We now pass from the star-clusters to the nebulæ properly so called. Till after the middle of last century it was an open question whether there was any real distinction between the two classes of bodies. Herschel had suggested the existence of a 'shining fluid,' distributed through space, whose condensations gave rise to those objects known as nebulæ; but it was freely maintained by many that the objects which could not be resolved into stars were irresolvable only because of their vast distance, and that the increase of telescopic power would result in the disclosure of their stellar nature. This view seemed to be confirmed when it was confidently announced that the great Rosse telescope had effected the resolution of the Orion Nebula, which was looked upon as being in some sort a test case. But the supposed proof of the stellar character of nebulæ did not hold its ground for long, for in 1864 Sir William Huggins, on applying the spectroscope to the planetary nebula in Draco, found that its spectrum consisted merely of bright lines, one of which--the most conspicuous--was close to the position of a nitrogen line, but has proved to be distinct from it; while of the other two, one was unmistakably the F line of hydrogen and the other remains still unidentified. Thus it became immediately manifest that the nebula in Draco did not consist of distant stars, but was of gaseous constitution; and Sir William Herschel's idea of the existence of non-stellar matter in the universe was abundantly justified. Subsequent research has proved that multitudes of nebulæ yield a bright-line spectrum, and are therefore gaseous. Of these, by far the most remarkable and interesting is the Great Nebula of Orion. The observer will readily distinguish even with the unaided eye that the middle star of the three that form the sword which hangs down from Orion's belt has a somewhat hazy appearance. A small telescope reveals the fact that the haziness is due to the presence of a great misty cloud of light, in shape something like a fish-mouth, and of a greenish colour. At the junction of the jaws lies the multiple star Theta Orionis, which with a 2- or 3-inch glass appears to consist of four stars--'the trapezium'--large instruments showing in addition two very faint stars.