SECTION V.

PHYSICAL OBSERVATIONS OF THE NEBULA IN ORION.

THE structure of this the most remarkable of the nebulae known to astronomers in Europe and North America—is of very great importance in its bearing upon the nebular hypothesis. The observations made here were largely embodied in drawings, from which the engraving of the nebula which accompanies this work was executed; but, besides, there are a great many allusions in the journals of observation to physical characteristics, which have been extracted and are here given; the observations previous to 1858, however, as a portion and a continuation of Prof. W. C. Bond's, were omitted here, to be given afterwards in Appendix I. As in the last section, the notices within brackets were supplied by the editor, as were also the foot-notes.

1858, Feb. 8. In the region preceding the trapezium, I was struck with the apparent increase of small stars in the parts where the nebulosity was brightest. I have before received a similar impression in the northern zones for the region preceding the nebula.

1859, Feb. 21, 8 P. M. Clear, but only tolerably good vision; Chart I. Commenced drawing the nebula in chalk, on the charts prepared by Mr. Hall, of all the stars on a black ground. There is a remarkable dark region extending from AR. - 0m 35 Dec. 11' 20" rel. to 6' to the Dec. 26' about, at which point bright nebulosity again appears. I have not thus far, since commencing in 1857, made any attempt to compare our results with previous observations.

1859, Feb. 22. The region about F", H", etc., has an appearance of resolvability. 1859, Feb. 23. Drew again on Chart I. There is a very large area north of the nebula, in which there is but little if any nebulosity. Under the best definition no star is to be seen within the trapezium. The edge ab [the S. E. edge of the Huyghenian region] seems to be just perceptibly (more strongly seen March 4th) brighter than the region within it; w is the brightest part of the nebula, x is very black, y bright. There is a curved bright outline as at pq.

1859, March 2. I think that there is often a disposition of the nebulosity to shrink from the immediate contact or neighborhood of some stars, and as evident a tendency to condense about others. I am disposed to think that, in general, where the stars are most numerous, there is also an increase of nebulosity.

1859, March 4. The moisture condensed on object-glass. The nebulosity preceding trapezium, distant say 1' at w, and towards a point half-way from a to q of Feb. 23d, is the brightest of all. Several stars about the trapezium as A, etc., are decidedly nebulous, or have nebulous attachments. This would be highly important to verify. (Subsequently verified, vide March 10th.) [A is here the star 62 Orionis = Y".]

1859, March 10. Admirable definition. Moon six days old in neighborhood. Two features were very noticeable. 1st. The great number (probably at least 20 could have been located) of very small stars in the region south of the three bright stars Y", B"B", and E"E", [H. 93, H. 101, H. 110,] as far as P''' and F"F" and h''h", and between this and the Great Proboscis. Another feature exhibited, and several times noted on other dates, was the nebulous appendages of the stars Y", B"B", and E"E", as above represented; that of Y" is very evident. On a scrap of paper I have indicated the region which has most bright stars, and it is remarkable that it is just where the small stars appeared in the greatest number on the 10th. That is in the region s. f. 0, and preceding the Great Proboscis, and is occupied with a bright diffuse nebulosity, mostly very evenly graduated. In the black regions near and n. f. 6, but a single very faint star could be seen. 1859, March 23. 236* 6h 57m, a patch of light just visible 40" preceding the centre of trapezium and a little south of the centre. 236-7h 00m, a patch of light 1'30" south of centre of trapezium and following 1'. Looked at the nebula in early twilight, and found that the bright part surrounding the trapezium is outlined

as in my water-color drawing, and as described Feb. 23.

[1860, Jan. 16th, occurs a diagram in which the bright portions of the nebula about 1' preceding the trapezium are marked as probably resolvable under very fine definition.]

1860, Feb. 21. 6h Sid. T. The spiral whirl of the nebula is readily distinguished when attention is once directed to this feature.

1860, Dec. 6. Have more than once noticed that there is a tendency in the nebulosity to dispose itself about the groups of bright stars in preference to other regions.

