324 Mr . Schroeter's Observations 
points, though somewhat more sensibly at the northern than 
at the southern ; and such also were the appearances after the 
inferior conjunction. These appearances will be thus ex- 
plained by the effects of a twilight. The bright prolonga- 
tion of the southern cusp, as it was seen on the 10th and 
12th of March, a , fig. 3, must be ascribed to the solar 
light illuminating a high ridge of mountains situated at this 
region, whence it appears why this prolongation might not 
be strictly spherical. The twilight that must have existed 
at this part, would naturally be eclipsed by the much greater 
brightness of the light immediately derived from the sun, in the 
same manner as, on our earth, mountains that face the rising or 
setting sun, are known to darken the twilight that ought 
faintly to illuminate the regions situated immediately behind 
them. Were this not the case, there is no doubt but that a 
true spherical arch of the limb of the dark hemisphere would 
have appeared faintly illuminated : and such did we see was 
the effect of this twilight produced at both cusps when the 
bright prolongation was not considerable, the appearance be- 
ing most decisive at that cusp where there was least of this 
prolongation. 
Should this pale streak of bluish ash-coloured light actually 
be the effect of a twilight, it will not only prove the existence 
of an atmosphere, but we shall also, from the length of the 
streak, as measured on the 12th of March, 1790, be able to 
draw some inferences concerning its density, and real extent 
on the surface of the planet. 
According to that observation, the apparent diameter of the 
planet measured 60", and the visible stretch of the twilight, 
as far as it could be seen with a power of 95, applied to a 
