 

 with sufficient distinctness to appear more expanded than the rest. 

 The brightest is the middle zone, which occupies the green colour, 

 and corresponds to the region comprised between the magnesium (b) 

 and the hydrogen (F), but much nearer the former ; the breadth of 

 this zone is very restricted, and does not exceed one-fifth of the dis- 

 tance of the two lines. When the atmosphere is particularly favour- 

 able, it is almost reduced to a single bright line, about as broad as the 

 nucleus of the comet. Another brilliant zone, though far less intense, 

 is met with in the greenish yellow, in the middle of the distance be- 

 tween the sodium (D) and the magnesium (b). Another band in 

 the red is sometimes met with, but it is fixed with difficulty. The 

 third zone of luminous intensity, about halfway between the two 

 former, is on the side of the blue, at about one-third of the dis- 

 tance from F to G, starting from F. This band is brilliant enough 

 to be well measured, and produces by scintillation the appearance of 

 a line. 



The following is the relative position of these bands referred to the 

 spectrum of Venus in terms of the micrometer : — 



Sodium (D) 5'13 



Comet, first luminous band 5*92 



Magnesium (b) 6*83 



Comet, second, very brilliant band. . 7'07 



Hydrogen (F) , 7*94 



Comet, last, rosy line - 8*52 



Line G 10'87 



These measurements are as exact as the feebleness of the comet's 

 light permits, which is scarcely that of a star of the seventh mag- 

 nitude. 



These observations lead us to interesting results. It seems at 



