rr jpw^ J^SL A i yt, lf ^r1H -X ! - University of California. T I FT OK THE NEBULA IN ORION ,:,!) L863 WASHINGTON ASTRONOMICAL OBSERVATIONS FOR 1878 APPENDIX I. MONOGRAPH OF THE [R. A. 5 h 2 8 m 2 4 8 .o; N. P. D. 95 29' io". 9 ; 1 860.0.] BY EDWARD S. HOLDEN, PROFESSOR OF MATHEMATICS, U. S. NAVY. WASHINGTON: GOVERNMENT PRINTING OFFICE. 1882. TABLE OF CONTENTS. Pag INTRODUCTION Object of the research System of nomenclature adopted Catalogue of stars Index-chart of the central parts List of the drawings referred to ' List of the memoirs, etc., on the nebula of Orion List of telescopes employed by the various observers PART I. History of the various researches on the nebula of Orion, in chronological order CYSAT'S discovery of the nebula (1618) HUYGHENS' observations (1656). See Fig. i HOOKE'S observations (1666) HUYGHENS' observations (1694). See Fig. 2 / . . MAIRAN'S observations (1731). See Fig. 3 PICARD'S observations (1673). See Fig. 4 LONG'S observations (1742). See Fig. 5 : LEGENTIL'S observations (1758). See Fig. 6 : Washington drawing of 1877, February 3, for comparison with early figures. See Fig. 7 : Comparison of the uncorrected outlines of HUYGHENS', PICARD'S, and LEGENTH/S drawings. See Fig. 8 . : Comparison of the corrected outlines of these three drawings. See Fig. 9 : MESSIER'S observations (1771). See Fig, 10 ' W. HERSCHEL'S observations (1774-1811). See Fig. n ' Discussion of the variations in form, etc., according to HERSCHEL ; Drawing by Mr. J. E. KEELER (1878) for comparison with HERSCHEL. See Fig. 12 ; Situation of the trapezium with regard to the adjacent nebulosity, according to all the early authorities . . ; LEFEBVRE'S observations (1779). See Fig. 13 : SCHROETER'S observations (1794-1798), See Figs. 14, 15, 16, 17 ; Discussion of SCHROETER'S observations ' BOOK'S observations (circa 1800). See Fig. 18 < FLAUGERGUES' observations (1802) * J. HERSCHEL'S observations (1824). See Fig. 19 POND'S observations (1826) ! LAMONT'S observations (1837). See Fig. 20 ! J. HERSCHEL'S observations (1837). See Fig. 21 ! DE Vico's and RONDONI'S observations (183^-1 841). See Fig. 22 ! KAISER'S observations (1844). See Fig. 23 COOPER'S observations (1847?) ! LASSELL'S obserTations (1847). See Fig. 24 W. C. BOND'S observations (1848). See Fig. 25 LIAPONOFF'S and STRUVE'S observations (1847-1851). See Fig, 26 LASSELL'S observations (1854). See Fig. 27 ' SCHMIDT'S observations (1860-1875). See Fig. 28 SECCHI'S observations (1862). See Fig. 29 TEMPEL'S observations (1862). See Fig. 30 LASSELL'S observations (1862). See Fig. 31 3 4 TABLE OF CONTENTS. PART I Continued. Page. STRUVE'S observations (1863) 77 WEBB'S observations (1863-1876) 78 BIRD'S observations (1866) 81 G. P. BOND'S observations (1859-1865). See Frontispiece and Fig. 32 8r Lord ROSSE'S observations (1867). See Fig. 33 85 SECCHI'S observations (1868). See Fig. 34 91 D'ARREST'S observations (1872). See Fig, 35 98 WINLOCK and TROUVELOT'S observations (1874). See Fig. 36 104 TROUVELOT'S drawing (1875). See Fig. 37 104 DOBERCK'S observations (1877, 1878) 104 TROUVELOT'S observations (1876?) 104 LANGLEY'S observations (1879). See Fig. 38 . .^ 105 PART II. Washington observations of the nebula of Orion, in chronological order (from 1874, January u, to 1880, January 29) 108 Synopsis of the preceding detailed observations 167 Collection of the separate results of observations of stars 179 Stars within the trapezium 179 STRUVE'S variable stars 180 Observations of variable stats 181 Synopsis of the more important measures made at Washington 184 Comparison of the positions of nebulous masses, as determined at Kasan and Washington 190 Reduction of photometric observations made at Washington igi I. Results from eye observations of the order of brightness of the principal masses 192 II. Observations with HASTINGS' photometer 191; Description of the instrument. See Fig. 39 ig6 Results from these observations 197 PART III. Summary of all the observations (1656-1880), which have previously been given in detail 200 Spectrum of the nebula, according to HUGGINS, LESUEUR, VOGEL, SECCHI, and D'ARREST 221 PART IV. Conclusions 224 ADDENDUM. Dr. DRAPER'S photographs ot the nebula in Orion (i88o-'8i-'82). See Fig. 40 226 NOTE. The important results obtained by Dr. HUGGINS from his photographs 01 the spectrum oi this nebula were known too late for insertion in the text. MONOGRAPH CENTRAL PARTS OF THE NEBULA OF ORION. The main object of this memoir is to leave such measures and descriptions of the brightest parts of the nebula of Orion as shall enable another person observing in after years with the same telescope, under like conditions, to say with certainty whether or no changes have occurred in these parts of this nebula. The brightest parts are chosen so as to avoid as far as possible any uncertainty in the conclusion then to be reached, and also because there is little to be added to the complete observations of Lord ROSSE and of Gr. P. BOND on the fainter portions. A second and an important object is to make a thorough discussion of the vast mass of material now on hand. The brilliant success attained by Dr. HENRY DRAPER in his photographs of this nebula leads to the hope that photographs may in the near future largely take the place of eye-drawings for such objects as comets and nebula?; the present is an appro- priate time for a resume of all observations made by the old methods. A complete research on this nebula might treat of the following questions: I. Its distance from the earth. II. Its connection with the stars contained in it. III. Its physical constitution. IV. Its proper motion. V. The variations in the shape of its parts. VI. The variations in the brightness of its parts. VII. Its possible rotation as one mass. VIII. The possible rotation of one or more of its parts. IX. The proper motion of one or more of its parts. The present memoir affords evidence relating to II, IV, V, VI, VII, VIII, and IX. II has also been treated in the Washington Observations for 1877, Appendix I, "On the Multiple Star 2. 748," where I have discussed a most complete set of measures of the six stars of the trapezium made by Professor HALL, The object to be attained could not have been reached by adding another drawing to the many excellent ones we now have, and my original plan of making micrometric and photometric measures of the various masses has been carried out without much change. All the observations at Washington have been made with the 26-inch Clark MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION, refractor, with magnifying- powers from 130 to 600 diameters. A full description (with plates) of this telescope is given in Washington Astronomical Observations, 1874, Appendix I. A view of it is given in the accompanying wood-cut from ANDRE and RAYET'S Astronomic Pratique, kindly furnished by M. GTAUTHIER-VILLARS. Probably no object outside of the solar system has received more attention from the best observers than the nebula of Orion. Before discussing the obser- vations of so many astronomers, each of whom has applied his own pecu- liar notation to the various parts of this nebula, it is necessary to fix upon one system of nomenclature which shall be used uniformly throughout the work. For the stars no doubt can arise as to the proper system to be adopted, as the Catalogue of Stars in the Nebula of Orion, published by Gr. P. BOND in vol. v of the Annals of the Harvard College Observatory, is bv far the most full that we possess, and is likely to remain so for many years. I have uniformly adopted the nomenclature of all stars in the neb ula from that great work, and while, in quoting from other authorities, I have given in most cases the letter or number of the star from the original source, I have added the synonym from BOND in square brackets, thus: [G. P. B., No 685, etc.] or simply [685]. I give, immediately following, a catalogue of all the stars referred to in the subsequent pages reduced to 1877.0 from BOND'S elements. This catalogue forms the basis of the present work. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 7 Catalogue of Stars in the Central Part of the Nebula of Orion for 1877.0. BOND. Mag. HERSCHEL and STROVE. BOND'S let- ter in his Zones. HERSCHEL. W. C. BOND. LASSELL. LlAPONOFK. 1877.0. Ad 1877.0. 479 IO.O 35 F' 3 56 y - 400.8 + 2 72" 5 523 10. I 45 P" r 5 40 / - 243.1 116.2 558 10.7 50 Q" 9 39 V - 159-7 -- 119.0 *5&7 13.9 51 I L 10 . . - 103.4 - 8.7 570 9-4 53 R" a 13 33 n 96.0 - 273.6 573 13-9 54 r*' . 12 /35 i - 87.4 - 1/9.4 *575 11.9 57 r" . II 45 85.4 22.7 581 14.2 rt' . 26 9i' 4- 22.0 + 37-4 *6<|8 14-3 X . 4- 23.6 - 9-3 *6si I3-I 0^75 i/) . 27 + 28.8 + 47-0 *6s2 13-9 76 c / 32 . f" 4- 30.0 4- 170.8 *6 5 4 12.3 78 u 31 4- 32.6 4- 9.2 *6 5 7 I3-I II . 7 80 84 s, 6 w 33 37 4 & + 39-4 + 55-1 4- 164.4 4- M6.3 *666 **/ 13.9 81 Xs" 30 . + 58.7 196.6 669 *6 7 i 9 .8 8? 88 Q' v 39 41 10 18 k 4- 62.9 4- 69.0 4- 99-2 25.2 *675 15-2 13. i ad 88 Y 43 a k K + 73-7 + 77-9 - 94-2 - 28.4 *6 7 7 *68i 685 *6S6 14.8 14.8 8.3 15.6 89 93 Xa" n Y" z e 34 45 44 26 e + 77-6 + 90.1 + 96-9 4- 99-0 202.2 4- 172.4 - 95.8 40.0 X-AQO 4- 105.0 19.0 *688 707 708 15.6 II. 2 9.6 .103 101 A" A" B" B" C 49 50 27 23 a f 4- 150.2 4- 150.6 - 254.5 ; 99.5 12. ^ IOO b" i>i" G 51 H 4- 152.1 - 137-4 724 741 746 784 88g j 10.5 IO.O 10.8 10.8 ii. 3 104 no in 123 142 C" C" E" E" "H" H" K" K" E K (*) 55 61 64 78 94 25 19 29 24 17 h n + 182.3 4- 225.1 4- 231.3 + 387.2 4- 800.4 - 177.0 - in. 7 - 585-0 - 287.4 256.0 -.*- C'T'Tl I* X* V The magnitudes are from G. P. BOND'S estimates. 8 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. It is equally necessary that a rather minute system of nomenclature should be adopted to distinguish the various bright masses, dark channels, spirals, etc., of the central portion, and the Index-Map, herewith, gives the nomenclature uniformly used throughout the present work, not only in referring to my own observations of 1874, '75> '7 6 > '77? '7^ '79? an( i l88o > b ut m tne discussion of the work of others. It is necessary to say a few words in regard to the nomenclature there adopted, as it is not all that could be wished. During a visit of M. TROUVELOT, formerly of the Harvard College Observatory, to Washington, he made, in connection with myself, the drawing of the central part of nebula Orionis which is reproduced in Appendix I, Washington Astronomical Observations for 1874. This was only a preliminary sketch, but it gave an idea of what could be seen with the 26-inch refractor. On his return to Cambridge I requested him to prepare a lithographed skeleton map of the central portions of the nebula, on which I proposed to insert letters, figures, etc., to designate the various bright masses, dark channels, etc. I intended to choose these symbols so as to preserve, not only the nomenclature proposed by Sir JOHN HERSCHEL in Memoirs of the Royal Astronomical Society, vol. ii, which has been adopted and added to by subsequent investigators, but also the nomenclature of the bright masses (<*, /?, 7, etc.) given in Lord ROSSE'S memoir of 1868 (Phil. Trans., 1868, p. 57), and in LIAPONOFF'S Memoir published by STRUVE in Memoires de V Academic Imperiale des Sciences de "St. Petersbourg, vol. v, 7th series, 1862. The brilliant labors of Lord EOSSE, LIAPONOFF, and STRUVE in this field demanded that the nomenclature adopted by them should not be lightly changed. On the return of the lithographed charts (which were executed by M. TROUVELOT at his own expense), however, I found on them a system of letters and numbers excellent in itself, -but varying from the nomenclatures of ROSSE, LIAPONOFF, and STRUVE. These charts were put into immediate use in my own work, and copies of them were sent to Dr. DOBERCK, Mr. PRITCHETT, Lord ROSSE, M. OTTO v. STRUVE, Dr. SCHMIDT, M. TEMPEL, M. TISSERAND, Dr. VOQEL, Dr. WINNECKE, and others ; and some of these were at once used in comparisons with the nebula by these astronomers. They were constantly used in my own work, and thus almost unavoidably a nomenclature was adopted which did not fulfill all the prerequisite conditions. Added to this was the fact that the nomenclatures adopted by former astronomers were not then as familiar to me as now, as it was my constant endeavor while the actual work was in progress to keep my mind as free from bias as possible, and to avoid too great familiarity with previous work. That this process, while advantageous from a purely scientific point of view, has also its disadvantages, the preceding remarks will show. I have seriously considered the question of changing my nomenclature through- out on these accounts, but the fear of introducing error, and the chance that these charts may also be used by the astronomers to whom they were sent has deterred me, and the index-chart herewith remains substantially as it has been during the series of observations. I have, however, added the nomenclature of LTAPONOFF in many cases, distinguishing his letters by inclosing them in a right angle. Lord ROSSE'S Greek letters are underscored in the index-map to distinguish them from M. TROUVELOT'S. U t III. with th i LON> ; w. } 1824 1826 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. DESCRIPTION OP THE INDEX-CHART. The stars are laid down from BOND'S Catalogue, and the numbers are throughout those of BOND. The stars inclosed in circles are those suspected by STRUVE to be variable, and were so distinguished in order to attract the eye, except the star h. Those stars in triangles were stars laid down by LASSELL, which my own early obser- vations had not verified. The letters, numbers, etc., were laid down by M. TROU- VELOT, and are very convenient for the purpose for which they were intended. I have kept the nomenclature of HERSCHEL, Sinus magnus, regio Huygheniana, etc., as it is now classic, and I have added 1 but one such term, and this was done almost by inadvertence. The sharp following point of 6 I have called " Spitze." The dimensions of the various masses in the Index-Chart are not strictly accu- rate, although nearly so. For accurate dimensions recourse must be had to my measures in Part III. The index-chart is simply intended as a key to the system of nomenclature, and to make verbal descriptions intelligible. The principal drawings referred to, with their dates, are given in the following list, in which the order is the same as that adopted in the subsequent discussion : Observer. Date. In what published, etc. Figure in this work. HUYGHENS . . 1656 I PlCARD .... HUYGHENS 1673 1694 Traite" de 1'Aurore Bore"ale, p. 248, and Hist, de 1'Acad. Roy. des Sci- ences, 1759, p. 435. 4 2 MAIRAN .... 1731 1742 Traite" de 1'Aurore Bore"ale, p. 249, and LALANDE'S Astronomic, p. 272 3 c 1758 6 1771 10 W. HERSCHEL . 1774 and later, I77Q P. T., 1811, p. 320, and MSS. in possession of the Royal Society of London. ii 19 I7OA 14 I "707 '08 15, 16, 17 BODE .... 1800? 1802 Anleit. z. Kenntniss d. Gest. Himmels, p. 166 and Plate i . . . . 18 1824 19 POND .... J. HERSCHEL . LAMONT .... 1826 1837 1837 TQ/1 "3 Mem. R. A. S., ii, p. 93 Obs. Cape of Good Hope, pp. 25 et seq Ueber d. Nebelflecken, p. 23, and MS. drawing kindly communicated by Dr. DOBERCK. 21 20 Io3r 22 1039 rQ AT 23 1844 24 LASSELL .... W. C. BOND . . 1847 1848 Mem. Amer. AcacL, vol. iii, p. 87, and Annals H. Coll. Obs'y, vol. 5 . 25 27 SCHMIDT . . . SCHMIDT . . . !54 i86o-'75j 1861 ] Two MS. unpublished drawings, courteously communicated by Dr. SCHMIDT. \ " APP. V- 10 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. The principal drawings referred to, with their dates, etc. Continued. Observer. Date. In what published, etc. Figure in this work. B. B. STONEY . . LIAPONOFF . . . 1851 1847 > Unpublished drawing. A photograph of this has been kindly given me by Lord ROSSE. 26 O. STRUVE . . . 1861 ) T8A/1 2Q SECCHI .... rQAo ? 3 LASSELL .... 1862 Unpublished drawing, on a large scale, made by Miss CAROLINE LAS- SELL, at Valetta, of which a full-sized fac simile has been most kindly communicated by the artist. 3i Frontispiece T QA*7 and Fig. 32. M 1868 Firenze Ital. Soc. Mem., vol. i, $d ser., pt. 2 14 rRT? e WINLOCK . . . TROUVELOT . . . | 1874 | Ast. engravings from Harvard College Observatory, and Annals Har- vard College Observatory, vol. viii. | 36 17 LANGLEY . . . 1075 1879 l88o MS. observations, kindly communicated by Professor LANGLEY, with the permission of the Superintendent of the U. S. Coast Survey. From photographs taken in 1880 38 4 I add here a list of writings on the nebula of Orion, reprinted with additions from my Index- Catalogue of Books and Memoirs relating to Nebulce Clusters, etc. Wash- ington, 1877. (Smithsonian Miscellaneous Collections, No. 311.) LIST OF THE MORE IMPORTANT BOOKS AND MEMOIRS RELATING TO THE NEBULA OF ORION. ARAGO : C. R., xiii, p. 450. : C. R., xxvi, p. 50. [Remarks on RONDONI'S drawing.] [BOND'S drawing.] BARNEBY : Mon. Not. R. A. S., vol. xxxiv, p. 248. [Variability of 6th star in trapezium.] BESSEL: B. J., 1808, p. 122. [CYSAT knew of the nebula of Orion.] BISHOP: [HIND] : BISHOP'S Astron. Obs., i839-'5i, p. 12 ; 1852. [Small star near 6 Orionis; mag. 13,^ = 1260.9(3); J=2 ".8(i). BODE: Anleitung z. Kenntniss des Gestirnten Himmels, p. 166, Plate i, p. 556. [Two drawings.] : Himmelskarten, Tafel 30. [Drawing.] BOND, G. P.: Annals Harvard College Observatory, vol. v, 1867. [With two steel engravings and two charts.] 4. : Mon. Not. R. A. S., xxi, p. 203. [Spiral structure.] : Mon. Not. R. A. S., xxiv, p. 177. BOND, W. C. : Description of the nebula about Orionis, Mem. Am. Ac. Arts and Sciences, vol. iii (1848), p. 87. [With steel engraving.] : Proc. Am. Ac. Arts and Sciences, i, p. 325. [Observations.] : Same volume, p. 342. [Resolvability of nebula of Orion.] See also Am. Jour. Sci., 2d series, iv, p. 427. CARPENTER and STONE : Mon. Not. R. A. S., xxiv, p. 92. [On G. P. BOND'S drawing.] CASSINI, J. D. : De Cometa Anni i652-'53. [Discovery of the 4th star in Orion's trapezium, etc.] : Decouverte de la lumiere celeste qui parait dans le Zodiaque. [Suspects nebula of Orion to be a star cluster.] See DELAMBRE. Hist, de 1'Astr. Mod., vol. ii, pp. 700, 709, 744. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION, 1 1 CYSAT: CYSAT, der erste Entdecker des Orions-Nebel. (1619.) [R. WOLF, 1853.] : Mathemata astronomica de loco cometae qui sub finem anni 1618, etc. D'ABBADIE : Mon. Not. R. A. S., xvii, p. 245. [PORRO'S new star in trapezium.] D'ARREST r^Undersogelse over de nebulose Stjerner, etc. 1872. 40. [With drawing of the nebula and detailed memoir.] : A. N., Ivii, col. 341. : A. N., hex, col. 337. [Notice of LEFEBVRE'S drawing.] : Om den store Orionstage. Kjobenhavn. Dansk. Vid. Selsk. Oversigt. (1867), pp. 236-241. [This paper relates to the connection between the <, 0, and c Orionis nebulae.] I. i and e are connected by two nebulous streaks : A. The co-ordinates of the middle of the first are J_6 S % -72-, -8o 8 , -8 4 8 , -85% -85% -83% -73-, -46', -28-; Ad 630", 720", 810", 900", 990", 1170", 1260", 1350", 1530", 1620". B, Those of the second are Ja-34 8 , -i6 8 , +3", +9% + i6 8 , +21"; A3 660", 810", 1180", 1 210", 1360", 1650". Professor SAFFORD has proposed for the whole system the name Corona Herschelii ; D'ARREST proposes for A the name Semita Bondiorum ; the name Paeninsula Othonis Struvii is proposed for a region there described. II. A threefold connection of 6 and c Orionis is described 1. Aa 49% 72% 79 s , 8i 8 , 68 s 46"; J 5+430", +720", +900", +1130" +1440", +1780". 2. The middle and faintest band is described. 3. The 3d is also described. The name Paeninsula Cysati is proposed for the region the co-ordinates pf whose middle point are A a = 56", A d = + 490". ; See DOBERCK. DAWES: Mon. Not. R. A. S., viii, p. 31. [New star.] DENNING : A. N., Ixxx, col. 299. [Ten stars in and near trapezium.] See SALTER. DE Vico : Mem. Oss. Coll. Romano, 1839, p. 31, Plates i and ii. [Drawing; new stars in trapezium.] : Same, 1840-41, p. 22. [Plate by RONDONI.] : C. R., xiii, p. 449. [Note on RONDONI'S drawing.] DOBERCK : A. N., xci, col. 335. [Remarks on COOPER'S drawing.] : Nature, vol. xvii, p. 311. [D'ARREST'S work on nebulae.] DOPPELMAYER: Himmels-Karten, Blatt 26. ENGELMANN, R. : Messungen 90 Doppelsternen, etc., p. 147. [Variability of stars.] FAYE: C. R., vol. Ix, 1865, i, p. 468. [Remarks on SECCHI'S observation of the spectrum of the nebula in Orion.] FLAUGERGUES : C. T., 1802 (An xi), p. 361. [Observations.] : Mem. de PInstitut, i (An vi), 1798, p. 106. GILL: Mon. Not. R. A. S., xxvii, p. 315. [Stars within the trapezium of Orion.] GLEDHILL: The variable (?) star h No. 78, near the trapezium of Orion. Observatory, 1880, p. 601. GOLDSCHMIDT: A. N., lix, col. 31. HAHN, VON: B. ]., 1797, p. 157; B. J., 1799, p. 235 [HALL] : Wash. Ast. Obs., 1877, App. I. Observations, etc., of S. 748. HERSCHEL, J. : Results of Astronomical Observations at the Cape of Good Hope, p. 25. [With a plate.] : Account, etc., of the nebula of Orion. Mem. R. A. S., vol. ii, p. 487- [ with plates.] : Mem. R. A. S., vol. iii, p. 189. [Fifth star of the trapezium.] See also same vol., p. 187. HERSCHEL, Capt. J. : Proc. R. S., vol. xvi (i867-'68), pp. 417. 45 1 - [Observations of spectrum.] HERSCHEL, W. : MSS. in possession of Royal Society, London. [Unpublished observations.] See Mem. R. A. S., vol. xxxv, p. 52. [There is an erratum in the Mem. R. A. S.: For 1780. 521 read 1780. 134 and for 22".4i read 22". 