Nebulæ are generally divided into two classes—the “white” nebulæ and the “green” nebulæ. The first, of which the Andromeda nebula is a striking example, give a continuous spectrum without dark lines, as if they consisted either of gas under high pressure, or of something in a solid or liquid state. The second, conspicuously represented by the Orion nebula, give a spectrum consisting of a few bright lines, characteristic of such gases as hydrogen and helium, together with other substances not yet recognised. But there is no continuous spectrum like that shown by the white nebulæ, from which it is inferred that the green nebulæ, at least, are wholly gaseous in their constitution. The precise constitution of the white nebulæ remains to be determined. It is only in relatively recent years that the fact has become known that the majority of nebulæ have a spiral form. There is almost invariably a central condensed mass from which great spiral arms wind away on all sides, giving to many of them the appearance of spinning pin-wheels, flinging off streams of fire and sparks on all sides. The spirals look as if they were gaseous, but along and in them are arrayed many condensed knots, and frequently curving rows of faint stars are seen apparently in continuation of the nebulous spirals. The suggestion conveyed is that the stars have been formed by condensation from the spirals. These nebulæ generally give the spectra of the white class, but there are also sometimes seen bright lines due to glowing gases. The Andromeda nebula is sometimes described as spiral, but its aspect is rather that of a great central mass surrounded with immense elliptical rings, some of which have broken up and are condensing into separate masses. The Orion nebula is a chaotic cloud, filled with partial vacancies and ribbed with many curving, wave-like forms. There are other nebulæ which have the form of elliptical rings, occasionally with one or more stars near the centre. A famous example of this kind is found in the constellation Lyra. Still others have been compared in shape to the planet Saturn with its rings, and some are altogether bizarre in form, occasionally looking like glowing tresses floating among the stars.