498 The Lunar Atmosphere (October, 
somewhat larger, as indeed is indicated by the observation 
made during the eclipse of 1870, the most favourable one of 
the two. It remains now to see whether, with this lunar 
semi-diameter, the observed occultation of stars by the 
moon’s dark limb indicate the presence of any lunar atmo- 
sphere; for the occultations at the bright limb are very 
inferior, and the re-appearances at both limbs are almost 
valueless. 
From a large number of observations of occultations of 
stars by the moon—made between 1850 and 1873 when very 
satisfactory lunar places had been obtainable, and observed 
at Greenwich, Oxford, and Cambridge—it has been shown* 
by the author that the lunar semi-diameter derived from the 
occultations at the dark limb of the moon is less than the 
adopted minimum semi-diameter by over two seconds of arc, 
corresponding usually to from five to ten seconds of time. 
The retardation of an occultation being less than twice the 
horizontal refracétion,t this would indicate the existence of 
an atmosphere exerting a horizontal refraction of a little 
over one second of arc; and the possibility of its existence 
appears beyond question, for no other method of detecting 
an atmosphere approaches this in delicacy.t 
The equations on which the physical conditions of the 
moon’s atmosphere depend may at once be employed to find 
the density of a lunar atmosphere whose horizontal refraction 
will be a little greater than one second at the tempera- 
ture of about zero. By putting the density of the moon’s 
atmosphere at about one three-hundredth of the earth’s, it 
will be found to satisfy this condition, keeping the value of f 
moderate. It will also be found that, as the average tem- 
perature of the moon’s bright limb is high, the refra¢tion 
exerted by the atmosphere is only about one-half of that at 
the dark limb; and, if allowance is made for the effects of 
the spurious telescopic disc at whose edge the star will dis- 
appear usually, it will be found that the observations of 
occultations at the bright limb give agreeing results. 
The great difference in the density of atmosphere 
arrived at for somewhat similar retardation of occultation— 
for Sir George Airy has long seen the possibility of an atmo- 
* Monthly Notices of the Royal Astronomical Society, vol. xxxiv., p. 356. 
t+ In the Monthly Notices, vol. xxxiv, p. 20, the late Sir John Herschel’s 
value has been taken, but it has since been found that the above is correc. 
This requires the suppression of the first sentence of the bottom paragraph of 
that page. 
t That further discoveries and observations may possibly modify this con- 
clusion is of course unquestionable, but this constitutes no reason for altering 
any deduction to be drawn from the circumstances as they exist at present. 