521.] : refers to the nebula of Orion in P. T., 1782, p. 129; 1785, p. 258; 1789, p. 249; 1791, pp. 7 2 > 75, 77; 1811, pp. 276, 320; 1814, p. 258. I2 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. HOLDEN: Mon. Not. R. A. S., vol. xxxvii, p. 231. [List of drawings.] Washington Astronomical Obs., 1874, plate vi, fig. 4. [Drawing.) Washington Astron. Obs., 1877, Appendix I. [Discussion of HALL'S observations of 2. 748.] HOOKE Micrographia, London, 1665, p. 242. [Discovery of the 4th and 5th? stars in trapezium.] HUGGINS : On the spectrum of the great nebula in Orion. Proc. R. S., xiv, 1864, p. 39 ; also, 1865, Jan. 26. Ditto, ditto. Proc. R. S., xx, 1872, p. 379. Ditto, ditto. Proc. R. S., xxii, 1873, p. 251. Mon. Not. R. A. S., xxvi, p. 71. [Nine stars in trapezium.] Am. Jour.^Sci., 3d series, v, p. 75. P. T..^i868, p.^541. [Spectrum.] HUYGHENS : Systema Saturnium. 4. 1659. [Drawing.] See KAISER. KAISER F. : Amster. Tijdsch. v. Wiss. en Nat. Wetens, i, 1848, p. 7. [HUYGHENS' drawing, 1694.] De Sterrenhemel, vol. ii, Plate 3, pp. 538, 542. [Original drawing.] LALANDE: Astronomic, i, p. 272. [With a figure, MAIRAN'S.] LAMONT : Ueber die Nebelflecken. Munich, 1837. 4. [With a plate.] LAPLACE: Exposition de la Systeme du Monde, p. 452. [Opinion that nebulae change.] LASSELL, W. : Observations of the nebula of Orion, etc. Mem. R. A. S., xxiii (1854), p. 53. [Plate.] Proc. R. S., xvi, p. 322. [Measures of stars.] A. N., xxxv, col. 386. Mon. Not. R. A. S., xiv, p. 74. Mon. Not. R. A. S., xvii, p. 68. [Relative visibility of 5th and 6th star in trapezium.] Mon. Not. R. A. S., xxii, p. 164. [New star in trapezium.] Mon. Not. R. A. S., xxix, p. 165. LEFEBVRE: ROZIER Obs. sur la Physique, xxii, 1783, p. 34. [With drawing.] LEGENTIL: Remarques sur les Etoiles ne"buleuses. Hist, de 1'Ac. Roy. des Sciences, 1759, p. 453 [ with several figures]. LESUEUR: Proc. R. S., xviii, pp. i, 242. [Spectrum.] : Proc. R. S., xix, p. 18. [Spectrum.] LE VERRIER: C. R., vol. xliv, 1859, pp. 1074, 1293-5. [PORRO'S new star in nebula of Orion.] LIAPONOFF : See STRUVE. : Mon. Not. R. A. S., vol. xxiii, p. 228. [Review of his memoir.] LONG : Astronomy, vol. i, p. 321, Plate 67, Fig. 96. [Observations and drawing.) MAIRAN : Traite de 1'Aurore Boreale, [p. 249; nebula Orionis varies in shape ; date of PICARD'S drawing given as 1673, March 20. MAIRAN'S drawing (Fig. xxvii), 1727-1733.] MESSIER: Nebuleuse d'Orion. Hist, de 1'Acad. R. des Sciences, 1771, pp. 435, 458. [Drawing.] NICHOL: System of the world, 1846, p. 55. [Lord ROSSE'S observations.] NOBILE : Osservazione del systema 748 2". Rendiconte d. Ac. d. Sci., 1877, May, No. 5. POND: On an appearance hitherto unnoticed in the nebula of Orion. Mem. R. A. S., iii, 1826, p. 93. [Recession of the nebula from the stars.] (See also same volume, p. 187, for an observation of J. HERSCHEL and RAMAGE on this point.) PORRO : Mem. dell'Osserv. Coll. Romano, i856-'57, p. 3. [Discovery of a new star in trapezium.] : A. N., xlvi, col. 171. [Same.] : C. R., xliv, p. 1031. [Same.] ROBINSON : Nature, vol. xv, p. 292. [Note on the resolvability of the central part of nebula Orionis.] RONDONI : See DE Vico. ROSSE (Fourth Earl of) : Account of observations on nebula of Orion, 1848-1867, P. T.; 1868, part i, p. 57 [Plates.] For a review of this, see O. STRUVE in V. J. S., 1870, p. 25. : Mon. Not. R. A. S., xxix, p. 165. SALTER : A. N., Ixxx, col. 299. [Ten stars in and near trapezium.] SCHMIDT : A. N., vol. xciii, col. 78. [Places of G. P. B. Nos. 746, 784, 822. 822 is certainly variable; at least between 9.7 and 13 mag.] SCHROETER : Aphroditographische Fragmente, p. 248. [Has a chart and memoir.] : Beytrage zu den neuesten astron. Entdeckungen, vol. iii, p. 429. [With figures.] : B. J., 1797, p. 198. [Observations.] MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. SCHROETER : B. J., i8oi, p. 128. [Changes in nebula of Orion.] SECCHI Mem. dell'Oss. Coll. Romano, i852-'56, p. 80, p. 92, and Plate v. [Drawing.] Mem. dell'Oss. Coll. Romano, i856-'57, p. 3. [New star in trapezium.] Bull. Meteor, d. Coll. Romano, 1865, January. Ace. d. Nuovo Cimento, serie 2 a , vol. v-vi, 1872, p. 20. [The solar Corona is brighter than the nebula of Orion.] Atti dell'Ac. d. N. Lincei, Anno xxv, sess. iv, 1872, p. 226. [Spectrum.] Sulla grande nebulosa di Theta Orione. 1868. 4. Mem. Ital. Soc. Firenze, vol. i. [Memoir; drawing; spectrum.] A. N., xlv, col. 60. [Sketch of nebula of Orion.] Mon. Not. R. A. S., vol. xviii, p. 8. Mon. Not. R. A. S., xxv, p. 153. [Spectrum.] Mon. Not. R. A. S., vol. xxviii, p. 162; xxix, p. 165. C. R., xliv,;p.~i279, and xlv, p. 170. [PORRO'S new star.] C. R., Ix, pp. 460,^543. [Spectrum.] C. R., Ixv, p. 63. C. R., Ixvi, p. 643. 1868. [Spectrum, etc.] Sugli Spettri Prismatici. Mem., i, ii, iii. SENARMONT: C. R., xliv, pp. 1075, I2 94- [PORRO'S new star.] SMITH : Opticks. 4. [HUYGHENS' drawing.] STONE, E. J., and CARPENTER : Mon. Not. R. A. S., xxiv, p. 92. [On G. P. BOND'S drawing of nebula of Orion.] STRUVE, O. : Obs. de la grande nebuleuse d'Orion, avec 4 planches. Mem. de 1'Acad. Imp. des Sciences de St. Petersbourg, tome v, No. 4, 1862. See M. M., ii, p. 517. [Abstract of above memoir.] Bull, de la Classe Phys.-Math. de UAcad. Imp. de St. Petersbourg, xvi, 1858, col. 113. M. M., iii, p. 535. [Observations at Malta.] M. M., iii, p. 550. [Variability of nebula of Orion.] Mon. Not. R. A. S., xvii, p. 225. [Stars.] Bestimmung d. Constante der Praecession, p. 40. [Proper motion of 6 Orionis.] V. J. S., 1870, p. 25. [Review of the memoir of Lord ROSSE.] W. : Rapport sur les observations de LIAPONOFF sur la nebuleuse d'Orion. Bull, de la Classe Phys.-Math., vol. xii, p. 316, and Melanges Math., ii, p. 45. Catal. Nov. Stell. Duplic., 1827, p. xiv. [Discovery of 5th star in trapezium.] Also, p. 242. [Sys- tem of Orionis.] TEMPEL : A. N., Iviti, col. 240. [Drawing.] : A. N., Ixxx, col. 29. [Trapezium.] : Unpublished drawing, made in 1876. TISSERAND : Bull. Inter. Obs. Paris, 1876, No. 119; also, C. R., Ixxxi, April 17, p. 891. TROUVELOT : Annals Harv. Coll. Obs'y, vol. viii. [Drawing.] : Wash. Ast. Obs., 1874, Appendix I, Plate vi, Fig. 4. [Drawing.] Vico : See DE Vico. VOGEL, H. C. : A. N., Ixxviii, col. 245. [Spectrum.] Also, Bothkamp Observations, vol. i, p. 56. VON HAHN : B. J., 1797, p. 157. -: B. J., 1799, p. 235. WEBB : Intellectual Observer, vol. xii, p. 258. [History.] : Mon. Not., R. A. S., xxvi, p. 208. [Account of his drawings and observations.] WINLOCK : Astronomical Engravings from the Observatory of Harvard College, Plate 24. [Drawing of central part by TROUVELOT.] See Annals Harv. Coll. Obs., vol. viii. WINNECKE : Melanges Math., iii, p. 499, and Bull, de 1'Ac. Imp., vii, p. 18. : Mon. Not. R. A. S., xxiv, p. 7. [New stars near 6 Orionis.] WOLF, C. : Sur la variabilite des nebuleuses. Association scientifique de France, No. 535, 1878, February 3, p. 277. [History of the nebula of Orion.] WOLF, R. : J. B. CYSAT von Luzern, 1853. : A. N., xxxviii. col. 109. STRUVE MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. List of Telescopes employed to observe the Nebula of Orion. Observer. Date. Telescope. Aperture, inches. Focus, feet. Maker. CYSAT . . . 1618 Refractor . ? 6, 10 ? HUYGHENS . . 1656-94 Refractor . *-33 12 or 23 HUYGHENS. HOOKE 1665 Refractor . 3-5 36 HOOKE. PlCARD . . . 1673 Refractor ? . p ? ? X MAIRAN . . - i73i Refractor . ? 7, 18, 22 ? LONG .... 1742 Refractor . ? 17 LONG.? LCGENTIL . . 1758 Reflector . ? 6 ? MESSIER . . . 1771 Refractor . 3-33 3-5 DOLLOND. W. HERSCHEL . 1774 to 1811 Reflectors . 4 J to 48 5i, 7, 10, 20, 40 HERSCHEL. LEFEBVRE . . 1779 Reflector . ? 3i ? SCHROETER . 1794-98 Reflectors . 6 to 19 7,13,27 HERSCHEL, SCHRADER, J. HERSCHEL 1824, 1837 Reflector . 1 84 20 HERSCHEL. POND .... 1826 Reflector . . 26 RAMAGE. LAMONT . . . 1839 Refractor . 10.5 15 FRAUENHOFER. De Vico . . . 1839 Refractor . <* H FRAUENHOFER. KAISER . . . 1844 Refractor . 6 ? ? COOPER . . . 1847 Refractor . 13-2 25 CAUCHOIX. LASSELL . . . 1847, 1854 Reflector . 24 20 LASSELL. W. BOND . . . 1848 Refractor . 15 23 MERZ. LlAPONOFF . . 1847 Refractor . 10 16 MERZ. O. STRUVE . . 1851 Refractor . 15 23 MERZ. SCHMIDT . . . 1861 Refractor J 6 6? DOLLOND. LASSELL . . . 1862 Reflector . 48 37 LASSELL. SECCHI . . . 1862-68 Refractor . 9.6 M MERZ. WEBB .... 1863 Refractor . 5 5 A. CLARK. G. P. BOND . . 1865 Refractor . 15 23 MERZ. WEBB .... 1867 Reflector . 9-4 ? WITH. BIRD .... 1866 Reflector . 12 ? BIRD. Lord ROSSE . 1867 Reflector . 72 55 ROSSE. D'ARREST . . 1872 Refractor . 10.5 15 MERZ. TROUVELOT . . 1874 Refractor . 15 23 MERZ. TROUVELOT . . 1875 Refractor . 26 32 A. CLARK & SONS. KEELER . . . 1878 Refractor . 2.5 ? ? H OLDEN . . 1874-80 Refractor . 26 32 A. CLARK & SONS. LANGLEY . . . 1879 Refractor . 13 ? Made by Frrz and re- figured by A . CLARK. DRAPER . . . 1880 Refractor . ii ? A. CLARK & SONS. HOLD EN . . . 1881 Refractor . 15.5 20 A. CLARK & SONS. I. HISTORY OF THE VARIOUS RESEARCHES ON THE NEBULA OF ORION IN CHRONOLOGICAL ORDER. The drawings and memoirs are here considered in chronological order. First a wood-cut of the original drawing is given, followed by extracts more or less copious relating to the observations. These wood-cuts were made in the following way: Good prints of the original drawings were selected and photographed on a scale of one English inch, equal to the distance between the stars Gr. P. B. 685 and 741; the scale MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 15 is thus about i inch 1 29" of arc. The copying-lens used produced no appreciable distortion. The wood cuts have, however, never been used as evidence. A set of photographic prints of the original drawings has been constantly used for comparison and for suggestions as to doubtful points. In every case reference has been made to the original drawing (when possible), or to the original engraving or wood-cut. The negatives so made were used to transfer the photograph on to wood, and the various revises of the cuts have been again compared with the original publication. They are therefore tolerably faithful representations of their originals, and will serve to recall them to those readers who have not these originals at hand. As they are nearly all on the same scale, and as the drawings made by reflectors have been inverted so as to present the appearances as seen in a refractor, they are all immediately comparable (except a few of the figures which, for special reasons, are on other scales), and they are interesting on account of the enormous differences at once apparent even among the more modern drawings. Everything relating to the Huyghenian region I have attempted to give fully, generally in the words of the author. Many of the memoirs contain, beside the records of original observations, a discussion of the results obtained by others, and these I have usually given in the words of the authors. Thus the discussion of the various drawings is partly completed in the progress of the work. I have resumed this discussion in the light of the Washington observations. By including these dis- cussions, the admirable resumes of 'LIAPONOFF, STRUVE, D'ARREST, and others, are available for immediate reference. It was first formally pointed out by Professor WOLF, of Zurich, that HUYGHENS was not, as commonly supposed, the first discoverer of the nebula of Orion,* but that this distinction must be assigned to CYSAT of Luzern, who saw it in 1618, 38 years before HUYGHENS published his account of it. It is now just visible to the naked eye under good circumstances, but it does not appear to have been seen by any of the early astronomers, not excepting the acute ABDEL RAHMAN AL SUFI, who was acquainted with the nebula of Andromeda before A. D. 950. CYSAT speaks of his discovery as follows : " Caeterum huic phaenomeno similis stellarum congeries est in firmamento ad ultimam stellam gladii Orionis, ibi enim cernere est (per Tubum) congestas itidem aliquot stellas angustissimo spatio et circumcirca interque ipsas stellulas instar albae nubis candidum lumen affusum."f This observation of CYSAT'S is mentioned by BESSEL in the Berliner Jahrbuch, 1808, p. 122. OBSEEVATIONS OF HTJYGHENS. Although HUYGHENS was not the first observer of the nebula of Orion, yet his discovery was made independently. His account of it is given in the Systema Sat- * Astronomische Nachrichten, vol. xxxviii, col. 109, and "]. B. CYSAT von Luzern." 4. Bern, 1853. t CYSAT Mathemeta Astronomica de Cometa Anni 1618, p. 75. i6 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. urnium (1659, p. 8), with a figure which I have reproduced from the original work.* HUYGHENS says that in examining the stars of the sword of Orion in 1 656 with a telescopef he perceived, instead of the three stars com- monly known, twelve in all, of which three were close together, and four others which shone "as if through a cloud," so that the space around them appeared much brighter than the rest of the sky, which was serene and quite black, so that the ap- pearance was produced of looking through an opening. All this he saw many times still unchanged in the same spot, and he concludes that this wonderful thing, whatever it may be, has there a place, apparently forever; the like of it he has never seen among the fixed stars. It will be ob- served that this figure, in the original, is uniformly shaded throughout, so as to appear to represent a uniform brilliancy in the nebula. It appears that HUYGHENS' observation did not escape HOOKE, for we find the following reference to this object in his Micrographia. For this reference I am indebted to the courtesy of HENRY B. WHEATLEY, esq., assistant librarian of the Royal Society of London, to whom all matters relating to the activity of HOOKE are familiar through his long study of his life. "In that notable asterism also of the sword of Orion, where the ingenious Monsieur HUGENS VAN ZULICHEM has discovered only three little stars in a cluster, I have, with a 36-foot glass, without any aperture [diaphragm] (the breadth of the glass being about some three inches and a half), discovered five, and the twinkling of divers others up and down in divers parts of that small milky cloud."! * For access to a fine copy of the original work I am indebted to Professor NEWCOMB. t ARAGO in Annuaire du Bureau des Longitudes, 1842, p. 268, states that the telescopes of HUYGHENS of 12 and 23 eet had an aperture of 2% inches (63 m. m.), and magnified 48, 50, and 92 diameters, t Micrographia, by ROBERT HOOKE, folio, London, 1665, p. 242. FIG. i. HUYGHENS, 1656. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 17 HOOKE'S 1 2 -foot telescope magnified 74 diameters, according to AEAGO (op. cit., p. 269), and it is likely that even a higher power was used on the 36-foot. Whether HOOKE really saw the fifth star of the trapezium, and the question as to how faint stars can be seen in the nebula with an aperture of 3 5 inches, I have dis- cussed at length in Washington Astronomical Observations for 1877, Appendix I, "On the Multiple Star 2 748," and the observations are given at length later in this work, The conclusions reached there are that from HOOKE'S language in the Micrographia, as well as from an entry in the MS. journal of the Royal Society of London (which is discussed by Sir JOHN HERSCHEL in Mem. R. A. S., vol. Ill, p. 189), no other stars than the fourth and fifth stars could have been meant by HOOKE; and yet, that the obser- vations made with the 26-inch refractor at Washington, with its aperture reduced to 3.5 inches, show that the 5th star cannot now be certainly seen with a telescope supe- rior to HOOKE'S. The obvious conclusion from these two facts, if accepted, is that the fifth star is now fainter than in HOOKE'S day (1666). I am not prepared to assert this, although I know of no way of explaining HOOKE'S early observation other than that which I have stated. HOOKE must have had the original of Fig. i before him, and " three little stars in a cluster" are only to be found in the trapezium. These three HOOKE declares to be five. OBSERVATIONS OF HUYGHENS (1694). In an account of HUYGHENS by KAISER* is a description of HUYGHENS' observa- tions on this nebula, and a drawing found among his MS. is given, which we reproduce in Fig. 2. "On the 8th of January, 1684, he first perceived that the group of stars in the nebula of Orion in which, up to this date, he had only been able to make out three stars with difficulty, was composed of four. Beside this note we find in HUYGHENS' journal another observation. This is the very last astronomical note made by HUYGHENS. " HUYGHENS discovered, in the year 1656, the nebula in the constellation of Orion, which, in later times, has been productive of so much research and speculation. That nebula was represented by him in his Sy sterna Saturnium, page 8, and is there exhib- ited as a spot of irregular shape over which the light is equally distributed. HUYGHENS certainly did not know of how much importance a correct representation of the nebula, made in his time, would be to astronomers in later years, and it is nearly certain that it could not have appeared so uniformly illuminated, or with such distinct edges as he has indicated. " Not long after HUYGHENS had published his discovery, a new representation of the nebula was given to the world by PiCARDf which coincides in many respects with that of HUYGHENS, but which ascribes to the nebula different external form. A century after HUYGHENS, LE GENTIL gave two illustrations which neither coincided with each other nor with the representations of HUYGHENS and PICARD, and differing widely from the beautiful illustration produced a fe^w^eari^ * Tijdschrift voor de Wis-sen Naturkundige Wetemchappen, vol. i, 1848, p. 7. 1 1 believe this was never published, but was communicated in MS. to GODIN. App. V - 3 i8 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. was represented for the first time as flowing and spreading, and with the light unequally distributed. The differences between all these drawings, between them- selves, as well as with the later efforts of SCHRCETER and HERSCHEL seems to indicate that the nebula underwent great variations ; from these one might even infer a doubt as to its very existence. "The younger HERSCHEL, however, did not allow himself to be misled by this seeming contradiction. He knew the difficulty of producing a correct representation of such an object, and from the illustrations given by his predecessors, which were probably made with imperfect means, he came to the conclusion that the nebula of Orion had undergone no considerable change since its discovery. For our knowledge of the higher regions of the heavens is certainly not so definite as to assure the possi- bility of indicating undemonstrable variations in the nebulae ; and every contribution which can serve to confirm or oppose HERSCHEL'S ideas about the nebula of Orion may be considered as an important gain to science. " Such a contribution HUYGHENS left us in his last astronomical note. This con- tains a representation of the nebula of Orion, until now entirely unknown, drawn with the pen in his journal of the 4th of February, 1694. There is certainly no more intractable instrument than a writing pen for giving an illustration of such an object, and yet the drawing of HUYGHENS expresses to us something of the form of its most illuminated portion and the play of its edges, with the relations of the surrounding stars. Of all the old drawings this approaches much more nearly the present appear- ance of the nebula in Orion, which is favorable to the younger HERSCHEL'S opinion. HUYGIIENS did not indicate the telescope with which he made this observation ; but it was most probably with that of 44 feet in length, which he had had constructed a short time before at his Hofwyk station at (or near) the Hague, which was a new and most suitable instrument. " We can be very certain that the illustration was not made by measurements, and therefore, to make the matter clear, can allow ourselves a slight variation between the present relative position of the stars in the nebula and their position as given by HUYGHENS. " The illustration, with a brief note, forms the contents of the last page of HUYGHENS' journal. We think we are subserving a not unimportant use in presenting a very exact reproduction of the page, in order to make this communication complete." This drawing of HUYGHENS is very pre- cious, since it is a fac-simile of the rough sketch in his note-book, untouched by the engraver. On it we find the trapezium correctly laid down. The Sinus Gentilii is plainly indicated; the Sinus magnus also, and the three stars [Gr. P. B. 685, 708, 741] are outside the light. Even the true north edge of the Sinus magnus is indicated, while the boundaries of the unshaded portions are similar to the appearances FIG. 2. HUYGHENS, 1694. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 19 seen to-day in small telescopes. The angle between the frons and occiput is 116; in LE GENTIL'S drawing (i 758), Fig, 6, it is 129. Its value to-day is about 91. The angle between the occiput and the north side of the Huyghenian region is according to HUYGHENS, Fig. 2, 1 10 ; in LE GENTIL'S, Fig. 6, 1 18 ; to-day it is about 120. OBSERVATIONS OF MAIRAN (1731). In Traite de TAurore Boreale (1733), p. 248, MAIRAN notes that HUYGHENS used telescopes 22^ Paris feet in length, and that HUYGHENS declares that it was only with such that the nebula of Orion was well seen. MAIRAN asserts, however, that he saw it in 1733 with a 7-foot telescope, from which fact he deduces the inference that it is denser [brighter] than in HUYGHENS' time. " Quant a sa figure, je crois aussi qu'elle varie ; et c'est qui m'a e'te' confirm^ par deux astronomes [MM. GODIN et GRANDJEAN DE FOUCHY] que j'avais pi-ie* d'y regarder avec moi M. GODIN m'a commu- nique* de plus un dessiri et une observation manuscripte de M. PICART de 2O me mars 1673, ou la forme exterieure de cet espace lumineux differe de celle de M. HUYGHENS." . . . . . After describing the position of the nebulous star n. f. Orionis [G. P. B. 757], MAIRAN proceeds (p. 249), "la figure xxvii represent ces objets renverse's et tels qu'ils m'ont paras le plus souvent depuis cinq a six ans avec une lunette de 1 8 et de 22 pieds." PICARD'S (from Hist, del 'Acad., 1759, Plate 21, Fig. 5), and MAIRAN'S figures are given in Figs. 3 and 4. In 1 733 the original MS. drawing of PICARD was in existence, and MAIRAN refers to it as a proof that the " exterior form" differs from that given by HUYGHENS. His own figure represents the " exterior form" uniformly filled with nebulosity. FIG. 3. MAIRAN, 1731. FIG. 4. PICARD, 1673. 