The nearest star of all, whose distance has been measured, is Alpha Centauri. It has been roughly calculated that a comet, passing direct from Alpha Centauri to our sun, would take about twenty millions of years for his journey. But here we tread upon very doubtful ground. Many matters, at present unknown to us, might greatly affect the result of such a calculation. Also it is by no means impossible that other stars lie really much nearer to us than Alpha Centauri, whose distance astronomers have not yet attempted to measure. [Illustration: BARON ALEXANDER VON HUMBOLDT.] “In order to complete our view,” says Alexander Von Humboldt, “of all that we have learned to consider as appertaining to our Solar System, which now, since the discovery of the small planets, of the interior comets of short revolutions, and of the meteoric asteroids, is so rich and complicated in its form, it remains for us to speak of the ring of zodiacal light. Those who have lived for many years in the zone of palms must retain a pleasing impression of the mild radiance with which the zodiacal light, shooting pyramidally upward, illumines a part of the uniform length of tropical nights. I have seen it shine with an intensity of light equal to the Milky Way in Saggitarius, and that not only in the rare and dry atmosphere of the summit of the Andes, at an elevation of from thirteen to fifteen thousand feet, but even on the boundless grassy plains of Venezuela, and on the seashore, beneath the ever-clear sky of Cumana. This phenomenon was often rendered especially beautiful by the passage of light, fleecy clouds, which stood out in picturesque and bold relief from the luminous background. “This phenomenon, whose primordial antiquity can scarcely be doubted, is not the luminous solar atmosphere itself, which can not be diffused beyond nine-tenths of the distance of Mercury. With much probability we may regard the existence of a very compressed ring of nebulous matter, revolving freely in space around the sun between the orbits of Venus and Mars, as the material cause of the zodiacal light. These nebulous particles may either be self-luminous or receive their light from the sun.