The gorgeous clusters we have been considering appear to the eye or the small telescope as little cloudlets of hazy light. One after another were resolved into stars; and the natural conclusion was, that all would yield and reveal themselves to be clustered suns, when we had telescopes of sufficient power. But the spectroscope, seeing not merely form but substance also, shows that some of them are not stars in any sense, but masses of glowing gas. Two of these nebulæ are visible to the naked eye: one in Andromeda (see Fig. 68), and one around the middle star of the sword of Orion, shown in Fig.78. A three-inch telescope resolves th Orionis into the famous trapezium, and a nine-inch instrument sees two stars more. The shape of the nebula is changeable, and is hardly suggestive of the moulding influence of gravitation. It is probably composed of glowing nitrogen and hydrogen gases. Nebulæ are of all conceivable shapes--circular, annular, oval, lenticular, [Page 218] conical, spiral, snake-like, looped, and nameless. Compare the sprays of the Crab nebulæ above z Tauri, seen in Fig. 79, and the ring nebula, Fig. 80. This last possibly consists of stars, and is situated, as shown in Fig. 81, midway between b and g Lyræ. [Illustration: Fig. 78.--The great Nebula about the multiple Star th Orionis. (See Frontispiece.)] When Herschel was sweeping the heavens with his telescope, and saw but few stars, he often said to his assistant, "Prepare to write; the nebulæ are coming." They are most abundant where the stars are least so. A zone about the heavens 30° wide, with the Milky Way in the centre, would include one-fourth of the celestial sphere; but instead of one-fourth, we find nine-tenths [Page 219] of the stars in this zone, and but one-tenth of the nebulæ. These immense masses of unorganized matter are noticed to change their forms, vary their light greatly, but not quickly; they change through the ages. "God works slowly." He takes a thousand years to lift his hand off. [Illustration: Fig. 79.--Crab Nebula, near z Tauri. (See Frontispiece.)] There are many unsolved problems connected with these strange bodies. Whether they belong to our system, or are beyond it, is not settled; the weight of evidence leans to the first view. [Page 220] _Variable Stars._ [Illustration: Fig. 80.--The Ring Nebula.] Our sun gives a variable amount of light, changing through a period of eleven years. Probably every star, if examined by methods sufficiently delicate and exact, would be found to be variable. The variations of some [Page 221] stars are so marked as to challenge investigation. b Lyræ (Fig. 81) has two maxima and minima of light. In three days it rises from magnitude 4-1/2 to 3-1/2; in a week falls to 4, and rises to 3-1/2; and in three days more drops to 4-1/2: it makes all these changes in thirteen days; but this period is constantly increasing. The variations of one hundred and forty-three stars have been well ascertained.