1861, Jan. 2. Thought I could recognize at least nine distinct instances where the nebulosity of Orion was expressly associated with bright stars or groups, sufficiently marked to arrest attention when directed to this particular aspect. There are included within a region 30′ on each side of trapezium, from Orionis, a nebulous group, and a group north of trapezium 1' more distant north of it than is south, and in nearly the same AR.

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1861, Jan. 28. Nearly clear sky. Definition pretty good. Now that attention is directed to the fact, the impression of a diverging spiral-like radiation of wisps of nebulosity from the bright region near the trapezium is very certain. There are one or two such near a [about 1' preceding trapezium, 0.15 south]. The three groups of stars constituting the stars of the sword-handle of Orion, 4, 6, and [c,] are foci of nebulous regions. The northernmost has about it a great mass of nebulosity which brightens up around several of the stars. 1861, Jan. 31. [Some differences of AR. are given among four stars, of which (c) is c1 Orionis, (d) is c2 Orionis, (a) is 536 of the present catalogue, and (b) 614 or 629. This region is brightly nebulous, especially about these four stars, and in the field preceding (c) and (d).] About (a) the accumulation of nebulosity is especially marked. Connection can be traced between this nebulosity and that about the trapezium. There is a decided accumulation of nebulosity about a star of 9th Mag. 11' south of the trapezium, and 35 seconds of time preceding it [probably H. 34=467 of the General Catalogue]. All the region for several fields

around Orionis is nebulous. Definition good. Eye-piece and object-glass clear. It is remarkable that about

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each group composing "sword-handle" of Orion, there is an accumulation of nebulosity quite evident even in the finder. See Obs. Feb. 21, 1863. There are, besides, several instances of a similar preference to the neighborhood of individual stars. The whirl-like arrangement of the drifts of nebulosity about the region of trapezium is very striking to-night.

1861, Feb. 6. Clear, with tolerably good definition. When the attention is given expressly to the dark

* Chronometer nearly indicating Sidereal time.

striae or furrows, their continuity becomes much more distinct, and the radiation more than ever evident. Those numbered [in a diagram accompanying] are clearly recognized as tending to give the wisp-like aspect and whirl of the brighter streaks of nebulosity. No. 3 is about 12' preceding T' [H. 108], and is 1' north of it. This streak tends a little to the south of T', above it is 4 tending to 2. This region about 2, 3, 4, and 5, has a twisted net-like aspect. 12 is remarkable, reaches half way from apex of nebula towards n. I propose to draw the dark features of the nebula with as little regard as possible to the bright regions, and vice versa. There is a field having only one or two stars visible, and those of the smallest visible, 15' prec. ‹ Orionis while about it are grouped a brilliant assemblage of all grades of brightness.

1861, Feb. 8. Very cold, [6° Fahrenheit,] and bad definition; not free from haze. One edge of the two dark channels (of the lower of them) passes at x.

1861, Feb. 12. After warm rain, cleared suddenly; not good definition, but clear. Object-glass covered with moisture twice. Dark channels y and x distinctly traced as represented.

1861, Feb. 13. Good definition. Slight haze in sky. Examined the dark wreath-like openings, and traced a large number on chart M (tracing linen); was more than ever convinced of their whirl-like disposition. The whole bright region n. p. trapezium, shows the tendency strongly, and I suspect a radiation in the bright regions, lying to the n. p. side of line joining the stars L", N", and R'. The region about T' has an indication of like tendency.

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N. B. That line from Orionis to c Orionis, and to northernmost stars of sword of Orion, is an axis of light (and stars) radiating from trapezium.

1861 [no date]. The examination of spiral structure was begun systematically about Feb. 6, in the Regio Godiniana, and proceeded regularly on successive nights round in the p., s. p., s. f., and f. quadrants.

1861, Feb. 25. Quite hazy. In finder [aperture 34 lines] the general indication of radiation of light from nebula is towards A, B, C, and D, noticeable in finder. [I cannot identify these points very exactly; A is about 12' from R. Huygeniana in AR. -3'; B is upon the Proboscis Major; C is about 9' north of the nebula Mairanni; D is perhaps a—8', 8+4'.]

