YOL. LXXXII.] PHILOSOPHICAL TRANSACTIONS. 237 



lected having 2 years ago, perceived such an appearance at the outward edge, near 

 the points of the cusps, though I did not then reflect on the cause of it. 



As all the requisite circumstances however do not often coincide, I thought my- 

 self particularly fortunate when, on the 24th of February, I was favoured with 

 a lucky combination of them. Though this be as yet only a single observation 

 of the sort, it is however in every respect so complete, and the inferences it leads 

 to, are, to me at least, so new and interesting, that I cannot withhold it from 

 those liberal minded men, who are zealous in the pursuit of genuine philoso- 

 phical knowledge. 



On the above-mentioned evening, at 5*' 40™, 2 days and 12 hours after the 

 new moon, when in consequence of the libration, the western border of the 

 grey surface of the Mare Crisium was l' 20'' distant from the western limb of the 

 moon, the air being perfectly clear, I prepared my 7 feet reflector, magnifying 

 74 times, in order to observe the first clearing up of the dark hemisphere, which 

 was illuminated only by the light of our earth, and more especially to ascertain 

 whether in fact this hemisphere, which, as is well known, is always somewhat 

 more luminous at the limb than in the middle, would emerge out of our twilight 

 at many parts at once, or first only at the 2 cusps. Both these points appeared 

 now, most distinctly and decidedly, tapering in a very sharp, faint, scarce any 

 where interrupted, prolongation; each of them exhibiting, with the greatest 

 precision, its farthest extremity faintly illuminated by the solar rays, before any 

 part of the dark hemisphere could be distinguished. But this dark hemisphere 

 began soon after to clear up at once at its border, though immediately only at the 

 cusps, where, but more particularly at their points, this border displayed, on both 

 at the same time, a luminous margin, above a minute in breadth, of a very pale 

 grey light, which, compared with that of the farthest extremities of the cusps 

 themselves, was of a very different colour, and relatively as faint as the twilight 

 I discovered on the dark hemisphere of Venus, and that of our own earth, when 

 compared with the light immediately derived from the sun. This light however 

 faded away so gradually towards the east, as to render the border on that side 

 perfectly undefined, the termination losing itself imperceptibly in the colour of 

 the sky. 



I examined this light with all possible care, and found it of the same extent 

 at both points, and fading away at both in the same gradual proportion. But I 

 also, with the same caution, explored whether I could distinguish any part of 

 the limb of the moon farther towards the east; since if this crepuscular light 

 had been the effect of the light reflected from our globe, it would undoubtedly 

 have appeared more sensibly at the parts most remote from the glare of the illu- 

 minated hemisphere. But, with the, greatest exertion of my visual powers, I 

 could not discover any part of the, as yet, wholly darkened hemisphere, except 



