ntre, they invariably show as bright
spots, then usually vanish as the background gains lustre, to reappear,
after crossing the disc, thrown into relief, as before, against the
dusky limb. But instances are not rare, more especially of the third and
fourth satellites standing out, during the entire middle part of their
course, in such inky darkness as to be mistaken for their own shadows.
The earliest witness of a "black transit" was Cassini, September 2,
1665; Roemer in 1677, and Maraldi in 1707 and 1713, made similar
observations, which have been multiplied in recent years. In some cases
the process of darkening has been visibly attended by the formation, or
emergence into view, of spots on the transiting body, as noted by the
two Bonds at Harvard, March 18, 1848.[1054] The third satellite was seen
by Dawes, half dark, half bright, when crossing Jupiter's disc, August
21, 1867;[1055] one-third dark by Davidson of California, January 15,
1884, under the same circumstances;[1056] and unmistakably spotted, both
on and off the planet, by Schroeter, Secchi, Dawes, and Lassell.

The first satellite sometimes looks dusky, but never absolutely black,
in travelling over the disc of Jupiter. The second appears uniformly
white--a circumstance attributed by Dr. Spitta[1057] to its high albedo.
The singularly different aspects, even during successive transits, of
the third and fourth satellites, are connected by Professor Holden[1058]
with the varied luminosity of the segments of the planetary surface they
are projected upon, and W. H. Pickering inclines to the same opinion;
but fluctuations in their own brightness[1059] may be a concurrent
cause. Herschel concluded in 1797 that, like our moon, they always turn
the same face towards their primary, and as regards the outer satellite,
Engelmann's researches in 1871, and C. E. Burton's in 1873, made this
almost certain; while both for the third and fourth Jovian moons it was
completely assured by W. H. Pickering's and A. E. Douglass's
observations at Arequipa in 1892,[1060] and at Flagstaff in
1894-95.[1061] Strangely enough, however, the interior members of the
system have preserved a relatively swift rotation, notwithstanding the
enormous checking influence upon it of Jove-raised tides.

All the satellites are stated, on good authority, to be more or less
egg-shaped. On September 8, 1890, Barnard saw the first elongated and
bisected by a bright equatorial belt, during one of its da