450 Unusual AppearuNce in the Sky. 



with the fact of two bows, not concentric, having been seen 

 by the same person at one time ? 



A very unusual Appearance in the Sky, (III. 199.) — The 

 Rev. W. B. Clarke will find an explanation of this appear- 

 ance in Sir D. Brewster's treatise on Optics, in Lardner's 

 Cahiiiet Cyclopcedia, p. 277 — 279. It is not very uncommon : 

 I have myself seen it several times towards sunset, and once 

 very distinctly ; on which occasion the eastern horizon was 

 more cloudy than the rest of the heavens. " This," says Sir 

 David, " seems to be necessary as a ground for rendering 

 visible such feeble radiations." Speaking of an instance 

 observed by himself, he adds : — "A few minutes after the 

 phenomenon was first seen, the converging lines were black, or 

 very dark ; an effect which seems to have arisen from the lu- 

 minous beams having become broad and of unequal intensity: 

 so that the eye took up, as it were, the dark spaces between 

 the beams more readily than the luminous beams them- 

 selves." On this I may observe, that an additional reason for 

 the dark spaces attracting the eye is, that the light of the con- 

 verging beams little exceeds, and, near the horizon, rather falls 

 short of, the average brightness of the rest of the sky. 



" The phenomenon is entirely one of perspective." The 

 beams of light arise from a portion of the sun's rays passing 

 through openings in the clouds, while the adjacent portions 

 are obstructed. These rays, practically, may be considered 

 parallel; and, consequently, according to the laws of per- 

 spective, must appear, to a spectator on the earth's surface, 

 to converge towards each extremity ; on the same principle 

 of perspective, clouds, carried along by the wind in lines 

 parallel to its course, seem to diverge from the windward 

 horizon, and to converge towards the leeward. 



The " singular Phenomenon " observed by L. F., and de- 

 scribed in III. 200., is noticed by Sir D. Brewster, in his 

 treatise on Optics, in Lardner*s Cabinet CyclopcEdia, chap. xiv. 

 p. 113., " on the colours of fibres and grooved surfaces." 



Mirage, — When I wrote the article in III. 484 — 486., I 

 had seen no explanation of the phenomenon, except that pro- 

 posed by Y., in III. 200. Since that time, I have seen many 

 notices of it, and several theories; but none of them do I 

 prefer to my own. In the present, I mean to confine myself 

 to the following, by Professor Jameson, in the eighth volume 

 of the Edi7iburgh Cabinet Library^ p. 254, 255. : — 



" Mirage, In viewing distant objects, it often happens, 

 under certain circumstances, that these objects present many 

 images which are straight, oblique, or inverted, and always 

 more or less changed in the contour. It is the appearance 

 of these images, without any visible reflector to produce them, 