20 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. OBSERVATIONS OF LONG (1742). ROGER LONG, of Cambridge, author of "Astronomy, in five books" (1742), was also an observer. In vol. i, p. 32 1 et seq., [Plate 67, Fig. 96], we find his account of the nebula of Orion: "HuyGHENS,who first discovered this wonderful appear- ance, as he justly calls it, has given us a draught of it, but the stars are all drawn nearly of equal magnitude, and the lumi- nous space is more defined than it ought to be; which faults were, in all likeli- hood, owing to the mistakes of the en- graver. I have, therefore, in figure 96, given another scheme of it, such as I have often seen it through a telescope of 17 feet, and have expressed therein the ap- parent magnitudes of the several stars." FIG. 5. LONG, 1742. OBSEKVATIONS OF LE GENTIL (1758). A memoir read to the French Academy of Sciences July 26, 1 758, by LE GENTIL, is the first in which a proper attention was paid to the observations of the details of a nebula, and it is indeed somewhat surprising to remark how carefully LE GENTIL'S observations were made and how caiitiously his conclusions were drawn. He had observed, in common with others, that the representations of the nebula of Orion by HUYGHENS and PICARD did not agree, and he adduces in this paper evidence to show that the nebula in Andromeda had also varied in brilliancy; so that he says, "Y seroit-il en effet arrive quelque changement, ou ne pourroit-on pas attribuer cette grande difference .... aux differentes longueurs des lunettes dont on se sera servi. . . .?" "C'est ce qui m'a engagd a employer les plus courtes et les plus lon- gues lunettes que j'ai pu avoir a ma disposition, pour comparer ensemble les differens effets que j'en pourrois tirer." "Ainsi nous avons tout lieu de croire que la plus grande partie de la difference qu'on trouve entre la description de SIMON MARIUS et celle que j'ai faite de la nebuleuse d'Andromede, vient de la diffurente longueur des lunettes dont nous nous sommes servis." LE GENTIL remarks that the nebula of Orion, as seen by him with various tel- escopes for several years, also varied considerably in form, and he gives his observations in detail. March 10, 1758, he observed, in company with M. JOLY and M. PINGRE, with a gregorian telescope of 6 feet, and this observation was repeated with the same telescope on the 3d of April. The drawing [Fig. 6] was made by LE GENTIL from observations with this telescope, and afterwards compared with the heavens. . MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 21 The Sinus Gentilii is there laid down distinctly, and faint nebulosity is described extending towards the north. This is the Regio Picardiana of HERSCHEL. It is to be remarked that LE GENTIL'S figure is not equally bright throughout, but that it is brightest along the following side of the Sinus Gentilii, along the frons and along the north shore of Sinus magnus. An inspection of a good print of this engraving will show that in the meridian of the star [G. P. B. 708] the end of the brighter part of the frons is shown [the point B of LIAPONOFP, the following point of Q in the index- chart], and that the trapezium is situated on nebulosity of the same kind as that near the northern limit of his drawing round star [G. P. B. 479 ?], which nebulosity he expressly describes as " faint." So much is at once evident, on 'a careful inspection of all the prints I have been able to see. I was at first inclined to suppose with Sir JOHN HERSCHEL that " these older representations are mere curiosities and present no points of exact resemblance," and that, therefore, such a shading as I describe was merely an accidental impression. I have frequently recurred to this drawing, not being able to believe that when so careful an observer as LE GENTIL expressly set about discovering whether the changes in the older drawings of this nebula were not due to a difference in the telescopes employed, he should be content to leave so extraor- dinary a figure of it as his witness of its shape in his day On examination of his figure with a magnifying glass, the explanation of the varied brightness of different parts of his drawing is at once manifest. He attempts to represent three grades of brilliancy, according to his own account: ist, the brightest central portions; 2d, the fainter northern portions near the star [479?] and near the Sinus Gentilii; and 3d, the black background of the heavens. The copper-plate engraver working most probably under LE GEN- TIL'S own eye, has chosen to represent the brighter nebulosity by wavy parallel lines about a sixtieth of an inch apart ; the lines run completely across the whole picture, over the black ground of the heavens as well as over the Huyghenian region. The nebulosity which LE GEN- TIL in his memoir explicitly declares to be "faint," the engraver represented by putting in a faint black line between each pair of the waved parallels, thus diminishing the brightness of the en- graving. Various parts of the central portion not explicitly declared by LE GENTIL to be faint, are, however, shaded precisely the same way, and these parts are in general those which to-day are faint. I have submitted this drawing to experienced engravers, and I am informed that there is no manner of doubt but that the engraver of this drawing FIG. 6. LE GENTIL, 1758. 22 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. intended that the portions in question should be fainter. I also can have no doubt but that the astronomer so drew them in his sketch, which was ''verified at the telescope," and from which the engraver worked. This drawing, so considered, will give us important testimony, and it extends our knowledge of the central por- tion of the nebula back from MESSIER (1771), formerly considered the first trust- worthy drawing, to 1758. It has been said that LE GENTIL gives PICARD'S figure also, and it will be found on a similar examination of PICARD'S drawing that a por- tion of the Eegio sulmebulosa is on that figure represented as fainter by the same conventional sign, and professional engravers assure me (as, indeed, any one can con- vince himself by the use of a magnifier), that this is not accidental, but designed HUYGHENS' figure is likewise given by LE GENTIL, but this is represented of equal brilliancy throughout, as was done in HUYGHENS' original plate. It thus becomes necessary to examine the evidence with regard to the drawing of PICARD. I consider these points quite important. To see how conclusive the proof is an examination of these figures should be made with a magnifier. The original drawing of PICARD was made March 20, 1673, and was communicated with a manuscript observation by GODIN to MAIRAN in 1731, or about that time. No account is given of the " observa- tion," but MAIRAN refers to this figure (but does not reproduce it), which he adduces to prove that " la forme exterieure" is different from his own. MAIRAN'S own drawing is uniformly shaded throughout. LE GENTIL speaks of MATRAN'S copy of PICARD'S figure, as follows: "M. MAIRAN se fonde encore sur un dessin de M. PICARD, dans lequel la forme exterieure de cet espace lumineux differe assez de celle que M. HUYGHENS nous a laissee du meme espace." In the explanation of his various figures LE GENTIL says, Fig. 5: "Nebu- leuse d'Orion suivant M. PICARD." There is nothing said of borrowing the figure or original drawing from MAIRAN. It is impossible, at this day, to decide whether FIG. 7. Drawing made at Washington Lfi GENTIL had aCCCSS to the Original drawing of PiCARD through tourmaline plates. , 111 riiii or not My own idea is that he had. I shall, however, treat the drawing of PICARD in both ways: ist, as if MAIRAN'S figure was correct; 2d, as if LE. GENTIL'S figure (including the fainter portions) was so. In this way we may arrive at some conclusion. " N(ibuleuse d'Orion telle que je 1'ai vue le 10 mars et le 3 avril 1 758, an soir avec un telescope de 6 pieds de longueur ; 1'ouverture A B c m'a paru de 65 degres environ, et les trois etoiles inferieures paroissent faire, avec le cott? cD, un angle de pros de 40 degres." The angle which LE GENTIL called about 40 is about 50 to-day. The stars of this drawing are [G. P. B., Nos. 741, 708, 685, 9, 969, and 479 ??]. During 1877 I made a number of sketches of the nebula through tourmaline plates, so as to reduce the light at will. One of these is given in Fig. 7 (see my obser- vations of 1877, February 3). One made 1877, January 30 (not given here), strikingly resembles Fig. 6, with one exception, viz, Fig. 6 puts the trapezium on darker ground ; my drawing puts it on brighter ground. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 23 If we regard PICARD'S, HUYGHENS', and LE GENTIL'S drawings as simple evidences of the exterior shape of the nebula in their times (1656, 1673, and 1758), we can best examine MAIRAN'S idea of a change in u la forme exte- rieure" by superposing these. Fig. ^8 shows the result of a superposition of the three drawings. The true posi- tions of the stars are denoted by a dot surrounded by cir- cle; PICARD'S positions by a dot and a square; HUYGHENS' by a dot and a circle of dots ; LE GENTIL'S by a dot and a triangle. HUYGHENS' outline is given by a dotted line; PICARD'S by a broken, and LE GENTIL'S by a full line. If we correct the distor- tion of each drawing by means of the true positions of the stars; L e., if we suppose that those portions of the nebula near a star are correctly drawn, while por- tions distant from stars are distorted by the same propor- tional amounts as the star positions, then we shall have outlines like Fig. 9. In this figure it is seen that HUY- GHENS* and PICARD agree as to "la forme exterieure," as we should ex- pect, since their telescopes were, in all likelihood, sim- FIG. 8. Comparison of the drawings of HUYGHENS, PICARD, and LE GENTIL, (uncorrected). FIG. 9. Comparison of the drawings oi HUYGHEXS, PICARD, and LE GENTIL (corrected). * A comparison of Fig. i and Fig. 9 shows that it is probable that HUYGHENS began his drawing of 1656 t the right hand side of the paper he was using. 24 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. ilar. The bounding lines of LE GENTIL do not agree with the others. His telescope was quite different in power from that of HUYGHENS. On Fig. 9 I have also put the bounding lines between the faint and bright por- tions of the nebula, as given by PICARD (broken line with two dots) and LE GENTIL (full lines, faint). There is no marked agreement, but it is to be noted that PICARD, thus corrected, agrees even more closely than before with both of HUYGHENS' figures. I have not included HUYGHENS 1694 and MAIRAN 1731 in these comparisons, as they would yield no additional evidence, but simply confuse the drawing. OBSERVATIONS OF MESSIER (1771). The observations and drawing of MESSIER are thus described by himself: " Le dessin de la n^buleuse d'Orion, que je pr^sente a I'Acade'mie a e'te' trace" avec le plus de soin qu'il m'a &t possible. La ne"buleuse y est represented telle que je 1'ai vue plusieurs fois avec une excellente lunette achromatique de trois pieds et demi de foyer, a triple objective, portant 40 lignes d'ouverture [3.33 Paris inches] qui grossis- soit 68 fois. Cette lunette [a e'te'] faite a Londres par DOLLOND. J'ai examine cette ndbuleuse avec la plus grande attention par un ciel entitlement serein : savoir, Les 25 et 26 feVrier 1773 ; Orion au meridien. Le 19 mars entre 8 and 9 heures du soir. Le 23 entre 7 and 8 heures. Les 25 et 26 du meme mois, a la meme heure. Ces observations combiners et les dessins rapproche's les uns des autres, m'ont mis a meme de rendre avec soin et precision sa forme et ses apparences. Ce dessin servira a reconnoitre, dans la suite des temps si cette nebuleuse est sujette a quelques changemens. II y auroit deja lieu de le presumer : car, si on compare ce dessin avec ceux donne's par messieurs HUYGHENS, PICARD, MAIRAN et avec LE GENTIL on y trouvera un changement tel qu'on auroit peine a se figurer que ce fiit le meme. ***** Le jet de lumiere, dirige de 1'etoile no. 8 a J'etoile no. 9 passant a cote* d'une petite e'toile de la dixieme grandeur, t'toit extremement rare, ainsi que la lumiere dirigee vers 1'etoile no. 10 et celle qui y est opposee oil sont les huit etoiles contenues dans la nebuleuse: ******* * M4m. de VAcad. dca Sci., 1771, p. 458. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION FIG. 10. MESSIER (1771). It may be remarked of this figure that the star-positions are quite erroneous, and therefore the shape of the nebula as a whole is somewhat distorted. Unfortunately MESSIER'S erroneous star-positions not only affect his own drawing, but those of SCHROETER (1794) also, who took MESSIER'S stars as a basis for his own drawing. The synonyms of some of MESSIER'S stars are given below : MESSIER'S Number. BOND'S Number. Remarks. MESSIER'S Number. BOND'S Number. Remarks. i 467 6 708 Star. 49- Not numbered. Star. 724 Not numbered. 2 505 7 734 Star. Star. 449 479 SOut of place in R. A. S Star. 741 784 Not numbered. Star. 523 Star. 822 Not numbered. 4 570 9 843 5 685 10 848 Apr. V- 2t> MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION, OBSERVATIONS BY SIR WILLIAM HERSCHEL (1774 TO 1811). (From the original papers in possession of the Eoyal Society, referred to l)y Sir J. F. W. Herschel in vol. 154 of the Phil. Trans., p. 6, No. 2.) [These papers were kindly put at my disposition by Sir GEORGE ATRY and by Professor STOKES, secretaries of the Royal Society, during a visit to London in 1876] " 1 7 74, March 4. [Journal No. i .] Saw the lucid spot in Orion's sword through a 5^ feet re- flector; its shape was not as Dr. SMITH has delineated in his Optics, though something resem- bling- it, being nearly as follows: From this we may infer that there are undoubtedly changes among the fixed stars, and per- haps, from a careful observa- tion of this spot, something might be concluded concerning the nature of it. 5. Nov. u, [17] 76 The lucid spot in Orion. The figure [see page 2 7] is only drawn for the description. The greatest glare is immedi- FIG. u.* HERSCHEL (i 774). ately about the four small stars. The 3 stars i, 3, 4, were almost (on the upper side of this figure) free from any glare. There was a total darkness in the corner by the 4 small stars. The stars i, 2 [685, 628], were of one size, those 3, 4 [708, 741], of the next, the two 5, 6 [640, 619], considerably less, 7 [669]? very small, 8 [624], rather smaller still. Instrument, lo-feet reflector; power only 120, but very distinct. 6. Jan. 25th, [17] 78. See figure, page 6. A tollerable (sic) exact delineation of the h h lucid spot At the eastern side the rays seem to make an equilatteral (sic) triangle with the stars i and 3 [685, 708]. It goes on in the direction i, 3, 4 [685, 708, 741], rather approaching to 4 [741] ; at 4 [785] it bends round in an angle of about 110 or 120 towards the east. From two [628] to 7 [669] the lucid part is concave; the concave part being turned towards 3 [708]. It goes to the northward about ^ of the distance from 2 to 7 beyond 7. From thence it turns to the west in an angle of about 75 or 70 deg. *Thie figure is not given in Journal No. i, but a reference is there made to the Phil. Trans,, 1811, from which the cut is copied. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 27 6. Jan. 26, [i7]78 6, 2, i [619, 628, 685], make a straight line [so in 1879]. 6, 8, 7 [619, 624, 669], make a straight line [not so, 1879]. 4, 5, 8 [741, 640, 624], make a straight line [so in 1879]. The lines 2, 5 [628, 640], and i, 3, 4 [685, 708, 741] diverge. 5 [640], a little larger than 6 [619] [so in 1879] 7. Feb. 7, [i7]78. The situation of the 4 stars in the lucid spot agrees perfectly well with the observation of Jan. 25. ? 25, [i 7] 78. No change in the situation of those 4 stars to be observed. [Paper torn here so that the date is illegible']. 7 Dec. 15, [i7]78, 6, 8, 7 [619, 624, 669], a straight line [not so, 1879] 6, 2, i [619, 628, 685], a straight line [so in 1879]. 4, 5, 8 [741, 640, 624], a straight line [so in 1879]. The lines 2, 5 [628, 640] and i, 3, 4 [685, 708, 741] diverge. This agrees exactly with Jan. 26, but there is a visible alteration in the figure of the lucid part. 28. Oct'r 7, [17] 79. Orionis see Fig., p. 6. The line 6, 2, i [619, 628, 685] is a little convex towards 5 [640], when that line is taken into the middle of the field; this I mention, as it is possible there might be a little curvature arising from the spherical figure of the eye-glass, tho' I believe there is not. If a line be drawn from 6 [619] to 7 [669], the star 8 [624] stands outwards, I suppose, no less than 15, so that 6, 8, 7 [619, 624, 669] is concave towards the side i, 3, 4 [685, 708, 741] [so in 1879]. The line 4, 5, 8 [741, 640, 624], I cannot very well compare, being rather too far distant by the power I now use, but I believe it is not far from a right line. I see a 9th star, which is marked in the annexed figure. [This figure gives i = 685 ; 2=628; 3 = 708; 4=741; 5 = 640; 6 = 619; 7 = 66958 = 624; 9 = 635. Thenum- bers of three figures are BOND'S.] 619 = 6 2 = 628 where, however, not the "" i = 685 l eas t exactness is in- tended. 624 / 5 = 640 .3 = 4 = 74i 635=9 / = 669 Altitude about 26 degrees, 14" 10'. The figure of the lucid part is very much altered 43. Dec. 5, [i;]79. 6, 2, i [619, 628, 685] concave, vid. page 6 and 28. The concave part turned to the south. 8, 6, 7 [624, 619, 669], still make an angle at 6 [619], tho' very small. I see the 9 th star [635] mentioned page 29. 45 Jan 22, 1780, io h 30'. The stars 6, i, 2 [119, 628, 685] instead of seeming concave towards the north appear convex. This may, however, be a decep- tion, as the star 2 [628] is the largest, and since there is a pretty strong aberra- tion on account of a fog, its diameter is more encreased than that of i 9 J, and, consequently, may give the balance towards the north. 2 8 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 46. ' Feb. 19, 1780. Exactly as described, page 28. 46. Feb 28, 1780. The two stars 6, 2 [619, 628], page 6, measure 22 20^ = 20% parts = 1 2 7/ .8 12 The measure is pretty narrow, but I believe true enough. The two stars 2, 5 [628, 640] measure 24^ i% = I5".2o8 .937 ~ I^'AJI. This is also a pretty narrow but just measure. The two stars 6, 8 [619, 624] measure about 16 i% =. 14^ =:9".o62. But this is doubtful on account of the obscurity of the star 8 [614] which is hardly perceptible when the field of view is illuminated properly so as to make the parallel hair very distinct. The two stars 5/8 [640, 624] measure 34^ \% 21.458 .937 2o".52i. This is also doubtful on the same account. [N. B. A new reduction of these measures, with the value i part = o".625i, shows no change to be required in the hundredths of seconds. ~\ 8 1. Oct. 10, 1780. The upper stars concave by the hair. The spot extremely fine. The 4 stars all full, round, and well defined. 84. Nov. 24, 1780. I view the nebula in the highest perfection. I perceive not the least change. 319. Jan. 31, 1783. The nebulous part is quite different from what it was last year. The 9th star [635] very strong, the nebula about it and the 7th (669) being much dispersed. 432. Sept. 20, 1 783. Has evidently changed its shape since I saw it last, 20 ft, 200. 441. Sept. 28, 1783. Nebula in Orion is surprisingly changed. 457. 15 Sw. Nov. 3, 1783. The nebula is beautiful, and I see several circumstances which I never observed with other instruments. Just close to the 4 stars it is totally black for the short space of a few seconds. In the open black part of the nebula is a small distinct nebula of an extended shape [o TT of the Index-Map}. The eastern branch of the great nebula extends very far ; it passes between two very small stars, and runs on till it meets a very bright star. The nebulous star below the nebula is not equally surrounded, but most nebulous towards the south. On the north of this lesser nebula is another smaller nebula joining to it, which is much fainter than the other, and it makes a rectangular corner by its meeting with the nebula surrounding the star. This faint nebula, as well as that contained within the dark part of the great one, other instruments did never show, and I suppose is not visible by them. 296. Sw. Oct. 1 6, 1784. The beginning of the nebula. 5 Monocerotis, p. 41' 6" n o ^43'- R- A. 5 h 23' 7", P. D. 95 30'. Sweeping Journal No. 2. Dec. 20, 1 784. 20 ft. The nebula as described, but moon- light, and therefore seemed to take hardly 3 A in extent. Sweeping Journal No. 3. Feb. 13, 1785. I examined the nebula in Orion with a new 10 ft. reflector, and with long attention could just perceive my small, faint nebula in the dark part of the great nebula. 