359 
on the Atmosphere of the Moon . 
It was very natural for Jupiter to diminish in brightness, 
when it approached so near to the moon, then almost at its 
full, as to be seen at the same time in the field of the telescope, 
which was, in fact, the circumstance of this observation ; but 
I could not observe any progressive variation of light in the 
eastern and western, equally luminous, disks, proportional to 
their distances from the limb of the moon, much less a real 
indistinctness ; and this neither when the limbs of the two 
planets were nearly in contact, nor when Jupiter was partly, 
or about one half, covered by the moon 
It was a sight truly gratifying to an eye accustomed to the 
light of the moon, or in general to similar observations, to be- 
hold how Jupiter, at its immersion, as well as emersion, being 
half, or more than half, covered by the moon, exhibited its 
belts and other parts, as distinctly, close to the limb of the 
moon, as it does at some distance from it : and had I not al- 
ready succeeded in my numerous observations on the atmo- 
sphere of the moon, and very recently in those which enabled 
me to determine its twilight, I should perhaps have adopted 
the doubts the ancient astronomers entertained concerning the 
existence of a lunar atmosphere; and this the rather, as when 
Jupiter, in its immersion, was so far covered, that the luminous 
spot /, fig. 2, was close to the moon, I could plainly distin- 
guish this spot, although it be in itself by no means very 
perceptible. 
Such, however, must have been the appearances, according 
to my new observations and measurements of the twilight of 
the moon : for if it be proved that the extent of this twilight, 
to where it loses itself in the light reflected from the almost 
wholly illuminated disk of our earth, amounts to no more 
3 A 2 
