very bad definition, and the reduced light of the nebula, owing to its low altitude, there is more resemblance to their description, and to Sir J. Herschel's drawing in this particular, than when the nebula is seen to advantage. There is a loss of precision in northern edge, although the terminus near is quite sharp. In bad definition, with feeble light, the dark channel which passes d, e, and , might be confounded, near Q', with the S. Magnus, and so explain Sir J. Herschel's figure.

1864, April 14. Just before it clouded, I prepared to sketch the brighter parts of the nebula, and noticed that the outline of Huyghenian R., the part strongly expressed on my engraving, is distinctly recognizable, and certainly, even in the part between AR. of Y" and B"B" at origin of Messierian Branch. The latter does not blend here with R. Huygeniana, nor does the light sweep out in definite curve, nearly so far in the parallel a little s. of 6', and towards following side, as is represented in Sir J. Herschel's drawing, but its limit even falls short of AR. of B"B" as in my engraving. In fact no part of the bright region shades off more indefinitely than is shown in my drawing, excepting it be about x, where the bright wisps of R. Gentil. originate, but even this is not noticeable. (April 15, seems to be outline at x as well, and a narrow darker space separates R. Huygeniana from origin of wisps 2 or 3" broad.) I had begun an outline noticing that ay was more readily traced than northern limit of S. Magnus, and that general effect of prec. margin of S. Magnus is that of a nearly straight, and sharply defined edge inclining to s. f. direction. More nearly straight than I have represented it in the engraving, where the bright mass in n. p. corner of S. Magnus is perhaps a little too bright, and too distinctly tending to s. p. direction. These appearances were noted hastily, but without thinking at the time of Messrs. Stone and Carpenter's remarks, in Report of Council of Astr. Soc.; but the correction here noticed would rather increase the squareness of preceding part of S. Magnus. [x is the preceding point of the Huyghenian Region; ay is the southern boundary of the Sinus Magnus.]

1864, April 15. Sky at first not clear; but improved, though it clouded again towards close of observations. Vision at first disturbed; improved towards middle of observations.

At 8h 50m 453, by Chro. 236. Can distinguish brighter masses of nebulosity especially preceding the trapezium.

At 8h 56m. Light preceding ◊ in parallel, may be traced 4.3. Centre of brightest part 2.25 [preceding 0], brighter mass is about 50" long.; axis [in] s. f. direction inclined 25° to hour circle. Brightest part [is] about in parallel of 0.

The south shore of Cape is south of Q' 19.2. Herschel's engraving has this limit north of Q'.

[G. P. B. allows 3" for the half breadth of the "Cape," and this, with the declination of Q', (H. 87) 100."0, from page 85, gives for the centre of "Cape" the declination +83."8.]

[The difference of declination between 6' Orionis and the north point of bright mass which reaches southward to (H. 75) is now measured, 69."8.]

It is about 15" s. of s. face of Cape; there seems to be an error of 10" in the engraving, which has ▲ 8 =81".

The limit of general terminus of Huyghenian Region from ' [measured in declination]. Ad=53.3. Following edge of light mass about (n. of) [measured] in AR. from 6'; A a=25."6. This edge is nearly in direction of AR. but very slightly inclines to s. f. direction. No other part of preceding edge of S. Magnus is so near to 6' in AR. My engraving is here to be corrected as to this mass axis.

* By " Cape," is meant promontory forming the southern boundary of S. Magnus; this edge trends a little n. of parallel

Also the n. p. corner of S. Magnus is to be filled in with more light. Noticed separation of wisps at origin from point x, April 14th, of Huyghenian Region; wisps emanating from x should be broader and more diffuse than in engraving, and brighter.

More light in S. Gentilii, by diffusing the masses there somewhat. The end of Cape at C, is well placed in AR. on engraving; greater than AR. of B"B" [+151."4, page 85, No. 708]. From C, the boundary of light is continuous, across interval between AR. of Q' and O', although dark canal from e, , and 8, is traced as intersecting it without by any means obliterating the light.