1861, Feb. 26. A fine clear night, with good definition. The best opportunity for viewing the nebula that has offered this winter. Set upon the nebula in strong twilight, and prepared for examining the quarter s. p. the trapezium with reference to the disposition of the wreaths or streaks of nebulosity. On previous evenings I have examined the n., n. p., p., and s. p., portions in succession. From the star R" a narrow brush or ray extended towards Q' as indicated on the chart M. in yellow. From E"E" there is a broad diffused mass of nebulosity extending towards F"F" not regularly curved from E"E". From A"A" towards trapezium the radiations seem to cross each other, forming a net-work, as it were, reticulated. Indeed, elsewhere, in many instances the wreaths, like smoke from wet weeds, grass, or hay thrown on coals, seem to intertwine in a way quite difficult to draw. On turning to the region bounded by the following stars, viz: h'h'2, K"K", I'I", H"H", W', U', and the trapezium, it was with great interest that I was able to trace, under a fine sky, very evident narrow wreaths especially crossing the line I'I" to W'. All of them could not be accurately located in the sketch on chart M; but the general aspect of long narrow wisp-like bright streaks, "Streifen," some of them 10' or 15' long, alternating and intertwined darker channels were very plainly seen and could not be overlooked after the attention had once been arrested upon the feature. Between K"K" (H"H"?*) and W' are as many as four dark streaks, and three or four, if not more bright wisps. The latter are rather more readily distinguished than the darker tracings.

The evidence afforded by the fine view of this evening leaves no doubt whatever of the structure of the nebula. It is impossible now to see it in any other aspect than as a maze of radiating spiral-like wreaths of nebulosity or filaments, tentacles; the centre of the vertex being about the trapezium. I think that the brightest region of all, that surrounding the trapezium, looked at in the light afforded by this disposition of the more distant nebulosity, shows itself to be nothing else than an intricate convolution of similar wreaths. T. H. S., who looked at the nebula, compared it to an appearance of auroral rays streaming up and out from the trapezium in an ascending spiral. The trapezium at the vertex, like a watch-spring, with its outer turn horizontal and centre drawn down, seen from above. H. P. T.* compared radiation to smoke from burning weeds when wet.

N. B. The character of the convolutions can be represented best by dropping a little ink in a tumbler of water; after the ink has diffused itself for eight or ten minutes the resemblance is exact, as to the aspect of the filaments. Earlier, they are too well defined and too narrow and threadlike.

N. B. The star R" is to be added to those having brushes of nebulosity appended on their southern sides.

1861, Feb. 28. A few glimpses between clouds. Noticed an interesting feature in parallel preceding trapezium. The bright striae from H", (K"?), 12th, can be traced as indicated on chart M. to the star under M (as in margin) in whirl towards trapezium. [The star under M (M'?) appears to be O' = H. 70.] 1861, March 4.

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Definition good; sky not always free from clouds. The striae between I"I", H"H", W', and U' well seen. Traced connection of light from d', as it passes between F' and G' across the sweep of the curve (from G' to below F') in continuation with a large bright mass directed from trapezium towards E. I also trace more alternating bands from C towards T.

1861, March 5. A very clear night. Definition is not better than usual, though good. A dark channel passes from F' between D' and H", and can be traced 12' beyond C', and D' and is quite distinct. The dark channel directed towards the small star nearest n. f. F' nearly in Dec. of c Orionis, has well-defined straight parallel edges, clean cut. I draw only the bright and dark striae which are to be superimposed on the general shading of the nebula. The sudden sweep of the striae in the region n. p. E' is well and certainly recognized. 1861, March 7. Very cold, but vision quite good, considering the low temperature. One of the clearest of winter nights. Noticed another instance of nebulosity radiating to southward from star of 6th (?) Mag. located on map M. near stars g1, g2, g, and 12' south of apex of light. This is perhaps a continuation of nebulosity from H"H", h"h", but the star is too bright to trace the connection. Dark channels of irregular width and tortuous, run up on either side of the nebulosity, trending from this star a little to left of Orionis. The dark canal was traced from t', past W' to almost as far beyond W' as W' is from trapezium.