458. Sw. Oct. 5, 1785. A wonderful phenomenon. 510. Sw. Jan. 1 8, 1786. The nebula observed. The place brought to the present from the Connoiss. des terns is R A. 5 h 24' 49", P. D.. 95 33' (var. from 1769 to 1786 in R. A. 4 9 ".8, in P. D. i' i".4). MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 29 528. Sw. Feb. 23, 1786. The 4 stars in the great nebula Orionis. 36 (v) Orionis f. 3' n", n i 56', R. A. 5 h 24' 42", P. D. 95 32'. 640. Sw. Nov. 28, [17] 86. The nebula which I saw by the front view was so glaring and beautiful that I could not think of taking any place of its extent. Rev. Jan. 14, 1801. Large X-foot telescope, power 1 20. As before described. Rev. Feb. n, 1806. Large 10 feet. The 4 stars are completely in the nebulosity. The 3 stars are intirely [sic] out of it with 270. With the double glass appear- ances are very different. Rev. Feb. 4, 1810. 10 feet. The nebulosity is intirely [sic] of the milky kind, and extends a great way. Rev. Dec. 31, 1810. 10 feet, double eye-piece. The 4 stars are within the nebu- losity. The star No. 7 [669] (see the figure of Oct. 7, 1779) is upon the borders of the dark vacancy. I see No. 9 [635] very well. The little star between 3 [708] and 4 [741] is still within very faint nebulosity. [This refers to 7 24 possibly, but I do not understand it.] The nebulosity reaches beyond 4 [741] as far as from i [685] to 4 [741] nearly It touches a very small star [793 1], and from that star goes on to two very bright ones [843, 905!] in the direction from the small star [793?] to the preceding one [843!] of the two. The black space near the 4 stars is much contracted. The neb- ulosity from i to 4 is concave, the concavity being to the following side. The parallel is nearly in the line of i, 3, 4 [685, 708, 741]. I can see 8 different condensations, notwithstanding the moon is very bright. The nebulous star [734] is pretty equally involved. It has the appearance of a star shining through a very faint mist. The star is a little larger than 4 [741]. The concavity from 2 [628] to 7 [669] goes beyond 7 [669]. Rev. Jan. 19, 1811. 10 feet. 2 of the 4 stars are within the nebulosity. No. 7 [669] is very near the borders of the black. The little star [724] between and following 3 and 4 [708, 741] is still within very faint nebulosity. The nebu- losity reaches beyond 4 [741] rather farther than from i to 4 [628 to 741]. X feet. I perceive 7 or 8 different condensations. The place near the 4 stars is much contracted. The nebulous star is exactly what we might expect to see if a star were to shine through whitish nebulosity. 40 feet. 5 h 1 6', B affected. 17', B much affected. 22', the 4 stars are intirely involved in nebulosity. The 7th [669] and 9th [635] stars are very bright. In the brightest part are four places brighter than the rest. I see the small detached nebula ; it is extremely faint. It is between the corner and a small star. The star called nebulous is within a nebulosity nearly detached; but the small stars marked nebulous in the figure of the 4th of March, 1774, are free from nebulosity. There is a very small, nearly detached nebulosity north of the nebulous star. The nebulous star has some resemblance to a star shining through a very thin mist [What follows, I suppose to be in Sir Wm. HerscJieTs handwriting. I suppose what precedes to have been written by his sister.] 3Q MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. Rev. Mar. 13, 1811. 7 feet; double eye-piece. The following or rather the southern branch (for I find the parallel nearly in the line i, 2, 3 [685, 708, 741]) goes towards the preceding star e [843] of the two large stars d e [905, 843], or rather a little preceding it, but it partly includes the star e and makes it appear a little nebulous. The light about the nebulous star b [734] is a little denser nearer the star than at a distance. A line from 5 [640] through 7 [669] goes to b [734], or rather a little south of it, and 7 is about ^ of the distance towards b. The star [724] south of 3 [708] and 4 [741] makes an equilateral triangle with them. The two large stars d and e [905, 843] are parallel to i, 3, 4 [685, 708, 741], nearly. A line from the 4 stars parallel to i, 3, 4 passes a little south of the small, formerly nebulous, star c [848]. There are many other stars connected with the nebula which I do not notice. Rev. Mar. 15, 1811. 7 feet ; double eye-piece. The northern branch is parallel to the stars ab [543, 734]. The nebulosity reaches nearly up to the stars g, h [570, 523]- A very faint nebulosity still joins the star b [734] to the northern branch, but b is more nebulous than the intermediate nebulosity. The southern nebulosity goes towards the star e [843], and some part of the very faint nebulosity incloses the star. Rev. Mar. 16, 1811. 10 feet reflector. Power, 100. The stars i, 3 are in the parallel; 4 is a very little south of their parallel. The nebulosity about b [734] is brightest about the star. In Philosophical Transactions, 1802, p. 499, Sir WILLIAM HERSCHEL alludes to changes in the nebula of Orion, as follows: "IX. Of Milky Nebulosity. 11 The phenomenon of milky nebulosity is certainly of a most interesting nature ; it is probably of two different kinds ; one of them being deceptive, namely, such as arises from widely-extended regions of closely connected clustering stars, contiguous to each other, like the collections that construct our milky-way. The other, on the contrary, being real and possibly at no very great distance from us. The changes I have observed in the great milky nebulosity of Orion, 23 years ago, and which have also been noticed by other astronomers, cannot permit us to look upon this phenomenon as arising from immensely distant regions of fixed stars. Even HUYGHENS, the dis- coverer of it, was already of opinion that, in viewing it, we saw, as it were, through an opening into a region of light. (See Systema Saturnium, pages 8 and 9.) Much more would he be convinced now, when changes in its shape and lustre have been seen, that its light is not like that of the milky-way, composed of stars. To attempt a guess at what this light may be, would be presumptuous. "If it should be surmised, for instance, that this nebulosity is of the nature of the zodiacal light, we should then be obliged to admit the existence of an effect without its cause. An idea of its phosphorical condition, is not more philosophical, unless we could show from what source of phosphorical matter such immeasurable tracts of luminous phenomena could draw their existence and permanency ; for, though minute MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 31 changes have beeri observed, yet a general resemblance, allowing for the difference of telescopes, is still to be perceived in the great nebulosity of Orion, even since the time of its first discovery." Sir WILLIAM HERSCHEL, in his " Astronomical Observations relating to the Con- struction of the Heavens," in the Philosophical Transactions for 1811, has a detailed discussion of his observations on the nebula of Orion, which I quote almost in full : " In the year 1774, the 4th of March, I observed the nebulous star which is the 43d of the Connaissance des Temps [G. P. B., 734], and is not many minutes north of the great nebula ; but at the same time I also took notice of two similar, but much smaller, nebulous stars : one on each side of the large one and at nearly equal distances from it [G. P. B., 543 and 848]. Fig. 37 [of the Plates to the Phil. Trans., 1811, Fig. 1 1 of this work], is a cop}'- of a drawing which was made at the time of observation. In 1783 I examined the nebulous star [734], and found it to be faintly surrounded with a circular glory of whitish nebulosity, faintly joining to the great nebula. About the latter end of the same year I remarked that it was not equally surrounded, but most nebulous towards the south. In 1 784 I began to entertain an opinion that the star was not connected with the nebulosity of the great nebula of Orion, but was one of those which are scattered over that part of the heavens. In 1801, 1806, and 1810 this opinion was fully confirmed by the gradual change which happened in the great nebula, to which the nebulosity surrounding this star belongs. For the intensity of the light about this nebulous star had by this time been considerably reduced by the attenuation or dissipation of the nebulous matter ; and it seemed now to be pretty evident that the star is far behind the nebulous matter, and that, consequently, its light in passing through it is scattered and deflected so as to produce the appearance of a nebulous star. A similar phenomenon may be seen whenever a planet or a star of the ist or 2d magnitude happens to be involved in haziness; for a diffused circular light will then be seen, to which, but in a much inferior degree, that which surrounds this nebulous star bears a great resemblance. "When I reviewed this interesting object in December, 1810, I directed my attention particularly to the two small nebulous stars by the sides of the large one, and found that they were perfectly free from every nebulous appearance ; which con- firmed not only my former surmise of the great attenuation of the nebulosity, but also proved that their former nebulous appearance had been entirely the effect of the passage of their feeble light through the nebulous matter spread out before them. The i Qth of January, 1 8 1 1 , 1 had another critical examination of the same object in a very clear view through the 4O-feet telescope ; but notwithstanding the superior light of this instrument, I could not perceive any remains of nebulosity about the two small stars which were perfectly clear, and in the same situation, where, about thirty-seven years before, I had seen them involved in nebulosity. u If then the light of these three stars is thus proved to have undergone a visible modification in its passage through the nebulous matter, it follows that its situation among the stars is less distant from us than the largest of the three, which I suppose to be of the 8th or 9th magnitude. * * * * From the very considerable changes which I have observed in the arrangement of its nebulosity, as well as from its great 32 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. extent, this inference seems to have the support of observation ; for in very distant objects we cannot so easily perceive changes as in near ones, on account of the smaller angles which both the object and its changes subtend at the eye. The following memorandum was made when I viewed it in 1774: 'Its shape is not like that which Dr. SMITH has delineated in his Optics, although somewhat resembling it, being nearly as in Fig. 37; from this we may infer that there are undoubtedly changes among the regions of the fixed stars ; and, perhaps, from a careful observation of this lucid spot, something may be concluded concerning the nature of it.' In January, 1 783, the nebulous appea'rance differed much from what it was in 1 780, and in September it had again undergone a change in its shape since January. "March 13, 181 1. With a view to ascertain such obvious alterations in the dis- position of the nebulous matter as may be depended on, I selected a telescope that, had the same light and power, which thirty-seven years ago I used when I made the above mentioned drawing ; and the relative situation of the stars remaining as before, I found that the arrangement of the nebulo&ity differs considerably. The northern branch N still remains nearly parallel to the direction of the stars a 6 [543, 734] ; but the southern branch S is no longer extended towards the star d [905] ; its direction is now towards the star e [843], which is very faintly involved in it. The figure of the branch is also different ; the nebulosity in the parallel P F of the three stars being more advanced towards the following side than it was formerly." DISCUSSION OF THE VARIATIONS IN FORM, ETC., ACCORDING TO HERSCHEL. In his memoir of 1811, Sir WILLIAM HERSCHEL recites three ways in which the nebula of Orion has changed between 1774 and 181 1. These are: (a) The stars 543 and 848 which were nebulous in 1774, as seen through a 5^- foot reflector, were afterwards, 1 8 1 1 , found to be free from nebulosity with a telescope of the same power, and no nebulosity could be detected about them even with the 40- foot telescope. (6) The Messierian branch which, in the drawing of 1774, was directed towards star 905, in 181 1 was directed toward the star 843, and (c) The figure of the lucid part had altered ; in particular the nebulosity in the direction of the parallel having moved further towards the east. These points could scarcely be intelligently considered as long as we possessed only the very.rough drawing of 1774, Fig. 1 1, as a record of HERSCHEL'S observations. The conclusions of HERSCHEL, which are never to be lightly doubted, were, in this case, not easily examined, since no knowledge was at hand as to the instrumental means, the manner of observation, or the amount of time spent upon the subject. The preceding copy of the observations in detail made by HERSCHEL upon the nebula, enables us to examine with more care the data upon which these conclusions rest. It must not be supposed, however, that all the evidence considered by HERS JHEL is there reduced to writing, since it is plain, from the memoir of 181 1, that these were the merest notes which served to remind him of former work done. We have from these notes, however, a much more certain clue to the data really available to HER- SCHEL himself, and we may with more confidence, though still with circumspection, MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 33 examine his conclusions. It must not be forgotten that Sir JOHN HERSCHEL has pur- posely omitted all the figures which accompany the memoir ( 1 8 1 1 ) of his illustrious father from his "List of figured Nebulae," in the Introduction to his General Catalogue of Nebulae, as he says "they do not profess to be resemblances." It is quite true that so much as this is stated by Sir WILLIAM HERSCHEL, on page 272 of this memoir, but it is evident that we must partially except from this rule Fig. 37 of the nebula of Orionj for Sir WILLIAM HERSCHEL expressly says that the nebula was, in 1774, "nearly as in Fig. 37," and he proceeds to predicate a change of shape on evidence derived from this drawing. The figure of this drawing is like a Greek 1, and I have been able to reproduce its general shape by viewing the nebula (1875) through the small finder of the 2 6-inch refractor of the Naval Observatory (2^ aperture, magnifying power about 15 diameters), and M. TROUVELOT has made a sketch of this appearance, which roughly agrees with Fig n. As is stated by SECCHI in his memoir of 1868, it is the very dark, straight channel between nebula Mairanni and the main nebula that determines the shape of some of the older drawings to be similar to that of a Greek omega with an elongated base. In HERSCHEL'S figure the north base of the omega is not sufficiently extended toward the west to agree with the present appearance. FIG. 12. KEELER, 1878. The accompanying Fig. 12 (for comparison with HERSCHEL'S) was drawn by Mr. J. E. KEELER, instructor in physics at the Johns Hopkins University of Baltimore, January 23, 1878, with a telescope having an aperture of 2.5 inches; power, 75. We may now consider the points (a), (6), (c) in detail. With regard to the first, which concerns the (formerly) nebulous stars 543 and 848, the MS. journal of obser- vations enables us to see that, although the nebula was observed 33 times til, 11 APP. V 5 34 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. no case is the nebulosity about these stars explicitly mentioned as observed, although its absence is remarked. Neither MAIRAN (1733), LONG (1742), LE GENTIL (1758), MESSIER (1771), LEFEBVRE (1779), nor SCHROETER (1794), mention this nebulosity, and however little negative evidence should in general avail, it would seem that here was sufficient to establish a strong probability, if not a certainty, that the observation of 1774 with the weak 5^-foot telescope was erroneous in ascribing a nebulous envelope to these two stars. It is to be noted that several of the above named authorities specially examined MAIRAN'S nebula (about the star 734), and the nebulos- ities about 543 and 848 could hardly have escaped them all. Moreover, the drawing bears the marks of being mainly a memorandum, as it is not possible to certainly iden- tify all the stars without his description. The 5^ -foot reflector was probably the first telescope that HERSCHEL himself made (in the winter of i773~'74), and before this time he had not had extensive opportunities to familiarize himself with the appearance of nebulae, so that, as STRUVE justly remarks (Obs. sur la nebuleuse d'Orion, page 97) this observation of 1774 is not of the same weight, as those made somewhat later.* The showing of nebulous halos to bright stars is a not uncommon defect of reflectors, and it seems quite possible, and in the light of contemporary work even probable, that some such accidental error is here recorded. With regard to (a), then it seems probable, from a consideration of all the evidence, that no material change took place in the appearance of these stars from 1733 (MAIRAN) to 1794 (SCHROETER), and in 1811 HERSCHEL describes them to be as they now are. (b). Direction of the Messierian branch. In 1771 MESSIER figures this branch directed to (and including) the star 843, as does also SCHROETER in 1 794. HERSCHEL describes it in 1774 as directed to 905, and as later he found it to be directed to 843, a change was supposed to have taken place. To admit HERSCHEL'S explanation would be to suppose the branch to have been in 1771 at the star 843, to have moved to 905 (distant about 4^0 by 1774 and back again to 843 by 1794. This is manifestly im- probable, and the error arose in the defective drawing of 1774, which, as before, served only to mislead. (c). With regard to the motion of the whole nebula toward the east, or rather that part of it in the parallel of J ', it is plain that the idea was first suggested to HER- SCHEL by comparing his drawing of 1774 with the nebula as he saw it later in the heavens. It may be at once declared that no such motion has taken place as his drawing, compared with modern observations, would indicate; but in order to test the question of any motion in this direction, I have excerpted from the various observations and drawings such details as bear on this and allied points, and they follow immediately after this paragraph. * L'argument principal de W. HERSCHEL en faveur des changements, consiste dans son observation que la ne"bu- Iosit6 qui entoure ]'e"toile de MAIRAN, lui a paru autrement disposed et beaucoup plus faible en 1810 qu'en 1774, et que deux petites e"toiles situ^es dans le voisinage de Pe"toile de MAIRAN avaient en 1810 entierement perdu la nelnilositd qu'il avait remarque"e auteur d'elles en 1 774. fivideinment de pareilles observations peuvent facilement avoir leur origiue dans quelque de~faut optique de 1'instrnment ou en d'autres deceptions ; mais avec un observateur aussi consommd et eirconspect que l'e"tait W. HERSCHEL m6me cette remarque perdrait tout sou poids, s'il n'y avait encore a conside"rer qu'en 1774 il ne posse"dait encore ni des instruments assez parfaits, ni 1' experience qu'il a gagne"e plus tard. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 35 SITUATION OF THE TRAPEZIUM WITH REGARD TO THE ADJACENT NEBULOSITY, ETC. 1656. HUYGHENS. The trapezium involved in nebula; the south # of Sinus magnus is filled with nebulosity. The north shore of Sinus magnus much as at present. 685, 708, 741 in bright nebulosity. 1673. PICARD. The trapezium much as in HUYGHENS as to following nebulosity. 628 and 619 are, however, in a dark space (V) surrounding them and extending toward the W. and S. The south y 2 of Sinus magnus filled with fainter nebu- losity : its north shore in the same general direction as at present. 685, 708, 741 in fainter nebula. 1 742. LONG. The trapezium involved in nebulosity. The south part of Sinus magnus filled with nebulosity. The north shore much as at present. 685, 708, 741 in bright nebulosity : 724 free from nebulosity. The parallel of & is involved east of 708, but not so far as 741. 1758. LE GENTIL. The trapezium in a dark space (V). The western end of Sinus magnus filled with nebulosity ; its southern y 2 also nebulous. The line of its northern shore somewhat as a very small telescope shows it to-day ; i. e., with the s. f. corner of ff cut off. 685, 708, 741 are in darkness. The extreme northern star of the drawing is probably 479. The parallel of & is involved east of 708. 1771. MESSIER. The trapezium involved, the following star being just on the preced- ing edge of Sinus magnus. No darker space about trapezium. The parallel through 9' and following is involved in nebulosity from & to beyond 741 ; i. e., the space called Sinus magnus at present has, according to MESSIER, its south part filled with nebulosity. The north shore is much as small telescopes show it to-day, the s. f. part of 6 being cut off. 685, 708, 741 on the southern edge of the bright nebulosity, but in the darker part. The Messierian branch goes to 843. 1 774. HERSCHEL (from the drawing). The trapezium is involved, but its two follow- ing stars are on the edge of the Sinus. The north shore of the Sinus is on the parallel of 6 1 9 and 640. The general shape of this gulf is much as it is to-day, but its situation is entirely different. 