343 
on the Atmosphere of the Moon. 
though gradually fading, and very undefined, yet brighter than 
that of our zodiacal light, when, in the months of March and 
April, it blends itself, comparatively with the remaining colour 
of the sky, with the terrestrial light, terminating in a very 
sharp point. 
The undefined and gradually fading appearance of this light 
was the cause that, though I had recourse to a dark projection 
table, I could not, however, take any accurate measurements of 
it. I found, nevertheless, by repeated comparisons, that the 
length of this pyramidal glimmering light, in which I could 
perceive no sensible inequality at the limb of the moon, 
amounted to about y of the distance between the two moun- 
tains c, /, fig. 1, which shone with the solar light. Comparing 
also this southern twilight with the northern, it appeared of 
the same length; and on measuring the distance cl, I found it 
repeatedly = 30 lines = 2' ; so that the length of the twilight 
must have amounted to 20 lines = 1' 20". 
Its greatest breadth at c could, on the other hand, because of 
the extent and greater density of its light, be easily ascertained 
by means of the immediate application of the projection table. 
This measurement gave at most y of a line, or full 2". 
Although I be positively certain of this very remarkable ap- 
pearance at both cusps, and of its perfect similarity, in all my 
observations, I could not, however, trace any vestige of a like 
crepuscular light at any other part of the terminating border : 
nor could I on the very next evening, being the 25th, and also 
on the 26th of February, perceive, even at the cusps, any of 
the twilight I expected to see there; the very thin, faint, lu- 
minous line which did indeed appear on the 26th, at the 
southern cusp between a and b, fig. 3, being undoubtedly the 
Yy 2 
