238 PHILOSOPHICAL TRANSACTIONS. [anNO 1792. 



one single speck, being the summit of the mountainous ridge Leibnitz, which 

 was then strongly illuminated by the solar light: and indeed 8 minutes elapsed 

 before the remainder of the limb became visible; when not only separate parts of 

 it, but the whole displayed itself at once. 



This alone gave me certain hopes of an ample recompence, and satisfied me 

 that the principles I had laid down in my Selenotop. Fragm. § 525, seq. con- 

 cerning the atmospheres of the planets, and especially of the moon, are founded 

 on truth. But a similar observation made on the 6th, after 7 o'clock, afforded 

 me several collateral circumstances, which strongly corroborate what I have there 

 advanced on this subject. The whole limb of the dark hemisphere, illuminated 

 only by the reflected light of our globe, appeared now so clear and distinct, that 

 I could very readily discern not only the large, but also the smaller spots, and 

 among these Plato, Aristarchus, Menelaus, Manilius, Copernicus, &c. and even 

 the small speck to the north-west of Aristarchus, marked b, tab. 27, fig. 1, of 

 the fragments. I could apply the usual power magnifying l6l times; and had 

 full leisure, and the means to examine every thing carefully and repeatedly, and 

 to take very accurate measurements. 



Though positively certain of this very remarkable appearance at both cusps, 

 and of its perfect similarity, in all my observations, I could not trace any ves- 

 tige of a like crepuscular light at any other part of the terminating border : nor 

 could I on the very next evening, being the 25th, and also on the 26th of Fe- 

 bruary perceive, even at the cusps, any of the twilight I expected to see there ; 

 the very thin, faint, luminous line which did indeed appear on the 26th, at the 

 southern cusp, being undoubtedly the effect of the immediate solar light, pro- 

 bably illuminating some prominent flat area, as yet situated in the dark 

 hemisphere. 



Thus far the observations: and now for the application of them. 



I need hardly insist on the proofs, that the very faint pyramidal glimmering 

 light, observed on the 24th of February, at the extremities of both cusps, could 

 by no means be the immediate effect of the solar light, all the circumstances of 

 the observations militating uniformly and decidedly against this supposition, 

 which, were it true, would oblige us to admit a most unaccountable diminution 

 of light, and thence also a density of the lunar atmosphere, that ought to exceed 

 even the density of ours ; a flict absolutely contradicted by all the lunar observa- 

 tions hitherto made. This light, indeed, was so very faint, that it disappeared 

 at 7'' 20*", when the moon approached the horizon ; while, on the other hand, 

 Aristarchus, which had no light but what it received from the earth, was still 

 very distinguishable ; and the summit of Leibnitz, (which, though far within 

 the dark hemisphere, was however illuminated by the immediate solar rays) <lis- 

 played a degree of brightness which, though wlien compared with that of the 



