1874.] and its Influence on Lunar Questions. 497 
time of the disappearance of the star, were there no 
atmosphere, is easily calculated; and if, therefore, it is 
found that the observed time of occultation is later than 
the calculated, then a lunar atmosphere—of a density that 
would cause this retardation—appears certain. 
Unfortunately the moon’s diameter is not known with 
exact accuracy for certain, although it is so within very 
small limits; and this somewhat complicates matters. Sir 
George Airy has determined, from transit circle observations, 
what is regarded as a very accurate value for the moon’s 
telescopic semi-diameter. This value has been considered 
very slightly too great, owing to the effect of irradiation at 
the lunar limb caused by the contrast between the bright 
moon and the sky; although a careful examination of the 
observation of the lunar diameter shows that this is ques- 
tionable, and renders it doubtful whether the variation found 
with different instruments is not due solely to the spurious 
telescopic disc. 
Observations made with the Great Equatorial, at Green- 
wich, during the solar eclipses of July, 1860, and December, 
1870, enable this to a great extent to be rectified, by showing 
the minimum value for the lunar semi-diameter to be some 
15’ 34°00’—a value in agreement with the observations of 
Greenwich and Oxford. For it is apparent that during >a 
solar eclipse there is an absolute reversal of the effects of 
irradiation, as they must then—to a much greater extent— 
diminish the apparent diameter of the moon than under 
ordinary conditions they can ever increase it,—as the con- 
trast between the dark moon and the extremely bright disc 
of the sun is vastly greater than the contrast between the 
comparatively feeble light of the moon and the sky. More- 
over, as the point to be determined in observation during a 
solar eclipse is where darkness ceases, we have absolutely 
the fullest action of irradiation, together with the effect of 
a maximum spurious telescopic disc; while under ordinary 
conditions vf observing the moon’s limb with the transit 
circle a smaller spurious disc comes into play, and it is 
questionable whether the effect of irradiation on these ob- 
servations has not been over-estimated. In, therefore, 
observations of a solar eclipse the moon’s apparent diameter 
is reduced to a much greater extent than under ordinary 
conditions it can ever be increased; and we have, accord- 
ingly, the value for the moon’s minimum diameter before 
stated. 
The minimum lunar semi-diameter may be considered as 
15/ 34°00”, although it is possible that the true diameter is 
