Connection between Meteors and Comets, 300 Physical and Chemical Constitution of Comets 314 II. The Zodiacal Light 318 III. THE STELLAR UNIVERSE 322 I. General Aspect of the Heavens 322 II. The Stars 330 The Constellations 330 Clusters 350 Double and Multiple Stars 355 New and Variable Stars 358 Distance of the Stars 364 Proper Motion of the Stars 365 Chemical and Physical Constitution of the Stars 371 III. Nebulæ 373 Classification of Nebulæ 373 Irregular Nebulæ 376 Spiral Nebulæ 384 The Nebular Hypothesis 391 IV. The Structure of the Stellar Universe 396 I. THE CELESTIAL SPHERE. I. _The Sphere._--A _sphere_ is a solid figure bounded by a surface which curves equally in all directions at every point. The rate at which the surface curves is called the _curvature_ of the sphere. The smaller the sphere, the greater is its curvature. Every point on the surface of a sphere is equally distant from a point within, called the _centre_ of the sphere. The _circumference_ of a sphere is the distance around its centre. The _diameter_ of a sphere is the distance through its centre. The _radius_ of a sphere is the distance from the surface to the centre. The surfaces of two spheres are to each other as the squares of their radii or diameters; and the volumes of two spheres are to each other as the cubes of their radii or diameters. Distances on the surface of a sphere are usually denoted in _degrees_. A degree is 1/360 of the circumference of the sphere. The larger a sphere, the longer are the degrees on it. A curve described about any point on the surface of a sphere, with a radius of uniform length, will be a circle. As the radius of a circle described on a sphere is a curved line, its length is usually denoted in degrees. The circle described on the surface of a sphere increases with the length of the radius, until the radius becomes 90°, in which case the circle is the largest that can possibly be described on the sphere. The largest circles that can be described on the surface of a sphere are called _great circles_, and all other circles _small circles_. Any number of great circles may be described on the surface of a sphere, since any point on the sphere may be used for the centre of the circle. The plane of every great circle passes through the centre of the sphere, while the planes of all the small circles pass through the sphere away from the centre. All great circles on the same sphere are of the same size, while the small circles differ in size according to the distance of their planes from the centre of the sphere. The farther the plane of a circle is from the centre of the sphere, the smaller is the circle.