disk. Hence, on ordinary occasions, when she seems very near on a
line with the sun, we see a very small part of the illuminated
hemisphere, which now presents the form of a very thin crescent
like the new moon. And this crescent is supposed to be a little
broader than it would be if only half the planet were illuminated,
and to encircle rather more than half the planet. Now, this is
just the effect that would be produced by an atmosphere refracting
the sun's light around the edge of the illuminated hemisphere.

The difficulty of observations of this kind is such that the
conclusion may be open to doubt. What is seen during transits of
Venus over the sun's disk leads to more certain, but yet very
puzzling, conclusions. The writer will describe what he saw at the
Cape of Good Hope during the transit of December 5, 1882. As the
dark planet impinged on the bright sun, it of course cut out a
round notch from the edge of the sun. At first, when this notch
was small, nothing could be seen of the outline of that part of
the planet which was outside the sun. But when half the planet was
on the sun, the outline of the part still off the sun was marked
by a slender arc of light. A curious fact was that this arc did
not at first span the whole outline of the planet, but only showed
at one or two points. In a few moments another part of the outline
appeared, and then another, until, at last, the arc of light
extended around the complete outline. All this seems to show that
while the planet has an atmosphere, it is not transparent like
ours, but is so filled with mist and clouds that the sun is seen
through it only as if shining in a fog.

Not many years ago the planet Mars, which is the next one outside
of us, was supposed to have a surface like that of our earth. Some