The least definite edge of bright light of Huyghenian R. is on the side n. and n. p. 6', though even here the light fades quite suddenly, so that line n. of Fig. A should be made a little more pronounced in engraving. The bays μ, μ', etc. (Fig. 3 of Herschel's engraving), do not exist apart from possible slight deficiencies of light which are not now to be recognized.

Star [H. 75] is far within nebulosity, not at its edge, as Herschel has it. His limit of nebulosity north of Q' is far too much north of it. The "nebula minima," he places in parallel of Q', when it is really far south of it

For the above I compared Herschel's engraving, reversed by reflection so as to present it as seen through

refractor.

From the foregoing observations it may be gathered that in all probability not only the stars of the trapezium (the multiple star 6' Orionis), but also many in the neighborhood, are physically connected with the nebula. This is especially true of the groups, which, to the naked eye, form 1, 0, c, Orionis. For we see that each of these groups is accompanied by a nebula. We have three nebulae in a nearly right line; the probability that mere chance should have superposed on these nebulae three bright star-groups, also nearly in a right line, and the extremes equidistant from the middle one, is very small indeed. The conclusion that the star-groups and nebulae are physically connected, each group with its nebula, and the three systems of stars together, is much strengthened by the manifest connection of the three nebulae inter se. Sir John Herschel, it is true, was not able, with his 18-inch reflector, even at the Cape of Good Hope, to trace the connection; but he carefully avoided deciding the point, as the fact appeared to him extremely probable, and as he had traced the nebulous connection between 0 and Orionis. The previous observations, as well as those made by Prof. W. C. Bond, confirm the connection of all three nebulae in the most decided manner; the luminous circle whose centre is near the star Herschel 34, and which encloses the principal great nebula, and nearly touches Orionis, connects ɩ and 0; while and c are joined by three filaments of light extremely attenuated.

ι

し

The name "Corona Herschelii," is suggested, in accordance with what are believed to have been Prof. Bond's wishes, for the luminous circle between and Orionis; as Sir John Herschel first traced a portion of it (I suppose the continuation of the Mes

sierian Branch), and as Prof. Bond's view of the whole subject accords so exactly with the wise suggestion or surmise of Sir William Herschel. The following quotations from these two great authorities, will be read with interest.

"Fifth Class. Very large nebulae. V. 35. Diffused m. nebulosity, extending over no less than 10 degrees of P. D., and many degrees of R. A. It is of very different brightness, and in general, extremely F. and difficult to be perceived. Most probably the nebulosities of the 28th, 30th, 31st, 33d, 34th, and 38th, of this class, are connected together, and form an immense stratum of far distant stars, to which must also belong the nebula in Orion." Sir W. H., in Philosophical Transactions for 1789, p. 249.

Again. "Table of extensive diffused nebulosity, No. 24. Visible and unequally bright nebulosity R. A.=5h 28m 31° P. D.=94° 22′, Par. 1° 48', Mer. = 2° 32', Area in square degrees 4.6. I am pretty sure this joins to the great nebula in Orion." Sir W. H. in Philosophical Transactions for 1811, p. 276.

"Although I have not succeeded in tracing any nebulous connection between this nebula (c Orionis) and the great one about 0 Orionis, yet as their distance is not much more than half a degree, it not improbably forms part of one great nebulous system extending southwards, through and beyond that nebula as far as Orionis, up to which star a pretty conspicuous branch of the great nebula runs. More powerful telescopes than mine must decide this point." Sir J. H. in "Results of Astronomical Observations made during the years 1834, 35, 36, 37, 38, at the Cape of Good Hope,” p. 11.

The connection of stars and nebulae is rendered additionally probable from the absence of nebulosity and of small stars in the same field; from the nebulous wisps attached to bright stars, and perhaps from the discovery by the Professors Bond, and by O. Struve, of variable stars among those of the General Catalogue, especially near 0 Orionis.

Portions of the nebula have been seen to approximate to resolvability; and in one place both Prof. G. P. Bond and Dr. Gyldèn appear to have detected a cluster of very faint stars, which were visible but by glimpses. This is especially interesting in connection with the researches of Huggins, on the nebula, as viewed by the spectroscope, and other such researches now in progress. The limits of visible nebulosity, as sketched by Professor Bond on a chart of all the brighter stars in the General Catalogue, are the following:

SECTION VI.

