served by the Jesuit Ricciolo, an opponent of
the Copernican theory, on January 9th, 1643. He describes the planet
as ruddy near the sun, yellowish in the middle, and of greenish blue
on the side remote from the sun; while he also noted the bow of light
limiting the dark hemisphere. Scarcely daring to trust his own
eyesight, he ascribed these appearances, although he recorded them, to
illusory reflection in the telescope.

[Illustration: VENUS AT HER GREATEST BRILLIANCY.]

They were again seen, however, by Derham about 1715, and six years
later by Kirch, in Berlin, who has the following entry in his diary
for Saturday, June 29, 1721:--"I found Venus in a region where the sky
was not very clear. The planet was narrow, and I seemed to see its
dark side, though this is almost incredible. The diameter of Venus was
65", and its sickle seemed to tremble in the atmospheric vapors."
Again, on March 8th, 1726, he records a similar observation. "We
observed Venus with the twenty-six foot telescope. I perceived her
dark side, and its edge seemed to describe a smaller circle than that
of the light side, as is the case of the moon." This effect is due to
irradiation, that is to say, to the glare from a bright surface,
giving a deceptive enlargement to its apparent area. He again saw the
dark side of the planet in October, 1759, as did Harding at Goettingen,
with Herschel's ten-foot reflector, on January 24th, 1806. This latter
observer saw it on this occasion stand out against the background of
the sky as of a pale ashen green, while on February 28th following, it
seemed to him of a pale reddish gray, like the color of the eclipsed
moon.

That the latter body should send to us from her nocturnal shadows
sufficient light to be visible is easily explicable, since she is then
flooded with earth-light reflected on her from a surface thirteen and
one-half times greater than her own, and probably casting on her an
illumination transcending our full moonlight in the same proportion.
But the secondary light of Venus admits of no such explanation, since
earth-light on her surface, diminished by 1/12000th part compared to
what it is on that of the moon, would be quite insufficient to render
her visible to our eyes. The phenomenon was therefore adduced as an
argument for the habitability of the planets by Gruithuisen, of the
Munich Observatory, who, writing early in this century, suggested that
the ashen light of Venus might be due to gener