perfect sharpness.[798]
The presence, however, of a "halo" was unmistakable in 1832, when
Professor Moll, of Utrecht, described it as a "nebulous ring of a darker
tinge approaching to the violet colour."[799] Again, to Huggins and
Stone, November 5, 1868, it showed as lucid and most distinct. No change
in the colour of the glasses used, or the powers applied, could get rid
of it, and it lasted throughout the transit.[800] It was next seen by
Christie and Dunkin at Greenwich, May 6, 1878,[801] and with much
precision of detail by Trouvelot at Cambridge (U.S.).[802] Professor
Holden, on the other hand, noted at Hastings-on-Hudson the total absence
of all anomalous appearances.[803] Nor could any vestige of them be
perceived by Barnard at Lick on November 10, 1894.[804] Various effects
of irradiation and diffraction were, however, observed by Lowell and W.
H. Pickering at Flagstaff;[805] and Davidson was favoured at San
Francisco with glimpses of the historic aureola,[806] as well as of a
central whitish spot, which often accompanies it. That both are somehow
of optical production can scarcely be doubted.

Nothing can be learned from them regarding the planet's physical
condition. Airy showed that refraction in a Mercurian atmosphere could
not possibly originate the noted aureola, which must accordingly be set
down as "strictly an ocular nervous phenomenon."[807] It is the less
easy to escape from this conclusion that we find the virtually airless
moon capable of exhibiting a like appendage. Professor Stephen
Alexander, of the United States Survey, with two other observers,
perceived, during the eclipse of the sun of July 18, 1860, the advancing
lunar limb to be bordered with a bright band;[808] and photographic
effects of the same kind appear in pictures of transits of Venus and
partial solar eclipses.

The spectroscope affords little information as to the constitution of
Mercury. Its light is of course that of the sun reflected, and its
spectrum is consequently a faint echo of the Fraunhofer spectrum. Dr. H.
C. Vogel, who first examined it in April, 1871, _suspected_ traces of
the action of an atmosphere like ours,[809] but, it would seem, on
slight grounds. It is, however, certainly very poor in blue rays. More
definite conclusions were, in 1874,[810] derived by Zoellner from
photometric observations of Mercurian phases. A similar study of the
waxing and waning moon had afforded him the curious discovery that
light-c