685, 708, 741 involved in bright neb- ulosity. 1776, Nov. ii. HERSCHEL (MS.). "The greatest glare is about" the trapezium. 685, 708, 741 "almost free from any glare." The Sinus magnus was totally dark. 1778, Jan. 25 HERSCHEL (MS.). "At the eastern side the rays seem to form an equilateral triangle" with 685, 708. ? At 74 1 "it bends towards the east at an angle of 1 10 to 1 20", much as in MESSIER. The north shore of Sinus magnus is concave towards the south, different from MESSIER. The angle between the south and east shores of G is about 75 to 70. In MESSIER it is about 90. 1783, Nov. 3. HERSCHEL (MS.). "Just close to the 'trapezium' it is totally black for the short space of a few seconds" (V). In the Sinus magnus "a small distinct nebula of an extended shape." (See observation of 181 1, Jan. 19.) 36 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 1806, Feb. ii. HEESCHEL (MS.). Trapezium completely involved. 685, 708, 741 completely free. 1810, Dec. 31. HERSCHEL (MS.). 724 in "very faint nebulosity." "The black space near the four stars" (Sinus magnus) "is much contracted." 1811, Jan. 19. HERSCHEL (MS.). " 2 of the 4 stars (of the trapezium) are within the nebulosity.'' The Sinus magnus " is much contracted." I include the results of LEFEBVRE (1779) and SCHROETER (1797-^78) in this com- parison. 1779. LEFEBVRE. The trapezium completely in the Sinus magnus. 685, 708, 741 free from nebulosity. 1 794. SCHROETER. SCHROETER made his drawing of 1 794 on a chart in which MES- SIER'S positions of the stars were taken as a basis, and this at the outset will account for a considerable amount of distortion in the drawing. Thus SCHROE- TER'S drawing gives G. P. B. 523 about 50" west of 479, while it is in fact east of 479 by over 150". Hence, in a part of the nebula quite close to 0', we may expect misplacement of the various minor features by as much as 3' in some cases. But when the features are important, and still more when two are to be compared which SCHROETER saw in the same field of view, we may rely with more confidence upon the drawing as it stands, and in nearly every case, though often only after much pains, I have succeeded in satisfying myself as to the portion intended to be represented.* From the drawing and text the fol- lowing may be established : The Messierian branch passes through 843 ; the three following stars of the trape- zium are in a dark space (V). 685, 708, 741 are free from nebulosity. The parallel through & and following it is entirely immersed in nebulosity ; that is, the south % of what is now the Sinus magnus is nebulous. The angle of 685-741, and the north short) of the Sinus, is 70. From all of this I conclude that HERSCHEL'S point (c) is not established. OBSERVATIONS OF LEFEBVRE (1779). ROZIEK'S Observations sur la Physique, volume xxii (page 34 and Plate I, Fig. 3), for 1 783, contains a letter to the Abbd ROZIER from M. LEFEBVRE, which I extract in full. "OBSERVATIONS SUR LES NEBULEUSES D'ORION; par M. LEFEBVRE, Pretre a VOratoire, Professeur de Physique du College de Lyon. "A 1'occasion de la ressemblance que M. de MAIRAN soup9onne entre la matiere de a lumiere zodiacale, celle de Paurore boreale et celle de ces n^bulosites qui accom- pagnent quelques e'toiles, j'ai eu la curiosite' de revoir la ndbuleuse de Pepee d* Orion dont la figure m'avoit toujours paru diffdrente de celle qui lui ont donnde MM. HUYGHENS et de MAIRAN. La voici, telle que j'ai cru Pappercevoir le 15 fevrier 1779, par un ciel tres-pur, a onze heures et demie du soir, ayant a-peu-pres 1 7 degrds de hauteur. Voyez Plane. I, fig. 3 [our fig. 13]. " Les sept e'toiles qui la composent m'ont paru entitlement hors du nuage, sur- * If it were worth while a corrected drawing could have been made, as in the case of HUYGHENS, etc. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 37 tout les trois inferieures qui larssoient entr'elles et le image un intervalle obscur bien tranchd, a 1'exception de celle qui en est la plus proche, qui peut me laisser quelque doute. "M. de MAIRAN croyait deja que, depuis 1756 [misprint for 1 1656] (temps de 1'observation de M. HUYGHENS) cette ndbulosit^ avoit eprouvc* quelque changement II seroit aujourd'hui bien plus considerable, et surtout la position des e*toiles, de'tache'es du nuage, sembleroit indiquer qu'au moins la n^bulositd n'est produite par aucune de ces Voiles. Voyez les fig. de la PI I. La fig. i re est celle de M. Huyghens, en 1656. La fig. 2, celle de M. Mairan en 1725. (Ces deux figures sont renversees.) La fig. 3 est droite, et represents la nebulosity telle qu'elle a paru cette annee 1779, observee avec un telescope de y/ 2 pieds." This figure is but eight years later in date than MESSIER'S. and by no means so detailed. The principal differences and resemblances are as follows : (a) Both have the Mes- sierian branch; in LEFEB- VRE the distance from d Orionis to the end of this branch is 5 times the dis- tance apart of stars 685 and 741 of G. P. BOND'S Catalogue [128"], while, according to MESSIER, the latter distance is 6 times the former ; (by the stars 685, 708, and 741 are detached from the nebula in LE- FEBVRE'S drawing, on the edge of it in MESSIER'S ; the outline of LEFEBVRE'S drawing would nearly fol- low a line of equal light on MESSIER'S, except in the Messierian branch; (c) the four stars of the trapezium are quite out- side of all nebulosity in LEFEBVRE, quite inside according to MESSIER. The angle of the opening ot the "jaws" is almost the same as given by LE GENTIL, namely, 75 in LEFEBVRE to 56 (LE GENTIL). FIG. 13. LEFEBVRE, 1779. 38 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. It is also noteworthy that LEPEBVRE uses the same conventional sign to represent a nebulosity gradually shading off to nothing, as does Pi CARD (see fig. 4), namely, the ragged and saw-shaped boundary. This drawing is hardly more detailed than HUYGHENS', although nearer the present shape of the nebula, as seen with very small telescopes. OBSEKVATIONS OF SCHKOETEE (i7 94 -' 9 8). In BODE'S Jahrbuch, 1798, p. 198, SCHROETER has a few observations on the nebula in Orion, from which I quote. These observations are subsequently given in full, but the following notes will serve to settle one or two doubtful points in the sub- sequent accounts : In "the central parts of MESSIER'S drawing" no less than 18 new stars were seen (Jan. 7, 1 794), " und atisser diesen iui Nebel nahe nord- und westlich bey den ein Trapezium bildenden 4 Sternen, Zwey kleine hellere Nebelflecken, in deren Mitte sich zwischendurch, ein hellerer doch sehr matten Nebelpunct zeigte. Auch haben Zwey gedachter 18 Sterne, gleich dem Huygenischen Nejbelsterne, ihren eigenen vom iibrigen getrennten Nebel um sich." These "Zwey" may be i and a of his drawing of 1 794 [Fig. 14], but of these only a answers to the description, and as no mention is subsequently made of a change it is quite likely that for north and west we should read south and east when c and b would be referred to. I know of no other explanation. He also makes the following notes : (1) 685, 708, 741 are in a dark space, different from MESSIER, 685 being just on the edge of the Frons. (2) The Sinus maynus is darker than the surrounding background of the heavens. (3) The Proboscis minor is described. In the Aphroditographische Fragments of SCHROETER (p. 243 and Plate II) may be found a resume of his work upon the Orion-nebula, from which I quote largely, partly because this work is now difficult to obtain,* and partly because of its intrinsic value. As OTTO VON STRUVE has justly remarked in his memoir on the same nebula, no doubt can be entertained of the good faith of SCHROETER, nor of his general acuteness arid accuracy as an observer. The apparent discredit into which his works have fallen seems to be largely due to the unwillingness of astronomers to follow him in his the- oretical conclusions, and partly also to the diffuse form in which he gives them.f "BEMERKUNGEN UBER ORIONS LICHTNEBEL. " Verschiedene Augen mit verschiedenen Fernrohren bewaffnet, diirften zwar diesen merkwiirdigen Lichtnebel unter verschiedenen Witterungs-und sonstigen * I am indebted to the courtesy of President ELIOT, of Harvard University, and to the kindness of the authorities of Harvard College Observatory, for the use of the copy from which I quote. t We find in Mem. R. A. S., vol. iii, p. 188, a foot-note to a paper of Sir JOHN HERSCHEL'S, in which his opinion of SCHROETER'S drawing of nebula Orionis is given, which we quote: "I have been guilty, I find, of a piece of invol- untary injustice to M. SCHROETER in omitting to mention him among the observers of this nebula. I am indebted to my esteemed and admired correspondents, Dr. OLBERS and M. HARDING, for a reference to his Aphrodttographisclie Fragmente, for his observations on this nebula; and the title of the work will plead my excuse for the omission. The representations of it there given are, however, so dreadfully bad as almost to convert the excuse into a justi fication * * * *" MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION, 39 Nebenumstanden immer etwas verschieden sehen ; bis jetzt sind wir aber in der Schopfungskunde zu weit zuruek, als dass es niitzlich sein sollte, alle Bruchstiicke von Beobachtungen solcher Art der Nachwelt zu umstandlichern Vergleichungen, Prufungen und Folgerungen aufzubewahren. In solcher Hinsicht konnen daher auch folgende Bemerkungen niitzlich werden, die mit zwei vorzuglich lichtstarken Instru- menten, iiamlich einem ^fiissigen besonders aber mit dem 27fiissigen Reflector angestellt sind. ^ Bei der da von verfertigten Tab. II befindlichen Charte, die in der Gestalt und Begranzung des Lichtnebels von den bisherigen bekannten Zeichnungen merklich abweichet, habe ich die in des Herrn Prof. BODE Vorstellung der Gestirne Tab. XXX, fig. 6, mit enthaltene Zeich- nung nach den Beobacht u n g e n des Herrn MES- SIER, in dem Un- terschiede der Aufsteigung und Abweichung, jedoch nach ei- nem g r 6 s s er n Maass-stabe zum Grundegelegt, so dass diejenigen altern, b is h e r darin bekanntge- wesenen Sterne, bei welch en nichts zu bemer- ken vorgefallen, o h n e Bezeich- nung geblieben, die hinzugekom- menen aber mit Buchstaben be- FlG. 14. SCHROETER, 1794. zeichnet nachgetragen worden sind. Eine wirkliche mikrometrische Messung fand ich theils wegen der Feinheit und Dunkelheit der Gegensttinde unsicher und fast unmog- lich, theils aber auch fur meinen Zweck uberfliissig, well em im Schiitzen geiibtes Augenmaass in solchen Fallen oft sicherer ist. Urn aber desto leichter die dunklern Sterne von den hellern zu unterscheiden und in der Folge wieder zu finden, ist die 4O MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. Characteristik der abnehmenden Lichtstarke, wenn sie auch gleich aus bekannten Griinden nur sehr beylaufig und einem veranderlichen Wechsel ausgesetzt sein diirfte, unter der Charte nach mehrern und wenigern Sternstrahlen ausgezeiget. " Vornehmlich habe ich mich beflissen, die ausserst verschwachenen Granzen und die Gestalt des hellern und schwachern Lichtnebels, die ich auch mit kleinern Tele- scopen und Vergrosserungen controliret und eben so gefunden haben, moglichst genau zu bestimmen. "Nach diesen vorlaufigen Bermerkungen sind die kleinen Sterne, welche ich bis jetzt ausser den von dem beriihmten Astronomen Herrn MESSIER ausgezeichneten nach und nach gefunden habe, folgende : "In a und b [D and A?] zeigen sich im 27fiissigen Reflector zwei sehr kleine Kernpunctgen, die beide gleich dem Huygenischen Nebelsterne in einen besondern vom iibrigen unterschiedenen Nebel gehiillt sind, besonders erkannte ich in a [D !] den hellern Nebelpunct den 6ten Janner 1 794, und ein ahnlicher schien westlich bey /3 durchzublicken [following part of C ?] : wenigstens war hier der Lichtnebel merklich heller, und es schien sich in diesen Puncten die Kraft des 27fiissiges Telescops der Auflosbarkeit des Nebels zu nahern. Einen ahnlichen vom iibrigen getrennten Nebel haben die beiden Sterne C [Not in MESSIER; our E !] und d [570] den ich besonders deutlich den 7ten Janner 1794, ab nach 10 U. mit 250 mal Vergrosserung des 27- fiiss. Telescops erkannte, und von welchen des erstern Nebel sich bios nordlich mit dem iibrigen Nebel zu vermischen schien. ***** Weiter fand ich nordlich zunachst unter 9 einen entfernten dunkeln Stern i [635] bei dem sich westlich ein etwas dunkler Strich im Nebel zeigte [W =: lacus secchii] * * * * * q [650 + 653 ?] liegt in einem langlichen dunkeln Striche, welcher den ostlichen Lichtnebel vom iibrigen trennt und ist auch im I3fiiss. Ren*, sichtbar. ***** Der Anblick des Lichtnebels selbst ist damit prachtvoll und erscheint in mehrere von einander getrennte Theile aufgeloset, die sich durch Zeichnung nicht ausdriicken, sondern nur sehen lassen, und zwiscben welchen man die dunklere Himmelsluft unterscheidet. So wie er unter der lichtstarken 183 mal Vergross. des 27fiiss Reflectors ins Auge fallt, ist er Zunachst bei 0^ und zwar von yd bis nordlich unter k und q hin am hellesten. Von a [D?] bis gegen den Stern [843] sprosst von ihm ein langer Zweig gegen Siiden ab, welcher auch mit schwachern Fernrohren sichtbar ist. Viel schwacher ist hingegen ein gegen Osten nach dem Stern /* [848] hin absprossender Lichtstrief, der mit dem I3fiiss. Reflector nur zum Theil erkannt wird. Er fallt gleich dem hellern siidlichen Streife gegen seine Spitze hin immer matter und zuletzt so ausserst matt ab, dass seine Endspitze mit dem 27fiissigen Telescope, dessen grosser Lichtstarke ungeachtet, nicht vollig genau bestimmt werden kann. Reizend ist sein Anblick der Vorstellungskraft des Naturforschers : denn hochst wahr- scheinlich hat er gleich dem siidlichen Lichtstreife gegen unser Auge eine schrage Lage und erstreckt sich in einem unermesslich entfernten Himmelsraume bis zu einer Entfernung fort, die sich der Fasslichkeit des Beobachters entziehet. P^ine almliche schrage Lage scheinet auch von d l an bis zur Endspitze des lichtern Nebels rj hin statt zu finden : denn auch heir wird ein gleicher matterer Abfall des Lichts merklich, und der Nebel fallt von a [D!] nach ;/ hin eben so matt und unbegranzt, als nach , MESSIER, and that here presented, it would seem as if the brighter portions of the nebula had formerly extended over this space, and were now contracting and receding towards the trapezium ; for in the figures of HUYGHENS and PICARD the three stars are represented as deeply immersed within the nebula, and the division which contains them is by far the most considerable portion of the whole ; but as in these there is no variation of shade, it is impossible now to say where they fixed the limit of what is to be considered as nebula. In MESSIER'S figure they are placed precisely on the edge of the most luminous portions, as above mentioned, while at present they are com- pletely detached from it, and the line which they form makes an angle of at least 45 or 50 with the frontal line. This appears conclusive, as the withdrawing of the nebula would seem to have gone on progressively. Unluckily, however, the observa- tions of LE GENTIL will not allow of this conclusion. This figure, drawn on Marcli 10, 1758, and therefore sixteen years before MESSIER'S, represents it in this respect just as it stands at present, the line of the three stars forming the same angle with the frontal line ; and to take away all doubt on the subject, he says expressly : "Les trois e'toiles que Messieurs HUYGHENS et PICARD, et apres eux GODIN et DE FOUCHY, ont vues en ligne droite dans ce que j'appelle la machoire inferieure, nous ont paru totale ment de'tache's de la ne'buleuse elles e*taient sur une meme ligne droite, et elles faisaient un angle considerable avec la machoire inferieure." On another night: "Je trouvai," he says, u * * * que les trois etoiles qui dans les figures de Messieurs HUVGHENS et PJCARD sont dans la machoire inferieure faisaient avec elle un angle d'environ 40 ;" the meaning of which he further explains by a reference to letters in his figure. The observation was made with a 6-feet Gregorian telescope, which of course would only permit the brightest part to be distinguished ; but on turning on it telescopes of 8 and 16 feet, he assures us that the three stars then appeared exactly as HUYGHEKS and PICARD represented them i. e., quite within the nebula, and of this appearance he gives a separate figure. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 51 1 These observations completely destroy the weight of any conclusion as to a change, drawn from the comparison of HUYGHENS' figure with MESSIER'S or my own. But how MESSIER could have overlooked the very remarkable and striking effect of the frontal line and its oblique position with respect to the three stars, with LE GENTIL'S figure and description before him, and his attention specially turned to the point, and with a telescope capable of showing the other peculiarities so well, is certainly sur- prising, and may lead to a suspicion that this line has really since become more definite, and that the nebula has retreated. The star ^ [G. P. B. 707] is not in this figure. "Sinus Gentilii. The totally dark recess designated by this name is represented by LE GHNTIL as a very long, narrow exactly rectangular cut, commencing, not as at present, at a considerable distance to the southwest of the star c [685], but nearly opposite it, so that the line joining [647], close along the borders of the Sinus at its bottom, is a part much brighter than the rest, of a small breadth. "Regio Derhamiana, etc From the trapezium there run out branches or tails like those of comets, in the direction of the star T [523] along the confines of this and the Huyghenian region, and across all that space between T [523] and o [479] and 4; [449], which lose themselves imperceptibly in the very faint, diluted nebulosity which fills the Fouchian region and adheres more or less in the form of wisps to all the stars in the latter, especially the stars n [335] and p. The stars o [479] and [449], on the contrary, are rather free of nebulosity, being situated in a darker portion, which forms a natural separation between the Fouchian and Godinian region. In the latter, the nebula dies away imperceptibly into total darkness." HERSCHEL'S remarks have been quoted from largely, as his memoir served to settle a nomenclature which has been since adhered to, and as they give a good general description of the different parts. In his first figure (Fig. 19) it is only necessary to call attention in passing to the nebulous filaments which he has represented on the north border of Sinus magnus near SCHROETER'S bridge. I conceive them to be, in fact, representations of the two bridges of SCHROETER imperfectly seen. OBSERVATIONS OF POND. (1826.) The recession of the nebula from the brighter stars is remarked upon by HERSCHEL in what immediately precedes The earliest notice of a recession of the light of the nebula from the neighborhood of the brighter stars in it is, however, due to POND, who, in 1826, communicated to the Royal Astronomical Society the results of his observa- tions with RAMAGE'S reflecting telescope.* * Mem. R. A. S., vol. 2, p. 93. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 53 After describing the situation of stars 685, 708, 741 of BOND'S Catalogue, he pro- ceeds as follows : " Now these three stars are neither situated on the edge of the nebula as represented in M. MESSIER'S plate, * * * nor are they parallel to the edge; but they seem to be insulated from the nebula, the light of which retires from them in a semicircular form, as if they had, in some incomprehensible way, either absorbed or repelled the light from their immediate vicinity. The same appearance is observable in the trapezium, round the four stars of which the light has also receded in a very analogous manner, leaving them on a comparatively dark ground. In both these cases the impression on my mind is that the stars have been the immediate cause of the disappearance of the light. ****** Another similar case is noticed a few minutes distant from the trapezium, and the Astronomer Royal concludes with an expression of his intention to communicate a drawing of this appearance to the society. I have not been able to learn anything regarding this drawing. Through the kindness of Sir GEORGE AIRY an unsuccessful search has been made for it at Greenwich and in the archives of the Royal Astronomical Society. OBSERVATIONS OF LAMONT (1837). LAMONT speaks of his own drawing (Fig. 20) as follows : " Meine Zeichnung be- zieht sich nur auf den glanzendsten Theil des Nebels, der das Trapez umgiebt. Ver- gleicht man sie mit der HERSCHEL' schen Darstellung [in Mem. R. A. S., vol. 2 (1826)] so ergeben sich nicht unbe- deutende Unterschiede ; dass merkwur- digste aber ist, dass der Refractor bes- timmte und begrenzte Abtheilungen erkennen lasst, wo Sir JOHN HERSCHEL nur im Allgemeinen ein ungleiches Licht gesehen hat. "Ubrigens bemerke ich, dass meine Absicht bei Beobachtung des Orion- Nebels dahin ging, die einzelnen Ab- theilungen, so weit es moglich war, micrometrisch zu messen, um liber kiinf- tige Aenderung bestimmt entscheiden zu konnen : dass aber auch eben desshalb in der Zeichnung vorzugsweise nur die messbareii Theile mit Sorgfalt darges- tellt, die schwacheren und unmessbaren Abtheilungen dagegen mit wenigerm Fleisse beriicksichtiget sind."