We do not have to depend upon the spectroscope for evidence that Venus has a dense atmosphere, for we can, in a manner, _see_ her atmosphere, in consequence of its refractive action upon the sunlight that strikes into it near the edge of the planet's globe. This illumination of Venus's atmosphere is witnessed both when she is nearly between the sun and the earth, and when, being exactly between them, she appears in silhouette against the solar disk. During a transit of this kind, in 1882, many observers, and the present writer was one, saw a bright atmospheric bow edging a part of the circumference of Venus when the planet was moving upon the face of the sun--a most beautiful and impressive spectacle.
Even more curious is an observation made in 1866 by Prof. C.S. Lyman, of Yale College, who, when Venus was very near the sun, saw her atmosphere _in the form of a luminous ring_. A little fuller explanation of this appearance may be of interest.
When approaching inferior conjunction--i.e., passing between the earth and sun--Venus appears, with a telescope, in the shape of a very thin crescent. Professor Lyman watched this crescent, becoming narrower day after day as it approached the sun, and noticed that its extremities gradually extended themselves beyond the limits of a semicircle, bending to meet one another on the opposite side of the invisible disk of the planet, until, at length, they did meet, and he beheld a complete ring of silvery light, all that remained visible of the planet Venus! The ring was, of course, the illuminated atmosphere of the planet refracting the sunlight on all sides around the opaque globe.
In 1874 M. Flammarion witnessed the same phenomenon in similar circumstances. One may well envy those who have had the good fortune to behold this spectacle--to actually see, as it were, the air that the inhabitants of another world are breathing and making resonant with all the multitudinous sounds and voices that accompany intelligent life. But perhaps some readers will prefer to think that even though an atmosphere is there, there is no one to breathe it.
[Illustration: VENUS'S ATMOSPHERE SEEN AS A RING OF LIGHT.]
As the visibility of Venus's atmosphere is unparalleled elsewhere in the solar system, it may be worth while to give a graphic illustration of it. In the accompanying figure the planet is represented at three successive points in its advance toward inferior conjunction. As it approaches conjunction it slowly draws nearer the earth, and its apparent diameter consequently increases. At _A_ a large part of the luminous crescent is composed of the planet's surface reflecting the sunshine; at _B_ the ratio of the reflecting surface to the illuminated atmosphere has diminished, and the latter has extended, like the curved arms of a pair of calipers, far around the unilluminated side of the disk; at _C_ the atmosphere is illuminated all around by the sunlight coming through it from behind, while the surface of the planet has passed entirely out of the light--that is to say, Venus has become an invisible globe embraced by a circle of refracted sunshine.