Great Nebula in Andromeda—after Ritchey.
Nebula M. 100 Comæ—after Roberts.
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.
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.