Moreover, 2ndly, about a quarter of an hour after the band was first observed, being then on the roof of my house and having a very uninterrupted view of the w^estern horizon, I noticed the for- mation of a small streak of cloud about the same apparent altitude, somewhat to the north of the pyramid of the zodiacal light, and therefore nearer to the place of the sun below the horizon. The direction of this streak was horizontal, not oblique, and its hue black, not white. This cloud enlarged and became projected as a dark space within the zodiacal light, and soon after others of a less defined character formed elsewhere, all, however, without exception, dark instead of luminous.
Srdly. At the rising of the moon, about half-past eight, the light of our band, already probably on the decrease, was almost wholly effaced. On the other hand, by this time numerous lines and cirrous streaks of light cloud which had been for some time in progress of formation, and had been either wholly unseen before or only noticed by their effacing the stars behind them, became illu- minated, and appeared as white streaks and patches, such as are usually observed in moonlight nights.
4thly, and lastly. Although the night was very calm, yet on watching narrowly the motions and changes of these real clouds with respect to the stars, they were perceived to rise very slowly from the west, i. e. in a direction nearly or quite contrary to that of the declining band.
From these united considerations, and from the extreme fixity of the band among the stars, I consider it impossible to regard it as a cloud illuminated by the sun through the medium of atmospheric refraction. The latter reason, too, is equally conclusive against its being classed with ordinary auroral bands and arcs, which, though they keep their position well enough to be regarded as at rest by a careless observer, yet, when compared with stars, are always per- ceived to be drifting, as it were, in some certain direction, or other- wise changing in figure and dimension.
If we look to an origin for this phenomenon beyond our atmo- sphere, we become involved in speculations, which, however inter- esting, it is not the object of this communication to enter into. On the other hand, its purpose will be answered if either it should be the occasion of eliciting corresponding observations of the same, or notices of similar phenomena already observed, or should lead to increased watchfulness on the part of meteorologists to avail themselves of occasions (which perhaps occur oftener than we are aware) of noting anything analogous in future.
I have the honour to remain,
My dear Sir, Your very faithful and obedient Servant,
J. F. W. Herschel.
Saturday, March 18, 1843.
P.S. — There having been no post today, and the above not having been finished in time for despatch last night, an opportunity is af-
