on the Atmosphere of Venus . 325 
seven-feet reflector, was estimated at 8", or eight of those parts 
of which 60 go to the diameter. This gives a circumference 
of 188,4, and the degree will be equal to 0,52 of these parts: 
and taking those 8" as a chord, the arc, h c, fig. 3, over which 
the twilight extends, will measure 15 0 19'; this being the 
portion of a great circle over which, in circumstances as 
favourable as those I was favoured with, the twilight will 
be seen extending over the dark hemisphere of the planet. 
This, and no more, is what I meant to express in one 
of my letters in which I mentioned the phaenomenon in ge- 
neral, without any accurate computation, or particular in- 
duction. 
But this pale luminous arc, h c, fig. 3, as it appears on the 
planet, not when it is at its greatest elongation, when we see 
half its illuminated surface, but at a time when it is approaching 
very near to its inferior conjunction, is in fact only the appa- 
rent, but by no means the real extent of the twilight, in a per- 
pendicular direction east and west from the circle terminating 
the dark and illuminated hemispheres.' 
This real distance must evidently fall much short of the 
length of the abovementioned arc. The following is the 
method I have adopted to deduce the real extent of the twilight 
of Venus, from the apparent one as seen at the points of the 
cusps. 
Let S, fig. 6, represent the sun, A D C B F the orbit of 
Venus, and T the place of the earth relatively to every aspect 
of Venus in its orbit. B will then be the point of inferior 
conjunction, C of the greatest western elongation, D of the 
superior conjunction, A of the greatest eastern elongation, and 
F nearly the point, not far distant from the inferior conjunction, 
mdccxcii. U u 