* The Fig. 20 here given of the nebula according to LAMONT, is copied from his engraving published in the work just cited. Through the kindness of Dr. DOBERCK, astronomer of Colonel COOPER'S private observatory at Markree Castle, I have had FIG. 20. LAMONT, 1837. Nebelflecken, Miinchen, 1837, p. 23.) It is unnecessary to go into an examination of LAMONT'S obser- vations here, as they have been fully discussed by LIAPONOFF in his elaborate memoir. We may, however, repeat LAMONT'S remark that the divisions of the Huyghenian region (E, F, G, H, etc., of the Index-Chart) are down with precision. 54 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. access to an original pencil drawing by LAMONT'S own hand, dated February, 1839, sent by him to Markree Castle for comparison with a sketch by EDWARD G. COOPER? esq. The engraving is a very faithful reproducing of the original, but it is to the latter sketch that I have referred in all comparisons throughout this work, and in general I have always referred to the original engravings and not to the wood-cuts herewith, which are inserted principally for the purpose of making text intelligible. OBSERVATIONS OF SIB JOHN HERSCHEL (1837). The central part of Sir JOHN HERSCHEL'S second and very elaborate drawing, made at the Cape of Good Hope in 1837, is given in Fig. 2 1 From his work, Astronomical Observations at the Cape of Good Hope (p. 25, et seq.), I extract the following : "I am aware of but four representations of this nebula which have ap- peared since 1824 one by Dr. LAMONT, published with his thesis " Ueber die Nebelflecken," read at the anniversary sitting of the Bavarian Academy of Sciences, August 25, 1837, and two by Sig. RONDONI, a Roman artist. The former, though rather a coarsely-executed figure, and con- fined solely to the denser part of the nebula, or those regions which I have termed the frons, occiput, and fauces, yet contains some valuable particulars respecting the apparent breaking-up of the nebula (especially about the frons and occiput) into patches and knots ; particulars very unsatisfactorily expressed in my figure of 1824, but in which my observations of 1834 and 1837 fully confirm Dr. LAMONT'S remarks. In his figure he has (perhaps intentionally) omitted to express the remarkable effusion of the nebula from the "frons" and "proboscis" into what I have termed the "Subnebulous region," and he has filled the interior of the trapezium with nebula, a particular in which we disagree decidedly. The two figures of Sig. RONDONI, which are given in the Report of Observations made at the Collegio Romano, by the associated astronomers of the Gregorian University, for the years 1840 and 1841, are perhaps rather to be regarded as curious specimens of lithography than as accurate representations of the nebula (such, at least, as I have ever seen it), which they resemble in fact hardly more than they do one another. * * * * * * FIG. 21. J. HERSCHEL, 1837. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 55 I purposely avoid all comment on the remarks which accompany these two repre- sentations, leaving- astronomers to form their own judgment on them. The other representation above alluded to is that of Sig. DEVICO himself, in the year 1839, printed in the Annals of the Collegio Romano for 1838, which, though much less inaccurate in many respects than Sig. RONDONI'S, is by no means free from objection on that score." On pp. 31 et seq. of the same work we find Sir JOHN'S discussion " Of evidences of change in the nebula." " To the reader who has never viewed this object through powerful telescopes, but who is familiar with the various representations which have from time to time been made of it (including my own of 1824), the number and com- plexity of the various branches and convolutions now first exhibited, and the different aspects under which even the portions best known are now presented, will no doubt tend to convey a strong impression of great and rapid changes undergone by the nebula itself. I am far from participating in any such impression. Comparing only my own drawings made at epochs (1824 and 1837) differing by thirteen years, the dis- agreements, though confessedly great, are not more so than I am disposed to attribute to inexperience in such delineations (which are really difficult) at an early period to the far greater care, pains, and time bestowed upon the later drawings and above all to the advantage of local situation and the very great superiority in respect both of light and defining power in the telescope at the latter over what it possessed at the former epoch, the reasons of which I have already mentioned. These circumstances render it impossible to bring the figures into comparison except in points which could not be influenced by such causes. Now, there is only one such particular on which I am at all inclined to insist as evidence of change, viz, in respect of the situation and form of the "nebula oblongata" which my figure of 1824 represents as a tolerably regular oval extended very nearly in a right line, or at most but a very little curved upwards between the two stars x = No. 120 [G. P. B. 781], and K = No. 136 [G. P. B. 848] of the Catalogue. Comparing this with its present appearance, as exhibited in Plate VIII, it seems hardly possible to avoid the conclusion of some sensible alter- ation having taken place. No observer now, I think, looking ever so cursorily at this point of detail, would represent the broken, curved, and unsymmetrical nebula in question (lying, as it does, in its whole extent, clearly out of the line of junction of the two stars above mentioned), as it is represented in the earlier of the two figures; and to suppose it seen as in 1837, and yet drawn as in 1*24, would argue more negli- gence than I can believe myself fairly chargeable with. "There is another point on which considerable stress might be laid were I satisfied that the earlier diagrams on which it turns were done with sufficient care. In 1837, the nebulous spur towards the end of the great proboscis, which terminates at K (No. in) [G. P B. 746], certainly was neither joined to the proboscis itself nor directed towards the star A (No. 135) [G. P B. 843], but rather towards a point about one-third of the distance from A (No. 135), to C (No. 126), near to where there is a small star 16 m (No. 131). Now I find two diagrams, one of December 25, 1832, the other of November 25, 1834, in which this spur is repre- sented as running directly from A to E, and forming a complete hook, no way dis- joined from the proboscis. But the chief attention on the first of these -occasions was 56 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. directed to the magnitudes and situations of the stars, and the hook seems to have been only roughly sketched in as a novelty to be further noticed in future, while on the last it was only very faintly indicated in a diagram of the stars adjacent to Orionis on all sides, preparatory to the formation of chart intended to take in both z Orionis, on the one side, and C Orionis on the other, which was subsequently discontinued FIG. 22. DE Vico AND RONDONI (1839-1841). (69) "Still less can we insist, as evidences 01 change, on such particulars as the curiously notched outline of the "nebula Mairanni" about the star ju (No. 108) [G. P. B. 734], now for the first time represented; or on the intricately rifted and broken state of the frontal and occipital region of the principal nebula. I ought to mention here that (owing, no doubt, to the difficulty of properly representing on paper and by lamp-light an object of the kind) I find a good deal of disagreement in respect MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 57 of the number, size, and distribution of the portions into which it may be considered as broken up, not only between my present figure and Dr. LAMONT'S, but between my own drawings of this part on several nights. But the most material difference between Dr. LAMONT'S figure and mine consists in the characteristic forms of these portions, which he represents as rounded masses more or less detached from or running into each other and into a general nebulous ground ; while in all my later drawings the effect is rather that of a tolerably uniform surface marked with branching rifts or channels like roads. There is one peculiarity in Dr. LAMONT'S figure which I can no way reconcile to my own impressions, viz, the strangely different form and magnitude which he assigns to the "Sinus Gentilii," from what I have always found it. This is a point which I trust he will be induced to re-examine." A full discussion of HERSCHEL'S drawing is given by LIAPONOFF and STRUVE, and will be best understood in connection with their observations. OBSERVATIONS OF DE VICO AND RONDONI (1839-1841). These are detailed in the Memoirs of the Roman College in the volumes for 1839 and 1 84 1 . Fig. 20 gives the best one of the three drawings. It was made by M- ROVERE and probably revised by P. DE Vico. Attention may be called to the comparative faintness of the region E (Index- Chart), the south point of the Huyghenian region. i OBSERVATIONS OF KAISER (1844). These are detailed in Die Sterrenhemel, vol. ii, Plate iii, Fig. i, and p. 538. I have not been able to see this work, but I owe to Dr. v. DER SANDE BAKHUYSEN a copy of the plate i . i . . . TT FIG. 23. KAISER, 1844. which is given in rig. 23. OBSERVATIONS OF COOPER (1847). Through the kindness of Dr. DOBERCK I have received an exact copy made by him of an outline sketch of the nebula made by COOPER about 1847. I* i g not repro- duced here, as the only points of note are : i st. E, of the Index-Chart, has at its s. p. corner a curved continuation like MES- SIER'S E, and similar in outline to LASSELL'S drawing of 1862. 2d. Three small projections are shown on the north shore of T (of the Index-Chart) similar to those shown in HERSCHEL, 1824 (Fig. 19). The east one of these is the longer, the west one the shorter. They undoubtedly refer to the bridges of SCHROETER. 3d. At the spitz (s. f. point of a in Index-Chart) COOPER has a small star marked No. ii. 4th. An oval space near where star 602 would be (it is not laid down) marks some special region. It is in all probability intended to discriminate the blank channel following I. OBSERVATIONS OF LASSELL (1847). In February, 1847, Mr. LASSELL made some studies of the nebula at his observ- atory, Starfield, near Liverpool. An oil-painting of this date was presented to the App. V 8 58 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. Royal Astronomical Society, and a copy also is preserved at Ray Lodge, his resi- dence in later years. These paintings were poorly copied in an engraving privately distributed, a portion of which is given in Fig. 24. The full title of the oil-painting which was made at Burlington House from LASSELL'S observations is, "The Great Nebula of Orion, with its Stars, as seen in February, 1847, with the Starfield Equatorial Reflector of 24 inches aperture and 242 inches focus. LASSELL." This was also reproduced in NICHOL'S Architecture of the Heavens, p. 106, Fig. x. Little need be said of this in this place, except to call attention to the peculiarities of light and dark within the Huyghenian region itself, and particularly near A (Index-Chart). FIG. 24. LASSELL, 1847. f . -. As a drawing, it does not compare with the later and admirable one made (1862) in Malta. See Fig. 31. OBSERVATIONS OF W. C. BOND (1848). The essential parts of W. C. BOND'S description of his observations with the Har- vard College Refractor in 1 848 are extracted below : " All such parts of the nebula in the vicinity of the trapezium as presented definite outlines susceptible of being measured were referred to #'." .... " The stars Nos. 10, 12, 26, and 27 [G. P. B., Nos. 567, 573, 647, 651] mark the present bound- aries of the Huyghenian region . . . very accurately No 10 [567] is situated close on the preceding edge of this bright region, and is closely followed almost in the same parallel [meridian f] by No. 12 [573], a star of the i7th magnitude, the latter being within the boundary. [I must believe that No. 1 1 m G. P. B. 575, marked i8th magnitude, is here meant, in spite of the evidence from the letter of the text to the contrary.] No. 27 [651] is as nearly as is possible to determine with our telescope on the very edge of the following side .... and is pretty closely preceded by No. 26 [647] of the 1 7th magnitude within the bright part* .... There is a great diminution of the light in the interior of the trapezium but no suspicion of a star. Sir JOHN HERSCHEL'S drawing shows the southern termination of the Huyghenian region (E) strongly preceding [0'], whereas I have repeatedly laid the micrometer wire upon it, and have found it to be of the same right ascension as [<9']. The difference of declination is .... 161"." " The bright portion of the Huyghenian region termi- nates abruptly and roughly at No. 50 [708]." [That is, the following point of Q is in the same R. A. as 708, or Ja = 1 50". 5.] "The preceding side of the Huyghenian region in [Sir JOHN HERSCHEL'S figure] has the light gradually softened away into the Regio * In Proc. Amer. Acad., i, p. 326, W. C. BOND says that a star which he calls h 91. but which really is G. P. B. 651 + 647 = ad 75, "has hitherto been taken for a single star of the I7th magnitude. This is double, and the direction is towards 6' of the trapezium. The following one of this pair [651] is as precisely as possible on the following edge of the bright part of the nebula at the bottom of the Sinus magnus" The last phrase identifies the stars as 647 and 651. The position of 651 is now practically the same as in 1848. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 59 Gentiliana. I here see a strong irregular outline extending from the Sinus Gentilii to a little beyond No. 10 [567]." [This is quite different from to-day, when J is a strongly marked mass, nearly all of which is dark in BOND'S description, and it accounts for the angle of position of the occiput in his drawing^ 147. 5 against p= 136 for G. P. BOND'S drawing of 1865.] A further discussion of these observa- tions, occurs in connection with LIAPONOFF'S and STRUVE'S measures. Fig. 25 is a copy of the steel engraving published by BOND. I would call attention to the convex outline of the from which BOND first correctly laid down. The mass A just preceding the tra- pezium is also faint. The other masses are well terminated, and appear differently disposed from their present arrangement. Through the kindness of Pro- fessor PICKERING, director of Har- vard College Observatory, I have had BOND'S original drawings arid observations in my possession for some months ; and although I have not the sketch from which , i j T i FIG. 25. BOND, 1848. this engraving was made, 1 have another nearly completed one. On this A is considerably brighter than in Fig. 25. E is quite bright, and there is a suspicion that the masses within the Huyghenian region were not laid down one by one and accurately, but dotted in to represent the general effect simply. From other original sketches, also unpublished, I find : 1847, Dec. 13. [Order of brightness appears to be D, B, A, and the regions round G, H, E ; region round F, I. These estimates are rather uncertain.] 1848, Jan. 17 [Order of brightness appears to be D ; part of J?, I; G?, HI, orF?; Q, N, A; north parts of E. These estimates are again rather uncertain.] Large completed drawing (no date). [Order of brightness D, E, A. These masses are plain, and others are indicated, but not sufficiently to deduce the order of brightness intended by the author.] OBSERVATIONS OF LIAPONOFF AND STRUVE (1847-1851). 20 [p. 68]. Observations sur differ entes regions de la nebuleuse. La nature et la con- stitution physique de la ne'buleuse ne pouvaient devenir, *\ cause de sa position australe et sous les conditions pen favorables de ma station, un objet d'observations detaillt'es et rdgulieres. L'extreme difficult^ des recherches de ce genre se fait dejt\ sentir en comparant superficiellement les resultats obtenus par les travaux de Sir J. HER- SCHEL [1837], M. LAMONT [1839], et M. BOND [1848]. En examinant ces resultats, 6o MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. on est le plus frapp^ par la grand difference qui existe dans les dessins de MM. HER- SCHEL et BOND, par rapport aux formes et a la constitution de la region centrale, la plus lumineuse et la plus definie de toutes les parties de la nebuleuse. II est presque impossible de concilier sous ce rapport les deux dessins sans ad- mettre la supposition d'un changement considerable qu'aurait subi cette region dans 1'intervalle e'coul<3 entre les ^poques des deux observations. L'eclaircisse- ment de ces discordances appartient certainement a un travail futur a entre- prendre dans ce but special a 1'aide d'un telescope de force suffisante et sous des conditions favorables dans un degrd pro- portionnd Saris pouvoir affirmer rien de positif sur ce sujet, je vais cepen- dant exposer les re'sultatsdes etudes com- paratives, que -fai eu 1'occasion d'exe- FIG. 26. O. STRUVE, 1862. , cuter dans le courant de mes observa- tions sur les dessins des trois astronomes nommes. 21 [p. 69]. Etudes relatives a la region centrale de HuygTiens, Cette region m'a constamment paru offrir sur ces limites des formes prononcees, qui se laissent definir par des lignes a pen pres droites. La planche II donne une representation graphique de sa figure g^nerale, qui est d^finie selon mes observations par une ligne rompue A, B, C, F, E, G, D. Les pointes A, B, C, etc., se presentaient dans ma lunette si bien termine'es, que j'ai trouve possible de fixer leur position par des mesures microme'triques avec une exactitude assez considerable. Les resultats d^finitifs de mes mesures sur la position des pointes observers s'eta- blissent maintenant : A. D. AW D w a [628] et D - 9-1-3 n - 39-9 a [628] et E 87.6 + 44-3 . . a [628] et G - 81.3 + 18.4 a [628] et A - 4-4 - 138.0 + o" to + i" - M5" a [628] et F 4- 26.5 + 69.6 . . + 94"? a [028] et C -t- 32.8 + 3-1 . . . .' a [628] et K + 143-6 + 7L5 + 1 66" + 79" a [628] et B + 146.5 22. + 150" - 28" The approximate positions of these points from Washington observations are given for comparison in the columns A w and D w . En comparant la figure que mes observations assign^pt a la region de HUYGHENS, MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 6 1 avec le dessin de Sir J. HERSCHEL, je retrouve ici les pointes A, C, F et la direc- tion AD, la position de la pointe A dans ce dessin parait convenir tres exactement aux resultats de mes mesures. D'un autre cot(j les pointes B, D, G, E manquent dans la figure que donne a cette region Sir J. HERSCHEL. Cependant selon .mes observations la pointe B [Q] est la plus prononce'e par une condensation particu- liere de la matiere n^buleuse dans son voisinage, nomme'ment dans un espace triangu- laire, qui attend k 1'ouest & partir de cette pointe. Get espace lumineux se retrouve dans le dessin de HERSCHEL, mais au lieu de se terminer par une pointe il se prolonge & Test jusqu'k la region de MESSIER. La comparaison avec le dessin de M. BOND [1848] donne h peu pres les memes r(3sultats, on n'y trouve que les pointes A, C, F, la pointe B [Q] y manque comme les autres, et la region pres de B y est plutot faible, que lumineuse. La forme de la region se niontre plus conforme k mes observations dans le dessin de M. LAMONT. Je retrouve ici mes pointes A, B, C, F et G. Parmi les mesures, qu'avait institutes M. LAMONT, voy. Obs. Astr. in Spec. Reg. Monach. InsL, vol. XI seu VI, p. 21 et 22, les suivantes se laissent comparer avec les miennes: P. d. d'. a et A 180 27 R. a A et D 32i 53 a et A D 7.027 [ 96.3] A et B 50 52 1 a et AB 8.310 [H3-9] B et C 282 4 Ay ant compare les distances observees par M.. LAMONT entre plusieurs e'toiles, avec les miennes, j'ai trouvd la valeur angulaire de sa vis micrometrique R 13". 745.* Il en resulte que les distances des directions AD, AB (occiput, frons) k Fe'toile & Orionis, observers par M. LAMONT, sont: q6".6 [96". 3] et 114'' '.2 [ii3".9]. Par un calcul je deduis de mes mesures les m^mes distances = 96". 4 et io6 // .6. Ainsi les differences de nos nisultats sont: LiapounovLamont. rfP. ad. a et A + 82 A et D - 255 [+ o.i] A et B + 87 [- 7.3] B et C + 26 [Washington observations of the distance of occiput from & (1877, January 5) give s io6 x/ .5(3) LIAPONOFF-HOLDEN =r io /7 .i. [p. 74]. Quant a la constitution physique de la region de HUYGHENS, mes obser- vations indiquent d'une maniere certaine 1'existence r^elle de plusieurs centres de con- *This value was really 13". 