on the Planet Venus. 
171 
sphere there, and our own, are favourable for the purpose : to 
this belong also the shadows sometimes seen by me at the 
southern horn, and which separated the extreme point of it 
wholly, or in part. It is possible likewise, that the atmosphere 
may be clear in one place alone of the boundary of light, in 
which case we should see something of a shadow there only, 
the boundary line appearing in the other parts as usual, not 
streaked with shade, but only not sharply terminated : so, 
for instance, it was on the 31st of July last year, when Dr. 
Olbers observed here with me. 
( b ) But if Venus be considerably more or less than half en- 
lightened, the shadows are not only shorter in themselves, and 
less perceptible in so small an image, but likewise we see them 
obliquely, and in a sensibly longer line through the illumi- 
nated atmosphere of the planet, which then covering the 
shadows more, renders them more difficult to be distinguished, 
and commonly quite invisible. It is, therefore, useless to ex- 
pect such appearances of shadow, in small crescent phases of 
Venus, although she be then vastly nearer, and her apparent 
diameter much larger. If there are at those times real sha- 
dows on her, we see the places, not as spots of shade, but as 
indentations ; and to this belongs the remarkable observa- 
tion, when the boundary arch of light appears irregular, some- 
times in larger and sometimes in smaller parts, and the point 
of one horn, nay even a considerable part of the horn, is 
evidently slenderer than the other. 
Here it will be readily understood, 
(c) That as our own atmosphere has an influence on the 
distinctness of all such phenomena, so accidental condensa- 
tions in the atmosphere of Venus may cause many bright parts. 
Z2 
