In 1843 when Lord Rosse’s giant speculum, six feet across, was turned upon the sky, a nebula was brought to light which was unlike any ever before seen. It was neither irregular like the great nebula in Orion nor round like the so called planetary nebulæ,—the two great classes at that time known,—but exhibited a striking spiral structure. It proved the forerunner of a remarkable revelation. For the specimen thus disclosed has turned out to typify not only the most interesting form of those heavenly wreaths of light, but by far the commonest as well. As telescopic and especially photographic means improved, the number of such objects detected steadily increased until about thirteen years ago Keeler by his systematic discoveries of them came to the conclusion that a spiral structure pervaded the great majority of all the nebulæ visible. Their relative universality was outdone only by the invariability of their form. For they all represent spirals of one type: two coiled arms radiating diametrically from a central nucleus and dilating outward. Even nebulæ not originally supposed spiral have disclosed on better revelation the dominant form. Thus the great nebula in Andromeda formerly thought lens-shaped proves to be a huge spiral coiled in a plane not many degrees inclined to the plane of sight. [Illustration: NEBULA ♅ I. 226 URSÆ MAJORIS—AFTER ROBERTS.] As should happen if the spirals are unrelated, left-handed and right-handed ones are about equally common. In Dr. Roberts’ great collection of those in which the structure is distinctly discernible, nine are right-handed, ten left-handed, showing that they partake of the ambidextrous impartiality of space. [Illustration: NEBULA ♅ V. 24 COMÆ—AFTER ROBERTS. Showing globular structure.] Lastly the spirals are evidently thicker near the centre, thinning out at the edge, and when the central nucleus is pronounced, it seems to have a certain globularity not shared by the arms, and more or less detached from them. This appears in those cases where they are shown us edgewise, and it has been thought perceptible in the great nebula of Andromeda. The difficulty in establishing the phenomenon comes from the impossibility of both features showing at their best together. For the globularity to come out well, the spiral must be presented to us nearly in the plane of sight; for the spirality, in a plane at right angles to it. Much may be learnt by pondering on these peculiarities. The widespread character of the phenomenon points to some universal law. We are here clearly confronted by the embodiment of a great cosmic principle, causing the helices it is for us to uncoil. It is a problem in mechanics.