slips by, as it were, a little on one side, thus cutting off
from our sight only a portion of his surface. An annular eclipse, on the
other hand, takes place when the moon is indeed centrally interposed,
but falls short of the apparent size required for the entire concealment
of the solar disc, which consequently remains visible as a bright ring
or annulus, even when the obscuration is at its height. In a total
eclipse, on the contrary, the sun completely disappears behind the dark
body of the moon. The difference of the two latter varieties is due to
the fact that the apparent diameter of the sun and moon are so nearly
equal as to gain alternate preponderance one over the other through the
slight periodical changes in their respective distances from the earth.

Now, on the 15th of May, 1836, an annular eclipse was visible in the
northern parts of Great Britain, and was observed by Baily at Inch
Bonney, near Jedburgh. It was here that he saw the phenomenon which
obtained the name of "Baily's Beads," from the notoriety conferred upon
it by his vivid description.

"When the cusps of the sun," he writes, "were about 40 deg. asunder, a
row of lucid points, like a string of bright beads, irregular in size
and distance from each other, _suddenly_ formed round that part of the
circumference of the moon that was about to enter on the sun's disc. Its
formation, indeed, was so rapid that it presented the appearance of
having been caused by the ignition of a fine train of gunpowder.
Finally, as the moon pursued her course, the dark intervening spaces
(which, at their origin, had the appearance of lunar mountains in high
relief, and which still continued attached to the sun's border) were
stretched out into long, black, thick, parallel lines, joining the limbs
of the sun and moon; when all at once they _suddenly_ gave way, and left
the circumference of the sun and moon in those points, as in the rest,
comparatively smooth and circular, and the moon perceptibly advanced on
the face of the sun."[152]

These curious appearances were not an absolute novelty. Weber in 1791,
and Von Zach in 1820, had seen the "beads"; Van Swinden had described
the "belts" or "threads."[153] These last were, moreover (as Baily
clearly perceived), completely analogous to the "black ligament" which
formed so troublesome a feature in the transits of Venus in 1764 and
1769, and which, to the regret and confusion, though no longer to the
surprise of observer