1 66 Mr. Schroeter's new Observations 
observation, the arch of a great circle, over which the twilight 
of Venus's atmosphere extends, as far as we can discern it un- 
der favourable circumstances, and which we may put in com- 
parison with our common twilight. 
If this result be compared with that of my older observa- 
tion of the 12th March, 1790, which was 4 0 3 6' 28", it will be 
seen that the two agree much more nearly than, could have been 
expected in such delicate observations , namely, to the very incon- 
siderable difference of one minute; and this is the more striking, 
as, according to the different situations of Venus, and the mo- 
difications of our own atmosphere, this crepuscular light is not 
likely to be ever observed, at different times, exactly of the 
same extent. 
Great, however, as this agreement is, I am far from regard- 
ing it as any thing but a lucky accident. Whoever considers 
the manner of measuring, and the nature of the observed ob- 
ject, will be easily convinced, that we can never determine 
quite exactly the length of the twilight of Venus. The most 
accurate measurements of this kind admit errors of in the 
projected extension ; and this alone would amount nearly 
to in the computed arch of the great circle. Moreover the 
crepuscular light gradually decreases, and I only pretend to 
shew how far it continued visible, in my observation, with my 
eyes and instruments, under the state of the atmospheres of 
Venus and the earth at that time : the part which was thus 
visible to me extended, according to the computation given 
above, over something more than 4 0 \ of a great circle. But 
I am convinced that, under favourable circumstances of the 
weather, situation of Venus, and perfection of instruments, the 
atmosphere of Venus might possibly be traced something fur- 