7036 according to a letter from Dr. LAMONT to Professor HUBBARD, U. S. N., and hence LAMONT'S measures, as here given, are too large in the ratio of 1,000 to 997. I have corrected the more important of 1 hese. 62 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. densation dans les limites de cette region. La planche II represents en or , a , & , c , d , A six centres d'une forte accumulation de la lumiere diffuse, dont je puis affirmer 1'existence reelle et dont je suis parvenu a fixer la position par des mesures microme- triques. Le centre a [in F] fut remarque des le commencement de mes observations en 1 848 et je le regardais a cette (3poque comme une petite etoile entouree d'une atmos- phere nebuleuse. En 1849 j'avais remarqu^ une condensation analogue autour des centres a et & [in D and G]. Cependant, des connaissances plus positives sur la dis- tribution de la matiere nebuleuse dans la region de HUYGHENS ne furent acquises qu'a 1'epoque de la derhiere seVie de mes observations, en 1851. Avant que je fus par- venu a reconnaitre la vraie nature de cette distribution, toute la region me prt3sentait ordinairement des apparences qui variaient d'un jour a 1'autre selon les circonstances atmospheriques. Meme a 1'epoque de la demiere serie, la plus favorisee par I'e'tat atmosphe'rique, j'e'tais encore longtemps ind^cis par rapport a ce sujet Ainsi je trouve dans mon journal la note suivante, inscrite le 24 fevrier 1851 : " J'ai cherch(^ en vain dans la region de HUYGHENS la repartition r^guliere de la ndbulosit^ en masses globu- laires, indique'e par MM. HERSCHEL et LAMONT, bien que je crois avoir remarqu^ quelque chose d'analogue anterieurement. Toute la region me parait offrir aujourd'hui les apparences d'une surface liquide qui se trouve en mouvement ondulatoire rapide." Quelques jours apres j'ai reconnu d'une maniere certaine les masses de HERSCHEL et ce r^sultat est inscrit dans mon journal dans les termes suivants: " Le 2 mars 1851. Aujourd'hui je vois distinctement dans la region de HUYGHENS les masses globulaires de HERSCHEL, trois dans la partie australe, A , , d [El, F, I], et deux un peu plus au nord, & , c [G, H]. En a [F] je supposais auparavant une petite e"toile n^buleuse, probablement c'est une masse analogue, qui presente une condensation tres forte pres du centre." Depuis ce jour ont commence les observations positives sur la re'gion de HUYGHENS et jusqu'a la fin de la seYie, en mois d'avril, je voyais constamment et sans difficult^ la distribution mentionri(3e de la matiere n^buleuse en masses globulaires condense'es. Les mesures microme'triques, ex^cute'es par moi pour fixer la position des centres reconnus de condensation par rapport a & Orionis, sont: [Omitted]. Ayaiit corrigt^ ces mesures pour 1'effet de la refraction, j'en tire les coordonndes suivantes : Centre. A A w D D w et il parait inadmissible de supposer que HERSCHEL ait pu se tromper d'autant des les positions respectives de la cime et de Petoile, qu'il ait place cette derniere sur la pente MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 73 australe, si, comme aujourd'hui, elle s'dtait Mfi trouvee en 1837 du cot<< boreal. [The position given by STRUVE for this star agrees with my own.] (h) Le coin boreal du Proboscis major. Dans la derniere annee toute cette partie parait avoir fait un mouvement vers le sud-ouest. Sans la supposition d'un change- ment il serait impossible de concilier, sur ce point, mes dernieres observations, avec les dessins des autres astronomes et m6me avec mes propres observations de 1857." The preceding observations and remarks have been given at some length both on account of their importance and because they contain a careful arid detailed com- parison of the works of LAMONT and HERSCHEL. LIAPONOFF was the first to carry out the idea suggested by SCHROETER of making a minute study of single parts of a nebula, and it appears to me that the faithful and accurate manner in which this critique was done has not received all the commendation it deserves. In this way, also, we may secure the advantage of the minute criticism which Dr. OTTO v. STRUVE has given to preceding memoirs. OBSERVATIONS OF LASSELL (1854). I extract from LASSELL'S account of his observations on the nebula of Orion, Mem. R. A. S., xxiii, p. 53, et seq., such notes as appear to throw the most light upon his views concerning it. These observations were taken with his admirable reflector of 2 feet aperture, at Malta. "Wednesday, Dec. 15, [1852]. * * * With 101 8, the wool- like masses appear as I have previously described them, and there is no disposition whatever in them to turn into stars." " 7th Jan., 1853. The nebula of Orion was surveyed under better circumstances than yesterday, without my seeing anything remarkably different from former observa- tions. The pea-green color of the nebula is very remarkable differ- ent in this respect from all others as, indeed, it is, I think, the bright- est of all the nebulae I have seen. * * * I tried several higher powers * * * on the brightest parts of the nebula, but they only strengthen my former impressions of my inability to resolve it with my utmost means." "In order to perpetuate as far as possible the results of these observations, I send, herewith, a painting, in oil, of this nebula on the same scale as my original draw- ings. * * * It is the work of my friend, Mr. HIPPISLEY, executed under my own superintendence f -it i ,i FIG. 27. LASSEI.L , 1854. and carefully compared with my APP. V 10 74 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. original sketches. I consider it a very faithful picture of what I saw, when placed in a proper light and well illuminated. Without attention to these conditions the nebula will appear too faint. * * * I send also, herewith, a similar drawing on a smaller scale. * * * I have endeavored in these drawings to represent the original as closely as possible, comparing and improving my original sketches night after night with the nebula itself." [This second drawing is given as Plate I of the volume already cited, and is reproduced as figure 27 of this text]. In the same volume of the Memoirs a portion of a private letter of LASSELL'S is quoted (p. 108), in which he says: "A comparison of Sir JOHN HERSCHEL'S, Mr. BOND'S*, and my own drawings of [the nebula of Orion] must, I think, suggest the idea of change in the nebula or varia- bility of the stars, or otherwise a less uniformity of delineation of the same thing than might have been hoped for." In this figure attention should be directed to the brightness, or want of brightness, of the parts just preceding the trapezium and of the n. f. parts of the Huyyhenian region. I add (from MS. by G. P. BOND put at my disposition by the Harvard College Observatory) a list of identifications of the small stars of LASS ELL'S chart. Small stars in LASSELL'S chart (Mem. R. A. S., xxiii). [The stars on this chart are G. P. B. 567, 575, 589, 595 (c), 608 (/), 612 (t), 618 (/*), 621 (c), 622, 625 (d), 636, 647 (/), 648, 651, 671, and 676 (&), #!!, 675 (a), 685, H?, 708, 741, besides the 6 stars of the trapezium. BOND has no stars corresponding to g and b; 601, 602, 642, 654, 686, 688 of BOND'S list are within the limits of LASSELL'S chart and not mapped by him. OBSEEVATIONS OF SCHMIDT (1860-75). The observations made by Dr. J. F. W. SCHMIDT, director of the Observatory of Athens, have been embodied in two drawings by him which he has most kindly communicated to me and put at my disposition. The first and most elabo- rate of these is given here- with (Fig. 28) The other relates to the nebula as a whole, and is preserved for reference. This work has not yet been pub- lished and no description has yet been given. For- tunately the drawing is complete in itselt. The different masses are well shown, and the amount of detail is quite striking for the 6-inch refractor used by Dr. SCHMIDT. FIG. 28. SCHMIDT, 1861. . MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. -r / D OBSERVATIONS OP SECCHI (1862). The drawing of SECCHT, 1862, given in the Astronomlsche Nachrichten, Band xlv, col. 60, refers more particularly to the nebula as a whole, and is reproduced in Fig. 29. FIG. 2.9. SECCHI, 1862. It is used here simply to give the general form of the whole nebula. It has been severely criticized by D'ARREST. The drawing is inverted. -6 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. OBSERVATIONS OF TEMPEL (1862). The early drawing- by TEMPEL,* given in Fig. 30, is also introduced for a com- parison with the drawings of the early observers, and being about contemporaneous with that of SECCHI may also be compared with it. FIG. 30. TEMPEL, 1862. OBSERVATIONS OF LASSELL (1862). LASSELL'S drawing 1 of 1862 was made by Miss CAROLINE LASSELL, an accom- plished artist, and one familiar with astronomical work of this kind, by means of the 4-foot equatorial of Mr. LASSELL'S construction during his celebrated astronomical expedition to Valetta. It was not published with the admirable collection of drawings made at the same timef on account of its large size. The scale was 100" = 1.194 inches. From Astronomische Nachrlchten, vol. Iviii, col. 240. t Memoirs Royal Astronomical Society, vol. 36. L (1862). ' accom- of the tronomical coli I' drawings >o" 1.194 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 77 A tracing of this was made by Mr. LASSELL himself in 1876, and after I had had an opportunity of inspecting the original at Ray Lodge, Maidenhead, the artist kindly offered to make a fac-simile of it, which is now in my possession. A photograph o'f the central portions only is reproduced in Fig. 31, and I cannot but regret that the drawing, as a whole, is not generally accessible, embodying as it does the results of great labor and skill employed under most favorable circumstances. It is to be remarked, that in this drawing there re-appears for the first time since SCHKOETER'S 1798 drawings (with the possible exception of HERSCHEL, 1826), the second bridge of SCHROETEE. It is there plainly laid down, as it was subsequently seen by Lord ROSSE and by myself, and much as described by SCHROETER. In the subsequent discussion (Part III) I shall recur to this feature. A strong suspicion of its variability in brightness is excited as it was seen by SCHROETER with a reflecting tel- escope of 19 inches aperture, and remained unseen until LASS ELL'S work, and was always unknown even to the indefatigable BOND, who observed the nebula under most favorable circumstances. OBSEEVATIONS OF STEUVE (1863). OTTO STRUVE in Melanges Mathematiques et Astronomiques, iii, p. 539, thus describes his examination of the Otow-nebula with the 4-foot telescope of LASSELL, which was at this time (1863) mounted at Malta. Comparison observations were made at the same time by Dr. WINNECKE at Pulkova. It will be remembered that the joint work of STRUVE and LIAPONOFF on this nebula was published in 1862, and that, therefore, all the details must have been fresh in STRUVE'S memory. His report is, therefore, of high importance: " Es ging jedoch meine Erwartung, hier an demselben allerhand neue For- men und Details zu sehen, die ich in Pulkowa nicht erkenrit hatte, nichtin Erfiillung: wie denn iiberhaupt der gesammt Eindruck des Bildes, der Charakter der Erscheinung, hier genau derjenige war, mit dem ich durch die vielfachen Beobachtungen an un- serem Refraktor vertraut geworden bin, nur etwa mit dem Unterschiede, dass hier eirizelne Theile etwas heller erschienen und dadurch bestimmter hervortraten als wie bei uns." Neither STRUVE nor LASSELL under favorable circumstances saw any indi- cations of the resolvability of this nebula, and in the Huyghenian region STRUVE found only five stars which he had not seen there in Pulkova. "In Bezug auf Veranderungen in der Nebelmaterie boten die beiden Nachten in Malta nur eine einzige aber sehr entschieden dastehende Thatsache. Am 8. Oct. erstreckte sich namlich pons Schoeteri kaum bis auf die Halfte des Sinus magnus und zeigte nirgends mehr concentrirtes Lichte, wahrend er am 10. Oct. bis nahe an die stidliche Begranzung der Bucht reichte und die gewohnliche Verdichtung von Nebel- materie nahezu auf der mitte deutlich hervortrat. In dieser Wahrnehmung stimnite Herrn MARTH'S Urtheil vollkommen mit dem meinigen uberein." STRUVE remarks that if either of the nights (8th and loth October, 1863), which he elsewhere speaks of as "herrlichen" was better than the other, that of the 8th was to be preferred. The description indicates a difference in position of the bridge of about thirty seconds of arc, and it is not at all probable that any real variation in shape could be remarked in 48 hours ; so that the difference in aspect must be ascribed to something- inside the earth's atmosphere. 78 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. "Der Stern 126, iiber dessen success! v veranderter Stellung zum Promentorium Herschelianum mein Memoire Andeutungen enthalt, stand jetzt erheblich auf dem nord- lichen Abhange jenes Vorgebirges, und war urn mehrere Secunden von dichter Nebel- materie iiberragt. Hierin scheint sich ein continuirliches Fortschreiten der angedeu- teten Veranderungen in diesem Theile der Proboscis major auszusprechen." With regard to the spiral nature of this nebula, which was pointed out by G. P. BOND, both LASSELL and STRUVE agree in certifying to the reality of the curved lines which BOND described. Their attention was particularly directed to the curve of light which starting fro'm the preceding side of the regio Huygheniana stretches far into the reglo Gentiliana. STRUVE is certain that this was not quite continuous throughout its length, and that it is broken in the narrow canal which joins the palus Bondii with the Sinus Gentilii. OBSERVATIONS OF WEBB (1863-1876). By the kindness of the Rev. T. W. WEBB, I have access to two original pencil- drawings by himself, as well as MS. notes of his observations. Such of the notes as refer to the central parts I give below, exactly as they were communicated, and beside these I have given in Part III inferences derived from the original drawings. The first drawing was begun 1863, Dec. 29, and was continued on the following dates: 1864, Jan. i, 4; Feb. 9, 12; March 4, Dec. 27, and Dec. 30. On the last two dates the following notes are written on the drawing: "Dec. 27. I think there is a dark channel," [connecting Sinus Gentilii with dark spaces toward the n p ] " The light n. p. the trapezium" is connected by a line which indicates the p. edges of J and B as they are at present. On Dec. 30 this dark channel was again seen. The drawing of 1866 was made Feb. 17, and has this note: "The three cumuli [F, G, H] nearest 93, [685 G. P. B.] form an equilateral triangle." H is marked "very feeble." The following observations on the great nebula in Orion were made by T. W. W^BB with a 5-in. objective, by ALVAN CLARK: 1863, Oct. 5 d . Rather flaring definition. I did not know exactly where to look for D'ARREST'S "bridging over" of the Sinus magnus, and definition was fluttering, and with considerable moonlight; yet 164 showed a nebulous bridge very feeble, bat quite certain, in the line between 2 stars marked in the diagram [669 and 685] ; now arid then I fancied that a minute star peeped through about the middle of the bridge [L 4 5, and BOND says . p. this line the Sinus magnus is filled with diffuse light, and edges are ill defined." 84 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. "Limit of dim outline of bay on south shore of Sinus magnus [i. e., south shore of T~\ reaches to declination of #'." "General direction of irregular [southern] outline of Sinus magnus (brighter light) * * * * is inclined 25 to parallel [^r=ii5]. This cuts off part of decided promontory meeting bridge" {(Pans Schroeteri;) this refers to R.] "Limit of dim outline of bay [r'] in south shore of Sinus magnus reaches to 10" or 12" south of 9', terminates with second bridge" [by "second bridge" is meant the preceding edge of 4; and not " SCHROETER'S second bridge"]. " South limits estimated with care." "Outlines of two bays dimly suggested on the north shore of Sinus magnus [these are T and r'\ Evidently the bay [r'~] has to do with the impression by the channel coming southward, or rather s. f. from [652, 657, 663, lacus Lassellii], and the separa- tion into two bays is as evidently suggested by the bridge l pons Schroeteri.'" North " outline of bay [r] dimly suggested." ******" The above sketch appears to me to suggest a more correct outline of Sinus magnus than the lines of my engraving, although in that it is the effect mainly which should be corrected. The suggestion of two ovals forming Sinus magnus is stronger when vision is bad, and no doubt HERSCHEL'S drawing [1837] aims at representing this." Lacus Secchii. " This prominent dark spot is the sudden limit of light of Huyghenian region, and from thence to [D] the limit, though indented, is precisely on the parallel." The parallel of i' north of 0' is nearly the limit of bright light from the trapezium northward between the R. A. of [647] and the R. A. of the preceding edge of [lacus Secchii]. D. The following side of D " is well defined and straight, forming nearly a straight line in the meridian with edges of masses south of it [i. e., of T], but the latter incline more to the south following" 647 is immersed in D, and 651 is on its edge. Preceding edges of J. and B. "This edge of nebula in meridian precisely through [575]." A. The south point of A runs up to 608 pretty exactly. 621-622-625 appear to be in darkness according to this sketch. From memoranda of features to be re examined. BOND remarks that the outline of the occiput is continuous across the mouth of Sinus Lamontii, although less bright. In the Memoir " On the Spiral Structure of the Great Nebula of Orion," pre- sented by G. P. BOND to the Royal Astronomical Society (Mon. Not., R. A. S., xxi, p. 203), BOND speaks of the small wisps or tails of light which are shown most plainly in his engraving in the Annals of Harvard College Observatory, vol. v, near the stars 685, 708, 741 of his Catalogue. That this was really seen by BOND we learn from his accurate description of them ; he speaks of " the large number of instances in which collections of nebulous matter are found associated with stars, frequently in the form of little wisps, shooting off in a southerly or south preceding direction." Other things are mentioned which point to a connection between the stars and the nebula as e. g., "the predominance of small stars in the nebulous regions," the " two remark- able instances where there is a deficiency of nebulous matter in close proximity to bright stars, which are yet closely encircled by it These are the bright groups of the trapezium, the central comparative darkness of which has been noticed by many observers, and i Orionis. Lord ROSSE'S figure of the latter is decisive on this point These features seem to favor the idea of a physical association of the stars with the nebula. The existence of a spiral arrangement of its component parts falls in with the suggestion of a stellar constitution, since, among the objects exhibiting this pecu- liarity are included, not only resolvable nebulae, but actual star-clusters, such, for instance, as the great cluster in Hercules, which has an unquestionable curvilinear sweep in the disposition of its exterior stars. In the Monthly Notices of the Royal Astronomical Society, vol. 24, p. 1 79, G. P. BOND replies with definiteness and with perfect justness to strictures which had been made upon his published engraving (same work and vol, p. 92), and institutes a com- parison between his drawing and HERSCHEL'S (1837), which had been quoted as evi- dence against his own. I quote certain portions of this as supplementary to what has already been given : " The only areas quite destitute of light which I have found in this part of the nebula, are: ist. An irregular opening with its center in the position Ja-\- 108", 4d + 50" [i. e., our r'], and 2d, a narrow channel having its axis nearly in the parallel, and a declination of 45 -f- 72" at the right ascension Ja = -+- 160"" [our r"]. * * ****** a -yy e ] iave a fi^ we u defined point of departure at the position 4 a -J- 145", 48 20" [our point Q ; LIAPONOFF'S B]. Of this there is no trace in HERSCHEL'S drawing. LIAPONOFF gives z/# 14.6". 5, 48 22" .o. HERSCHEL makes the breadth of the bright light here still 40" to 50", and continues the curve * * * 180" beyond its actual limit." " We find, then, the following instances of discrepancy between HERSCHEL'S delineations of the region in question and the actual appearance of the nebula : " ist. The absence of a definite limit to the bright light of the Huyghenian region on its eastern side, etc. "2d. The bright light on the southern shore is carried 10" to 15" too far north." ******* "4th. In its best defined part the western shore is placed 12" too far to the west. " 5 tn - [go] is 15" to 20" too far north, etc. "6th. All the features of the northern shore [of Sinus magnus] to the east of pons Schroeteri are represented in positions 30" or 40" north of their true locality at the same time that the direction of the principal lines is largely in error." OBSERVATIONS OF LORD ROSSE (1867). (These observations are extracted from Phil. Trans., 1868, p. 57, et seq.) " The observations upon this nebula, recorded in the journal of the observatory at Parsonstown, date from 1849. From that time till February, 1858, there are entries of 54 observations. 86 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. In the year 1852 Mr. BINDON STONEY made a drawing of the Huyghenian region, which is a very interesting record.* Mr. BLNDON STONEY was a highly educated civil engineer, well accustomed to use his pencil. His drawing was made with great care, and he was engaged upon it the whole season. It was compared by several persons with the nebula, and was considered exact. When we compare this drawing with the nebula as it is (Figure 33) at present, there are strong indications of change. FIG. 33. ROSSK, i865-'67. Between February, 1860, and February, 1864, there are 74 entries of observa- * A photograph of this interesting drawing has been kindly sent me by Lord ROSSK, to whom my thanks are due for this and other similar kindness. