The other naked-eye nebula is in the constellation Orion. In the telescope it is a more striking object, perhaps, than the Andromeda nebula; for it has no well-defined geometrical form, but consists of an immense odd-shaped mass of light enclosing and surrounding a number of stars. It is unquestionably of a very complicated structure, and is, therefore, less easily studied and explained than the nebulæ of simpler form. There is no doubt that the Orion nebula is composed of luminous gas, and is not merely a cluster of small stars too numerous and too near together to be separated from each other, even in our most powerful telescopes. It was, indeed, supposed, until about forty years ago, that all the nebulæ are simply irresolvable star-clusters; but we now have indisputable evidence, derived from the spectroscope, that many nebulæ are composed of true gases, similar to those with which we experiment in chemical laboratories. This spectroscopic proof of the gaseous character of nebulæ is one of the most important discoveries contributed by that instrument to our small stock of facts concerning the structure of the sidereal universe. Coming now to the smaller nebulæ, we find a great diversity of form and appearance. Some are ring-shaped, perhaps having a less brilliant nebulosity within the ring. Many show a central condensation of disk-like appearance (planetary nebulæ), or have simply a star at the centre (nebulous stars). Altogether about ten thousand such objects have been catalogued by successive generations of astronomers since the invention of the telescope, and most of these have been reported as oval in form. Now we have already referred to the important addition to our knowledge of the nebulæ obtained by recent photographic observations; and this addition consists in the discovery that most of these oval nebulæ are in reality spirals. Indeed, it appears that the spiral type is the normal type, and that nebulæ of irregular or other forms are exceptions to the general rule. Even the great Andromeda nebula, as we have seen, is now recognized as a spiral. The instrument with which its convolute structure was discovered is a three-foot reflecting telescope, made by Common of England, and now mounted at the Lick Observatory, in California. The late Professor Keeler devoted much of his time to photographing nebulæ during the last year or two. He was able to establish the important fact just mentioned, that most nebulæ formerly thought to be mere ovals, turn out to be spiral when brought under the more searching scrutiny of the photographic plate applied at the focus of a telescope of great size, and with an exposure to the feeble nebular light extending through three or four consecutive hours.