on the Atmosphere of Venus. 323 
contrast with the whitish more vivid light which is seen im- 
mediately on the cusps, as the ash-coloured light reflected 
from our earth on the dark limb of the moon does, when com- 
pared with the solar light on its phase. This pale light 
in the dark hemisphere, it must be owned, faded away to- 
wards its termination, in the same manner as the solar light 
did at the edge of the bright phase: but had this faint streak, 
like the more vivid light, been an immediate emanation from 
the sun, the gradual diminution would have been throughout 
progressive in a continued proportion ; and the light in the 
dark part, immediately contiguous to the points of the cusps, 
must have had nearly the same degree of brightness as the points 
themselves, which was by no means the case. 
Every circumstance, therefore, seems to evince that this 
phenomenon is occasioned by a light reflected by the atmo- 
sphere of Venus into the dark hemisphere of the planet, being 
in some measure the light of the atmosphere itself, when illu- 
minated by the rays of the sun, or, in fact, a real twilight. 
But this will appear still more manifest when 
4. We compare, according to the abovementioned observa- 
tions, the alternate relative appearances of the cusps of Venus 
reciprocally with each other. On the 9th and 12th of March, 
1790, when the southern cusp extended, not in the true sphe- 
rical curve of the limb of the planet, but in a somewhat 
hooked direction, into the dark hemisphere, the pale bluish 
ash-coloured streak appeared only at the point of the northern 
cusp, from whence it proceeded, in a true spherical curve, along 
the dark limb of the planet. On the 10th of March, on the 
other hand, when the southern cusp did not penetrate so far 
into the dark hemisphere, the pale streak was perceived at both 
