"It is a wonderful sight," says Peck in "The Constellations," "the whole field of view is filled with an irregular mass of green shining mist, which is apparently broken up into flocculent masses, delicate clouds of light, sprays and wisps, and standing out from the cloudy background, like a sprinkling of diamond dust, are seen faint, glittering stars." Vast masses of dark nebulæ in the Nebula of Orion are also effectively revealed in telescopes. The dark nebula is visible in the photographs as veil-like clouds obscuring the light of the stars beyond them, or, again, as definitely outlined black blotches lying like some huge object on a brilliantly glowing background. Such a 'blotch' is the Dark Bay or Dark Horse Nebula in Orion whose photograph is so fearfully suggestive and wonderfully beautiful. Nebulous clouds are, in general, dark rather than luminous. These clouds are believed by many astronomers to be cosmic dust-particles driven out from bright stars. These particles finally collect in great clouds which, when far from the light of stars, are dark. When the accumulating clouds of dust become so heavy that gravitation begins to draw their particles together, heat is generated and the mass begins to glow, although sometimes dark masses, if they are close to stars, will glow with reflected starlight. The gaseous clouds are devoid of solid or liquid matter, and consist, the spectroscope shows, of glowing gases, apparently nebulium and hydrogen. Thus the Nebula of Orion, which is a gaseous nebula, is never spoken of as a "universe," like the Nebula of Andromeda, which may be resolved into stars. [Figure 117: THE DARK BAY OR DARK HORSE NEBULA IN ORION. Photograph by Mount Wilson Observatory through the 100-inch Hooker telescope.] The finest example of an irregular nebula is the Nebula of Orion; of a ring nebula, that found in the constellation of Lyra; and of a spiral nebula, the one found in the constellation of Canes Venatici, just below the handle of the Big Dipper. The spiral of Andromeda, which is turned edgewise to the earth, is the largest nebula in the sky. ORION'S TWO DOGS--CANIS MAJOR AND CANIS MINOR Canis Major, the "Great Dog" of Orion, lies in a straight line southeast of his Belt Stars. Canis Minor, his "Small Dog," lies across the Milky Way, 20 degrees northeast of Betelgeuse, the pink star on his shoulder. The three stars form a large and easily traced triangle.