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1861, March 11. Night perfectly clear, definition very fine. Traced the terminus of bright nebulosity on following side of trapezium towards K"K". The brightness continues without much diminution to line from I"I" to W'. It is plain that the termination of the brighter nebulosity on the n. p. side of the nebula is the bright mass indicated on chart M. which sweeps around E' to D'.

The opening south of E', F', to D', H", K", is not at all dark, it is rather indicated by its outlines than by deficiency of light. Whereas beyond the sweep preceding D', E', the falling off is abrupt and decided. There is, however, a distinct continuation of the nebulosity of the region s. f. E to just two fields (24′) preceding S, where it sweeps up.

The long wreaths from the region P", Q", to M' particularly well seen and delineated on chart M. There

* H. P. Tuttle, Esq., now Assistant Paymaster U. S. N.

is plainly a long stria made up of several filaments, having a general origin near the bright mass close preceding M', which extends almost to the parallel of L". The most interesting observation of the evening was the aspect in which the whole bright region close about the trapezium presented itself as a mass of wisps, curling out towards the eye, and to the left in short tufts curving back. I associate plainly into one group of wisps all the bright regions on the preceding side of the trapezium, even the abrupt vertical ray B [the preceding or W. boundary of the Huygenian region] is clearly connected as a whirl from the ray, shooting out more faintly towards P".

1861, March 20. Was satisfied of the whirl character of bright masses s. p. trapezium. 1862, Jan. 31. Notice that both c1 and c2 Orionis are decidedly nebulous, i. e. have well marked aggregations of nebulosity about them. There are also two other stars within 4' or 5' of them having a similar character. The spirality is most distinct in the region preceding trapezium, and in same Dec. or near it, say 3' either side.

1862, Feb. 18. Clark's 18 inch object-glass. Good definition, calm and clear. Temp. about 32° [Fahrenheit]. Obtained a very fine view of the nebula with the great object-glass. The cirrus-like filaments sweeping outward from the Regio Huygeniana perfectly distinct, far more so, indeed, than with our 15-inch object glass, and entirely unquestionable as to their wisp-like curve or spirality in the sense attached to this term by Lord Rosse. The filaments were more numerous, distinct, and cirrus-like than I have before seen them. In the R. Gentiliana, Derhamiana, and Picardiana, this disposition was very strikingly evident. The R. Huygeniana was resolved into a confused assemblage of wispy masses verging on resolution. The region south of R. Subnebulosa towards, and following Sinus Gentilii, abounds with very small stars. The nebulous light shooting southward from the bright stars in the R. Subnebulosa, more especially the preceding one, are very plain, also the nebulosity of the nebulous star s. of R. Gentiliana has same tendency very plainly seen. As the result of this view I was entirely confirmed in my previous impressions of the spiral structure of this nebula..

1862, March 26. [Group of stars AR. 0' Dec.-16' noted as strongly nebulous.]

1862, March 27. N. B. That nebulosity tends to aggregate about the groups of small stars. That the smallest stars in the bright masses of nebulosity about the trapezium are easily seen in strong twilight, and before others in darker regions come into sight, although when the sky becomes dark the latter are much more easily seen. This shows that the small stars near the trapezium are really much brighter than they appear to be, their light being commonly overpowered by that of the nebula. This fact is important as evidence of a clustering of stars about the bright nebulous regions.

1863, Jan. 18.

1863, Jan. 19.

Bridge over Sinus Magnus easily seen, with brightening up in the middle.

The bright ridge (in parallel in Sinus Magnus) was distinct.

1863, Jan. 25. The dark streaks from the vicinity of R" towards L' are very remarkable. Region about H" has a strong appearance of resolution.

1863, Feb. 7. 3h 1m. Outline of nebulosity prec. tr. just visible. [Later in evening] in the field following in Dec. of L'"', bringing L'" to preceding edge of field, are fifty stars, large and small. All this region, and thence to H"H" and between N'" and Y" is strewed thick with very small stars which could be kept steadily in view. There are hundreds visible, not on charts. North prec. T' are great quantities of stars; evidently the mass of stars is in line of AR. of 6'. But the field following T has many very small stars. The large starless areas two or three fields following S"" are remarkable in contrast. I several times found fields destitute of the minutest star under the finest definition. 2 fields following the bright stars S and V' is a field