184 Prof. Challis on a Theory of the Zodiacal Light. 



is vertical, at which times it looks almost like another Milky 

 Way. It is very evidently the zodiacal light. This luminous 

 arch, I know, is not a new discovery ; for Baron Humboldt saw 

 it in the seas off Mexico, and Professor Brorsen has noticed it 

 more fully in Germany. I also had glimpses of it in my late 

 cruise in the eastern seas. Here, however, it is developed with 

 a remarkable degree of distinctness, and I am giving it particular 

 attention." At the time of his writing, his observations at 

 Quito had been of eleven weeks' continuance. He further states 

 that the light of the luminous arch has little reference to change 

 of position as regards the ecliptic, and that " while the zodiacal 

 light over the horizon tapers off in intensity as it ascends up- 

 wardly, the luminous arch, after this other ceases, is uniform 

 quite across the sky." 



In a first letter, contained in vol. xlii. of the Astronomische 

 Nachrichten (col. 219), and dated from Senftenberg, 1855, Nov. 

 13, Prof. Brorsen states that in that place, and also in the Isle of 

 Alsen in the Duchy of Schleswig, he had observed during the 

 two preceding years a luminosity, which he calls a reflexion 

 (Geffenschein) of the zodiacal light ; and he then gives, as follows, 

 the result in substance of the two years' observations. "The 

 reflexion is seen at the times of both equinoxes, but is more per- 

 ceptible in spring than in autumn. The first faint trace may be 

 remarked as early as February, after which, in March, April, and 

 the beginning of May, it increases continually in brightness and 

 extent. The much fainter and smaller autumn reflexion is ap- 

 parent during the months of September, October, and No- 

 vember. Respecting both, it is a fact of which I have convinced 

 myself by repeated observations, that the brightest part lies 

 exactly opposite the sun's place, so that the estimated point of 

 greatest intensity agrees within half a degree with the opposition- 

 point of the sun. Further, the observations indicated that the 

 spring reflexion towards the middle of April connects itself, by 

 a very faint beginning, becoming by degrees brighter, as a 

 scarcely perceptible light streak, with the westerly zodiacal light; 

 while the autumn reflexion in the first half of November appears 

 stretched along the ecliptic to the westerly horizon in a faint, 

 scarcely recognizable girdle of light, which gradually, by in- 

 creasing brightness and more extended base, passes over into the 

 known appearance of the winter-evening zodiacal light. But 

 from this time to the beginning of March, its point remains 

 quite stationary about the position of r x and r 2 Arietis. Both re- 

 flexions can be distinctly seen here at the same time of the year 

 on each evening that the sky is clear and the region of the 

 heavens in which they are situated is moderately high, even after 

 the commencement of moonlight." 



