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.