otal solar eclipse at Sohag of a supposed
thickening at the moon's rim, of certain dark lines in the solar
spectrum, is now acknowledged to have been illusory. Moonlight, analysed
with the prism, is found to be pure reflected sunlight, diminished in
_quantity_, owing to the low reflective capability of the lunar surface,
to less than one-fifth its incident intensity, but wholly unmodified in
_quality_.

Nevertheless, the diameter of the moon appeared from the Greenwich
observations discussed by Airy in 1865[921] to be 4" smaller than when
directly measured; and the effect would be explicable by refraction in a
lunar atmosphere 2,000 times thinner than our own at the sea-level. But
the difference was probably illusory. It resulted in part, if not
wholly, from the visual enlargement by irradiation of the bright disc of
the moon. Professor Comstock, employing the 16-inch Clark equatoreal of
the Washburn Observatory, found in 1897 the refractive displacements of
occulted stars so trifling as to preclude the existence of a permanent
lunar atmosphere of much more than 1/5000 the density of the terrestrial
envelope.[922] The possibility, however, was admitted that, on the
illuminated side of the moon, temporary exhalations of aqueous vapour
might arise from ice-strata evaporated by sun-heat. Meantime, some
renewed evidence of actual crepuscular gleams on the moon had been
gathered by MM. Paul and Prosper Henry of the Paris Observatory, as well
as by Mr. W. H. Pickering, in the pure air of Arequipa, at an altitude
of 8,000 feet above the sea.[923] An occultation of Jupiter, too,
observed by him August 12, 1892,[924] was attended with a slight
flattening of the planet's disc through the effect, it was supposed, of
lunar refraction--but of refraction in an atmosphere possessing, at the
most, 1/4000 the density at the sea-level of terrestrial air, and
capable of holding in equilibrium no more than 1/250 of an inch of
mercury. Yet this small barometric value corresponds, Mr. Pickering
remarks, "to a pressure of hundreds of tons per square mile of the lunar
surface." The compression downward of gaseous strata on the moon should,
in any case, proceed very gradually, owing to the slight power of lunar
gravity,[925] and they might hence play an important part in the economy
of our satellite while evading spectroscopic and other tests. Thus--as
Mr. Ranyard remarked[926]--the cliffs and pinnacles of the moon bear
witness, by their unworn