349 
on the Atmosphere of the Moon. 
breadth of the twilight did not lay, like the side/c, along the 
edge of the limb, but nearly at right angles to it, in a direction 
immediately tending towards the eye: whence its orthographic 
projection could by no means amount to 10,5 lines, but must, 
according to well known principles, be less in the proportion 
of the sine of 2 0 34' 12" to the cosine = 10,5, whence, by the 
following computation, 
Log. kc=fg— L. 10,5= L. 2,0211893 
x Log. sin. 2 0 34' 12" - - = L. 8,6516648 
== L. 0,5 
This greatest apparent breadth of the twilight at the cusps 
could not, therefore, exceed 0,5 lines, or full two seconds ; and 
this in fact was the breadth given it by the observation. It is 
scarce possible, that in the determination of so nice an object, 
theory should coincide more accurately with observation. 
3. But it appears farther from these observations, why, not- 
withstanding all the pains I took, I could perceive nothing of 
this twilight at the terminating border of the falcated phase, 
and why, on the succeeding evening, this light disappeared 
likewise at the cusps. The brightness of the phase will, even 
on the first days after the new moon, occasion a strong glare 
in the field of the telescope, and this particularly at the 
spherical border, terminating the light ; so that the margin con- 
tiguous to this border, which is only illuminated by the light 
reflected from the earth, may, indeed, appear luminous, but 
must necessarily be very undefined. The smaller the phase, 
moreover, the less will be the projection of the twilight at 
the terminating border, so that on the third day after the 
new moon it cannot well amount to much more than 2" at 
the cusps ; and it will then be so far obscured by the stronger 
mdccxcii. Z z 
