339 
on the Atmosphere of the Moon. 
On the abovementioned evening, at 5 h 40', two days and 
twelve hours after the new moon, when in consequence of the 
libration, the western border of the grey surface of the Mare 
Crisium was 1' 20" distant from the western limb of the moon, 
the air being perfectly clear, I prepared my seven-feet reflector, 
magnifying seventy-four 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 two cusps. Both these points ap- 
peared 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 ex- 
tremity 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 parti- 
cularly 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 co- 
lour, 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. 