ON THE SPIRAL CHARACTER OF THE GREAT NEBULA IN ORION.

Extracted from the "Proceedings of the American Academy," Vol. V. pp. 227-230.

[PROF. G. P. BOND exhibited a drawing of the great nebula surrounding the star Orionis, representing its appearance in the twenty-three foot refractor of the Observatory of Harvard College.]

The feature to which attention was particularly directed was the spiral structure of the principal masses of light, or, more correctly, the tendency to an arrangement in elongated wisps or whirls, sweeping outward from the bright region of the trapezium. A disposition of the nebulosity in some localities to radiate from the vicinity of the trapezium, noticed in the Memoir published by Prof. W. C. Bond in 1848, has repeatedly attracted attention in subsequent years. The idea of a spiral character in the radiations had even been suggested, without however presenting itself definitely to the mind as the true conception of the leading features of the nebula.

During the past winter, opportunities were taken to review the whole region, with particular reference to this peculiarity; attention being given exclusively to the arrangement of the diverging wisps of nebulosity, and the alternating dark spaces by which they are separated from each other. A particular scrutiny of the latter was of considerable assistance in tracing the fainter convolutions. The form and disposition of the whirls were thus defined by two independent processes, the nebula being first sketched as a bright object on a dark ground, and, again, its darker openings and channels as dark objects on a white ground.

The quarter designated in Herschel's chart* as the Regio Godiniana, was first explored. The nebulosity was here resolved into an assemblage of three or four long wisps, interlaced with each other, or crossed by offsets; these were ultimately traced from a point near the northern margin of the Sinus Magnus, over the whole length of

* Mem. Astr. Soc., Vol. II.

the Regio Picardiana and the Regio Godiniana, forming a sweep of 120°. After passing the well defined northern boundary of the last-named region, and beyond Herschel's stars o and E, these wreaths bend rather suddenly, and tend towards the southpreceding direction. Indications of their presence in this quarter are imperfectly suggested in Lassell's and in Sir J. Herschel's latest drawing. From this point feeble traces exist for 10′ or 15' in a south-preceding direction. Their course over the R. Picardiana gives a decidedly reticulated aspect to the whole region; but, though bright, they are here so closely intertwined and connected by offsets, that it is a matter of no little difficulty to gain a clear comprehension of their proper relations. The complexity of the details is further increased by several offshoots from this quarter, which cross over into the adjacent R. Derhamiana; still the general effect is easily recognized.

From the southern corner of the Regio Picardiana, and from those parts of R. Derhamiana and R. Huygeniana which lie near the trapezium on its north-preceding and preceding sides, a number of narrow and bright branches diverge, their extremities tending also to the south-preceding direction. Some of these cross the R. Gentiliana and seem to merge together, forming a nebulous mass, which can be followed through an arc of 10′ or 15'. Others, which are less curved, originate near the Sinus Gentilii; these are narrow and somewhat tortuous.

It is to be noticed that the initial direction of the wreaths (Nebelstreifen) changes continuously from an angle of position of 330° on the northern margin of the Sinus Magnus, to one of 220°, or less, at the S. Gentilii, and the sweep of the curve correspondingly diminishes, so that throughout the whole nebulous region preceding the sharply defined apex of the R. Huygeniana, the extremities of the filaments have a pretty uniform tendency in the angle of position 220°. As soon, however, as we pass to the fields on the following side of the apex, a change is immediately apparent, the ultimate direction being about in the angle 160°. The principal group of wisps results from the resolution of the R. Messieriana, and the region between the trapezium and the Proboscis Minor, including both these features, into four or five distinct wreaths, having a common initial direction in the angle of position 110°. The very bright nebulosity lying between the S. Gentilii, the trapezium, and the R. Subnebulosa, cannot be resolved into a regular structure, but three or four condensed spots, constituting the most brilliant part of the nebula close on the south-preceding side of the trapezium, are plainly distinguished as tufts or curled offsets from a prominent wisp of light which extends from its origin, near the trapezium, across the R. Gentiliana.