The assumed revolution of one of these stars about the other--thus destroying all the known analogies of the universe, as exemplified in our own system--may be accounted for in the same way. With stupendous planetary systems revolving about each of these apparently double stars, they must respectively have a revolution, real as well as apparent, about their own centres of gravity--not one and the same centre, but different and far distant centres. Lying in nearly the same line of vision, with planes of movement at right angles with it, they would necessarily present the appearance of one star revolving about the other--an _apparent_ motion only. And the writer here ventures an explanation of the phenomena of _temporary_ stars, or those making their appearance in the heavens, flaming up into stars of the first, second and third magnitudes, and then disappearing altogether. The most remarkable of these stars, or _apparent_ stars, was that of Tycho Brahe in 1572, presenting its maximum brilliancy at the very first, but gradually diminishing in size until the end of seventeen months, when it disappeared, without change of place, from the heavens. This temporary star was visible in Cassiopeia, on the verge of the Milky-way, within whose swarm of stellar worlds most of these apparent stars have made their appearance. Tycho Brahe, in seeking to account for this stellar phenomenon, advanced the theory that stars might be "formed and molded out of cosmical vapor," or "vapory celestial matter," as the elder Herschel put it, "which becomes luminous as it condenses (conglomerates) into fixed stars." But any such rapid condensation of "vapory matter," in the light of Laplace's "nebular theory," is manifestly too absurd for scientific recognition. A more satisfactory explanation may be here suggested:--Supposing the apparent relative position of any six or seven stars of the sixth magnitude in the Milky-way, should be so changed by the combined motions of our sun-system and of the stars themselves, as to throw them into one and the same line of vision, but so clustered together as to show their several star-discs as one, we should unquestionably have a star of the first magnitude, which would continue as long as this extraordinary stellar conjunction should last. As one after another of these stars should fall out of line, by reason of the combined motions named, the apparent star would be diminished from the first to the second magnitude, and so on until it reached the sixth magnitude, when it would pass beyond the reach of unaided human vision. But as the star of Tycho Brahe suddenly appeared at its fullest brilliancy, it may be objected that this suggested theory fails to meet the required conditions.