on the Atmosphere of Venus. 331 
so that the sun do not rise and set much earlier and later than 
the planet ; and should, at the same time, our own crepuscu- 
lar light be very great ; should the planet, I say, when the 
sun is some degrees below our horizon, have no great altitude, 
it will then be either not seen at all, or only appear in our 
strong twilight at or near the horizon. As, even besides all 
these favourable circumstances, it is moreover requisite that 
the atmosphere be uncommonly clear, it cannot be thought 
extraordinary that this very instructive phsenomenon,on which 
I never reflect without particular satisfaction, should not have 
been seen by me sooner in the long and uninterrupted series 
of observations I have made, but it will rather be allowed to 
have been a most fortunate combination of incidents which 
enabled me to observe it for four days successively. It will 
also appear, 
4. Why this phaenomenon did not shew itself again after 
the conjunction with the same degree of splendour it did be- 
fore ; the northern declination of the planet being then daily 
decreasing, and the length and brightness of our twilight as 
gradually increasing. This will also point out the reason why 
it may not be seen again for some time. The fact, however, 
being now known to exist, there is no doubt but that some 
traces of it may, and I flatter myself will, be perceived, even 
under other perhaps less favourable circumstances. 
This discovery, lastly, throws also some light on the con- 
stitution of the atmosphere of Venus, and the physical con- 
struction of that planet. 
It appears, from the abovementioned observations, that how- 
ever faint the last perceptible degree of evanescent light be 
In this phaenomenon, we do not however, even with the 
