332 Mr. Schroeter's Observations 
keenest eyesight, aided by the very best telescopes, distinguish 
the final termination of the twilight, but can only ascertain 
the border where its most luminous part begins to fade. If it 
be then mathematically demonstrated that this more luminous 
part, which may in some respects be compared to our own 
twilight, does actually, a few possible but trivial errors in the 
measurement excepted, extend 4 0 38' 30" into the dark hemi- 
sphere, there can be no doubt but that this twilight does, in 
its whole extent, approach nearly to the extent of ours, which 
has been estimated at 18 0 ; and that there is hence a material 
similarity between the two. 
This may be inferred from the uncommon brightness of the 
solar light on this planet, and the comparatively very bright 
appearance of the more luminous measured part of its twilight, 
which also accounts for our seeing this more luminous part so dis- 
tinctly even in the midst of our own twilight, and perhaps points 
out why even the pale bluish-grey light which in this, as well 
as in other proper phases, is seen to terminate the luminous 
part, forming an evanescent margin of 1 or i-j seconds, may af- 
ford some indication of a twilight gradually blended with the 
luminous terminating border. When this twilight was mea- 
sured on the 12th of March, 1790, soon after six P. M. about 
twenty-six minutes after sun-set, the sun was at most only 
4 0 below o.ir horizon, and the angle cTS, fig. 10. Tab. VII. 
could, according to the above calculations, not measure above 
12 0 4 7' 45". 
Hence it appears that, under such circumstances, the eva- 
nescent margin at the termination of the brighter visible part 
of the twilight of Venus, was at least as luminous as our own 
crepuscular atmosphere at that point of the heavens. And it 
