If we compare the regions of space to one of the island-studded seas of our planet, we may imagine we see matter distributed in groups, whether of unresolvable nebulae of different ages condensed around one or more nuclei, or in clusters of stars, or in stars scattered singly. Our cluster of stars, or the island in space to which we belong, forms a lens-shaped, flattened, and everywhere detached stratum, whose major axis is estimated at seven or eight hundred, and its minor axis at a hundred and fifty times, the distance of Sirius. If we assume that the parallax of Sirius does not exceed that accurately determined for the brightest stars in Centaur (0.9128 sec.), it will follow that light traverses one distance of Sirius in three years, while nine years and a quarter are required for the transmission of the light of the star 61 Cygni, whose considerable proper motion might lead to the inference of great proximity. Our cluster of stars is a disc of comparatively small thickness divided, at about a third its length, into two branches; we are supposed to be near this division, and nearer to the region of Sirius than to that of the constellation of the Eagle; almost in the middle of the starry stratum in the direction of its thickness. The place of our solar system and the form of the whole lens are inferred from a kind of scale--_i.e._, from the different number of stars seen in equal telescopic fields of view. The greater or less number of stars measures the relative depth of the stratum in different directions; giving in each case, like the marks on a sounding-line, the comparative length of visual ray required to reach the bottom; or, more properly, as above and below do not here apply, the outer limit of the sidereal stratum. In the direction of the major axis, where the greater number of stars are placed behind each other, the remoter ones appear closely crowded together, and, as it were, united by a milky radiance, and present a zone or belt projected on the visible celestial vault. This narrow belt is divided into branches; and its beautiful, but not uniform brightness, is interrupted by some dark places. As seen by us on the apparent concave celestial sphere, it deviates only a few degrees from a great circle, we being near the middle of the entire starry cluster, and almost in the plane of the Milky Way. If out planetary system were far outside the cluster, the Milky Way would appear to telescopic vision as a ring, and at a still greater distance as a resolvable disc-shaped nebula. _IV.--On Earth History_