blage of curved wisps of luminous matter, which, branching outward from a common origin in the bright masses in the vicinity of the trapezium, sweep towards a southerly direction, on either side of an axis passing through the apex of the Regio Huygeniana, nearly in the angle of position 180°. About twenty of these convolutions have been distinctly traced, while others giving a like impression are too faint or too intricate to be subjected to precise description. It may therefore be properly classed among "the spiral nebulae," under the definition given by their first discoverer, Lord Rosse; including in the term all objects in which a curvilinear arrangement, not consisting of regular reëntering curves, may be detected.

That the existence of this feature in the great nebula of Orion should have hitherto escaped notice, after the many careful scrutinies to which it has been subjected, with the help of the largest instruments and the most skilful observers, may seem scarcely credible; a few words of explanation on this point will not therefore be amiss. It is to be ascribed partly to the confusing effect produced by the crossing and intersection of the principal striae and of their offsets, which the eye cannot unravel without the aid of some clue to their mutual relation and significance; and partly also to the faintness of some of the details, which are, nevertheless, very essential features in a correct apprehension of its structure, supplying, as they do, what would otherwise appear as breaks of continuity, and assisting materially in the recognition of a principle of regularity pervading the whole structure. Until the law of relation and continuity in the several parts of such an object is entertained in the mind, it must remain an incoherent, confused assemblage of material, having no orderly or connected arrangement.

The change from the previous notion of its configuration is not more considerable than that which took place with reference to the celebrated nebula 51 Messier, in which the original discovery of the spiral arrangement was made. This object had been subjected to a careful examination and description by both the Herschels, but neither their drawings nor descriptions furnished the slightest intimation of a spiral structure. It deserves particular notice, too, that there was no want of sufficient optical power to exhibit the appearance in question; for the spirality of 51 Messier is seen with perfect distinctness in a refractor of 15 inches' aperture, and must certainly be within reach of the twenty-foot Herschelian reflectors. Nor can it for a moment be thought that the earlier observations and delineations were in any proper sense erroneous. They were simply made at a great disadvantage, in the absence of a clear conception of the general plan of structure presented in the object. Some of the details indispensable to its recognition, being only faintly presented, were overlooked,

or, appearing by mere suggestions and glimpses of vision, they conveyed an erroneous impression; in this way the mutual relation of the various parts came to be entirely misconceived. The missing links were supplied by the larger optical power of Lord Rosse's telescope, too plainly not to insure notice; and the nebula then presented itself under a totally different aspect. Instances of similar revelations, completely at variance with previous conjectures, have indeed so often occurred in the history of astronomical discovery, that the process ought to be regarded as the ordinary rule, rather than as an unusual exception.

[The notes of the observations referred to will be found in the text of the previous Section, at the dates 1860, Feb. 21st, 1861, Jan. 28th, Feb. 6th (when the regular examination mentioned in the early part of the present Section began), Feb. 13th, Feb. 25th, Feb. 26th, Feb. 28th, March 5th, March 7th, March 11th.

The paper bears date March 12th, 1861, but underwent some revision after that, before it was finally printed. In 1862, observations upon Feb. 18th, made with the 18 inch object-glass, by Alvan Clark, now belonging to the Dearborn Observatory of Chicago, and at that time temporarily mounted in Cambridgeport, confirmed the views the author had previously expressed. There are also some notes bearing on this point, 1864, March 9th.]

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