became a test of their power. But spectrum analysis finally entered the lists with new methods of investigation, and the comparatively tiny spectroscope at a single leap passed far beyond the utmost limits of the highest telescopic vision, and at one blow struck the whole category of nebulæ into two widely different classes,--those composed of discrete stars grouped like the suns of our own Milky Way, and exhibiting the characteristic spectra of such bodies, and those composed of diffused gaseous matter not yet condensed into suns, and showing the disconnected spectral lines of simple elemental gases. The line of division was clear, direct, positive, and beyond all dispute. Yet beyond these two classes further research has disclosed certain vast nebulæ in which some portions exhibit true solar spectra more or less modified and others true gaseous spectra, each apparently merging into the other by gradations so faint and delicate that the inference is irresistible that in these nebulæ we see the processes of galactic and solar creation at various stages of their development. Of these nebulæ, Professor Ball says, "In one of his most remarkable papers, Sir W. Herschel presents us with a summary of his observations on the nebulæ, arranged in such a manner as to suggest his theory of the gradual transmutation of nebulæ into stars. He first shows us that there are regions in the heavens where a faint diffused nebulosity is all that can be detected by the telescope. There are other nebulæ in which a nucleus can be just discerned, others again in which the nucleus is easily seen, and still others where the nucleus is a bright star-like point. The transition from an object of this kind to a nebulous star is very natural, while the nebulous stars pass into the ordinary stars by a few graduated stages. It is thus possible to enumerate a series of objects, beginning at one end with the most diffused nebulosity and ending at the other with an ordinary fixed star or group of stars. Each object in the series differs but slightly from the object just before it and just after it." And of these composite nebulæ, he adds, "The great nebula in Orion is known to be the most glorious body of its class that the heavens display. Seen through a powerful telescope, ... the appearance of this grand 'light stain' is of indescribable glory. It is a vast volume of bluish gaseous material with hues of infinite softness and delicacy. Here it presents luminous tracts which glow with an exquisite blue light; there it graduates off until it is impossible to say where the nebula ceases and the black sky begins."