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. g; tions. In February, 1860, Mr. HUNTER, who was then the assistant, being an accom- plished artist, commenced a new drawing, and was engaged upon it till February, 1864 As a groundwork for his drawing, Mr. HUNTER laid down all the stars given in " Observations de la grande nebuleuse d'Orion faites a Cazan et a Pulkova, par 0. STRUVE, St. P^tersbourg, 1862," in the positions given at page 1 18 of that treatise ; the nebulosity was gradually filled in by eye as correctly as possible with reference to the stars given in that memoir, and twenty-eight additional stars from the 9th to the 1 5th magnitude were inserted by eye-estimation During the season 1864-^5 the nebula was often examined with the view of verifying the drawing made by Mr. HUNTER, and in i865~'66 some additions were made to it. During the season 1866-67 these measures were completed, the additions of the previous season verified, and the drawing extended. [The Figure 33 is copied from the drawing black on white published in sections, and not from the large engraving.] Very little need be said on this subject, as the drawing will speak for itself; it may, however, be well to call attention to the apparent connection between some of the stars and the nebulosity near them. In some places the stars appear to have either repelled or absorbed the nebulosity, for instance at the trapezium, at 32* and 35, and so on; and in other places the nebu- losity is denser, as if the star had attracted it, for instance at 2 lt 4, 34, and 108. Around the star 108 [734] the nebulosity s6ems to have a spiral character, and the same appearance, though much less decided, may be seen round 4. Round the stars 46^ 46 U , and 99 1? the nebulosity seems to have been concentrated, but close to them there appears to be an absence of nebulosity ; and in the case of 99^ the dark hole is situated eccen- trically with respect to the principal star, its nearer companion being close to the opposite side of the hole; but in the case of the double star 46^ 46 U , the hole is nearly symmetrically situated, but the nebulosity is brightest at the north preceding side. We can hardly, therefore, account for these numerous coincidences, except by sup- posing some at least of the stars to be situated nearly at the same distance from us as the nebula ; in fact immersed in the nebulous matter. [This point, as brought out by Lord ROSSE, and confirmed as it is by a telescopic examination, appears to be a conclusive proof that we have, at least, some of the stars associated with the nebula.] Variability of form and intensity of the nebulosity. On this subject it is impossible to speak decidedly. On comparing the following six drawings Sir J. HERSCHEL'S of about the year 1825, Sir J. HERSCHEL'S " " 1837, Mr. BOND'S 1848, M. LIAPONOFF'S " " 1850, Mr. LASSELL'S 1854, Mr. HUNTER'S " " 1863, great discrepancies exist in almost every part, but these are probably to be at- tributed in a great measure to the difference of power in the instruments used and * Only those stars within the limits Aa = 300" and A,l = 2oo" have been marked with BOND'S number.-E. S. H. 88 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. the amount of labor expended on the drawings, as no continuous change seems to be shown by them. In the case of the spiral nebula round 108 [734], BOND'S, LASSELL'S, and HUNTER'S drawings appear to agree tolerably well, allowance being made for the difference of size of the instruments, but when we go back to HERSCHEL'S drawing of 1837, we find a considerable discrepancy. HERSCHEL'S drawing of 1825, however, as far as it goes, is in this place more like the latter drawings. With regard to the following extremity of the Huyghenian region, all the former drawings, with the excep- tion of LIAPONOFF'S, represent the frons as curving round to meet the Proboscis major, which latter also Curves round to meet the former, whereas Mr. HUNTER represents both these parts as curving slightly in the opposite direction. This I am satisfied is their present appearance. If, however, the night is not good, they acquire very much the appearance of the other drawings, the light of the brighter portions being scattered, to a certain extent, over the intervening space. In the case of the Huyghenian region, HERSCHEL'S drawing (1837) agrees much more nearly with Mr. HUNTER'S than any of the others, although the interval (30 years) is so much longer than in the case of Mr. BONDS and Mr. LASSELL'S drawings (15 and 9 years, respectively). With reference to the relative brightness of the various parts, I find recorded by Mr. HUNTER, February 22, 1861 : " In bright moonlight the degrees of brightness are " i. The Huyghenian region. " 2. The nebulosity immediately south preceding it. " 3. The Mairanian region. " 4. The subnebulous region. u 5. The south Messierian branch, and the nebulosity immediately north of the Huyghenian region" And again : " The observation of February 22, 1861, gives very different degrees of brightness for the various regions from what they had this season (i863~'64). " i. The Huyghenian region. " 2. The nebulosity immediately south preceding it. " 3. The nebulosity immediately north of it. " 4. Subnebulous region. " 5 The south Messierian branch and the Mainmian region nearly equal." Mr. HUNTER on two occasions estimated, as nearly as he could, the relative bright- ness of the various masses of nebulosity of the Huyghenion region. The following are his estimations. (See diagram.)* FEBRUARY 13, 1864. a [A], r [L], v [Q], y \F] nearly equal; brighest of these is perhaps 6. JT[CJ. co [in A], e [H], <5 [G], ft [I] ; ft is the faintest of these four. a [E], t [in Q], ^ [2], A [in Q]. , 2}^, which MAIRAN saw surrounded by mist, has certainly a pretty decided outline in the MONOGRAPH OF THE CENTRAL PARiS OF THE NEBULA OF ORION. 95 form of an inverted comma. On March 31, 1856, I find noted that this mist has an undoubted spiral aspect, notwithstanding the tail is separated by a thinner veil, which makes it at times appear detached and double. The convolutions indicated by HtR- SCHEL and by BOND are different from ours, and do not agree among themselves. Between this nebula and the region of PICARD of the principal nebula is the nubecola oUongata of HERSCHEL, which almost touches the other small star, itself also surrounded by mist The most singular thing is that the space which separates this nebula has been found perfectly black by us, so that it was absolutely necessary, on those evenings when we noted the fundamental forms, to cancel utterly from the map every trace of thin mist which had previously been drawn upon it. I was surprised by so much darkness, and took note of it. This canal, so straight and black, was certainly the one which at the end of the last century determined the giving to the nebula a figure of a capital, elongated omega (see fig. of 1 774), and thus it really does present itself in a small instrument which does not separate the details. However, this is so much the more singular since O. STROVE says positively that he saw some nebulosity between the region of PICARD and the nebula oblongata, and to us as well it would have appeared nebulous had we not traced that mist which subse- quently we were obliged to cancel. Here, then, is one of those points to be re-exam- ined in the future." SECCHI sums up his results as follows: "ist. From the comparison of our observations with those of preceding astron- omers, it appears that the nebula is sufficiently known in its general structure. The coincidence of the principal points is now assured, and their relative variability remains only to be fixed by more exact measurements. The labors of LIAPONOFF, STRUVE, BOND, and HERSCHEL, confronted with ours, put the latter beyond controversy as to the points of greatest brightness and of the first order. 2d. The differences which are met with occur principally in the parts of the second order, where the weakness of the light, the power of the instruments, the sensi- tiveness of the observer's eye, and the state of the heavens exercise an immense influ- ence. The nebula being green, all eyes have not the same sensitiveness for this color, and considerable diversities in the drawings must follow. Taking all these circum- stances into account, the divergencies will be seen to sensibly diminish 3d. It is not yet proved, however, that in these parts all the differences are effected by the extrinsic causes mentioned above, and that some real variation may not take place. Worthy of principal consideration are the Sinus Lanxmtii and the pons Schroeteri, where it is difficult to attribute everything to accidental and extrinsic variations. 4th. The resolution into little stars of the bright masses does not at all prove that they are agglomerations of true stars The spectral diversity is fundamental, and that assures us that the physical constitution of nebulous bodies is gaseous. The form with distinct points may occur in a mass even of this nature and give rise to a similar appearance, but from this alone a true stellar constitution cannot be concluded. A sim- ilar concentration occurring in any less dense part may have given origin to some of 96 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. the bright points observed as stars by more than one. The very absence of con- tinuance in these appearances proves the justice of this explanation. 5th. The mass being 1 recognized as gaseous, it is impossible that it should be in constant equilibrium, hence nothing is more easy to explain by real movements, not only the aforesaid agglomerations, but also a large part of the variations presented in its aspect. 6th The confusion which is felt on first observing the nebula is only an ordinary case of what happens at the first sight of an irregular object before one has become accustomed to it and has made of it a certain order of regular figures. This happens even when looking at the starry heavens before knowing the constellations ; and in the (stellar groups) clusters, before studying them, all appears confusion, and not until after a certain time does order and regularity seem to enter. Then, however, the number of the objects seems to decrease, but this diminution is only apparent. Artists know in practice this effect, and it is not to be feared as a defect that a drawing made after much study should seem to decrease in parts because a certain order is gained. 7th. The nebula of Orion is not the only mass of cosmic matter, gaseous and irreg- ular, which occupies space ; it is only one of the more dense agglomerations of cosmic matter which extend in some parts of the heavens, specially in Sagittarius and in Orion. If the general clearness of the heaven prevents the discernment of the presence of this matter, it can, however, be shown by various devices, and by the help of certain spaces in which its presence is wanting. The great zone which extends over these regions seems to protract itself even to the northern hemisphere in which the black space or coal-sack in Cygnus may easily be carried out between the Milky Way and a luminous zone in continuation of those of Orion, to the pole in form of a very elongated M- Hence, the splendor of the heavens is due in these regions not only to the stars, but to an immense stratum of nebula in which our solar and stellar system is immersed. 8th. This matter interposing itself between us and the stars may give origin to the aureoles which surround them in some portions and influence their spectrum. Probably the green color of the stars in the neighborhood of Orion, and the extreme fineness of their black lines, depends upon this stratum whose action tends to paralyze the effect of the absorption of the atmosphere proper of the stars Sirius, which does not share this influence, would be outside of this mass. 9th. Yet if these masses are destined some day to form stars, there is very little hope that we can ever calculate the successive phenomena to be developed in such concen- tration. If the relations of time and space are in equal proportions, the movements here must be of the slowest, and the variation imperceptible beyond our imagination. The supposed enormous mutations in other objects is less believed in proportion to the more powerful means and more diligent care used in computing the forms, and what remains of uncertainty is more due to the imperfection of the study given than to the well-proved reality of changes." After this was published, SECCHI received a proof of the engraving of Gr P. BOND (frontispiece), and he compared this critically with his own work. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 97 NOTES ON BOND'S ENGRAVING. " First of all BOND recurs, in the region Huygheniana, to the system of superposed cumuli as in HERSCHEL, the different series of which form a pyramidal and almost imbricated figure. The three orders of cumuli essentially agree with our series, except in the special details obtained by us during moonlight ; and as he retains the too small scale used in the other drawing, what we said of the cumuli of HERSCHEL is applicable here. Their flat forms and the narrow canals which divide them are due certainly to the great power of his instrument ( 1 6 inches), which by increasing the light very much in the fainter parts, causes the half shades of the globular forms, which come out better in the moonlight, to disappear. It is singular that BOND, who had often observed in the twilight, should not have noted this difference of intensity. But he, also, was a victim to the common prejudice of observing nebulae in complete darkness in order to see it better. The figure being positive, that is, the ground of the plate being black and exquis- itely cut, is eminently suitable for an exact comparison with the sky. We have given the reasons why we could not use this system. The Sinus magnus is barred towards the bottom by the pons ScJiroeteri that has a luminous mass in the middle, exactly as we said in our Memoria, at page 20, it was seen by us in 1857, but which we have not succeeded in seeing again since. The epoch of BOND'S drawing being given as 1859 to 1863, would be a strong confirmation of the variability of this bridge and of the precision of our former observations. Hence, it is clear that this portion of the nebula should be watched. The bottom of the gulf from the bridge up is nebulous, as we also found formerly. It has a little nebulosity at the mouth, but is not barred as at 'present. The Sinus Lamontii is quite black and has a double curvature on the left side, but is wider at the mouth than we found it to be ; it, however, approaches more nearly to our figure than other drawings. This, also, is a region to be watched. In the region palm Bondii long, continuous, spiral filaments are found, which, however, occupy the whole region Picardiana and Derhamiana. They have a pro- nounced spiral inclination, and start from side 57 of our polygon from below the prin- cipal of left base of the large triangle Huygheniana near the trapezium. The author states that it cost him much labor to trace these spiral convolutions in the midst of the labyrinth of the nebulous mass. If we should sincerely express our opinion, however, we believe that this preconceived idea of reducing the nebula of Orion to nebulous spirals, applying to it the principle of Lord ROSSE, may have slightly forced the observer's judgment, inducing him to give prominence to certain traits which, perhaps, have not all the strength that they show in the drawing. The reticulation in the region H, G, F, K, of our polygon, is certainly very confused, and a preconceived idea can easily distort the fancy; but we do not remember ever to have seen lines so continuous and easy to trace as those drawn by the illustrious deceased, and tlu-y cannot be imagined from our drawing. In BOND'S drawing the large arc (H, 7) of the region Fonrhunm is well traced, and App. V 13 g% MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. continues in a large oval that has in the middle the brighter mass represented by us in (D, 3 a 7). As our drawing in this part of the nebula is more limited than his, we cannot make a comparison throughout the whole extent. This confirms, how- ever, the exactness of our figure in these parts." OBSERVATIONS OF D' ARREST (1872). " ON THE NEBULA IN ORION AND ITS SPECTRUM. " BY PROFESSOR D'ARRE.ST, 1872. "[Translated from the Danish by Dr. WILLIAM DOBKRCK.] What follows was kindly communicated to me by Dr. DOBEREK, and it has been slightly condensed through the care of Miss ELIZABETH HARRIS, who is familiar with this nebula from the assistance rendered by her to GEORGE BOND during his director- ship of the Harvard College Observatory. It is given here almost in full, as in its original form it is not generally accessible to English-speaking astronomers. "CHAPTER 1 1. "The brightest and most interesting part of the nebula, HUYGIIENS' region, with its environs, is represented on the plate at the end of the work, and may be consid- FIG. 35. D'ARREST, 1872. ered as the result ot my observations in the winters from 1865 to 1871. It may MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 99 especially be compared to the representation given thirty-five years ago by M. LAMONT,* as the large refractors in the observatories of Munich and Copenhagen may be con- sidered as perfectly identical in optical respects ; and we have not, to my knowledge, hitherto possessed two drawings of the ^da-nebula made with exactly similar instru- ments after a considerable interval, although not a few valuable drawings have been made within this interval. While plates published by LASSELL and Lord ROSSE rep- resent the details of the nebula as seen about 1853 and 1864 in the three largest reflectors which have ever been directed to the heavens, SECCHI'S drawing of 1865 was made with a Q-inch refractor, and GEORGE BOND'S, from 1857 to 1865, with a 1 2-inch object-glass [14 Paris inches. E. S. H.]. The appearance of the nebula is known to depend in a considerable degree upon the optical power of the instrument. The first two named telescopes, especially that of Lord ROSSE, surpass surprisingly in their effects on this field all existing refractors ; older contemporary ones are consequently strictly comparable with LASSELL'S and ROSSE'S only when it is certain what alterations have occurred in the ^efa-nebula, and in what regions they have occurred.f " To my graphical representation I have added a general view, which gives the necessary information on the nomenclature and designations used at present. There was no occasion to introduce new names in the region referred to ; those now used arise all from Sir JOHN HERSCHEL,J 0. STROVE, and ROSSE. We shall anon speak about certain parts previously discussed, for instance, Hemicyclmm Liapunovii, which we are no longer able to recognize under the slow variations of light which doubtless take place in the nebula. "A comparison between the two perfectly adequate representations of the central part (LAMONT'S and my own) shows that HUYGHENS' region, in conformity with what else is known for certain, has, on the whole, not materially altered its form and appear- ance. The separation, however, in the southwestern part between the forms a, ft, x [E, I, F], which is but feebly and indistinctly indicated, appears so much more dis- tinct that it is hardly possible that the divisions can have presented thirty- five years ago the same sharp and certain outline for which they are at present remarkable- This suspicion is indeed confirmed by HERSCHEL'S first drawing of 1824; but in the Cape observations, Vhere certainly the nebula was seen under favorable circumstances, there appear such definite traces of the existing main separations that we, on the other hand, dare not assign their first origin to so late an epoch. The attention of astron- omers will therefore be directed to this point in the [immediate future]. a In LAMONT'S figure there is no trace of the strong condensation in the north- western corner of the great body, about the place where STROVE put his lien* Secchii : an object which, however, I have not been able to identify from the description.^ I have " * LAMONT. Uebcr die Ncbelflecken (Academisctif Schrijt) Miiuchen, 1837. JRegio Hugeniana, Fig. XI. " t This opinion is shared by O. STRUVE, the most competent authority in this respect; compare : Vlert. Jahr. d. astr. Gesellnch. V, page 26. Jan., 1870. "t Memoirs R. A. S., II, page 490, ei seq., and plate VIII. STRUVK and Lord ROSSE, in the works mentioned In the first section. The star-numbers on my general view are given from O. STIU'VE'S catalogue in the paper on the nebula in Orion (1862). " I have repeatedly, after STRUVE'S description, and according to his measured positions and * 70 and * c, sought for this black circular spot of 15" diameter. I never succeeded in finding this little dark opening, although lacus Secchii no doubt was to be found in October, 1857. Everything lion- points to a permanent alteration in the distribution of luminous matter. I do not know that any other astronomer h:is ten* s ,-n this Secchtan f.alr. JOO MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. every reason to consider my own representation as trustworthy. As I am on this point in relatively good agreement with Gr. BOND, no doubt can prevail that a substantial alteration has occurred here ; so much the less, as the present sharply appearing and precise bounding is also altogether wanting on HERSCHEL'S drawings of 1824 and 1835. "Among the most extraordinary differences between LAMONT'S and my representa- tion, I further class ' pons Schroeteri' in Sinus magnus, which is often found mentioned in older reports. LAMONT has nothing of this bridge across the gulf but the small [base], which, like a promontory, is attached to the north side of the gulf, while on my drawing is to be seen a perfect communication, with two brighter points about midway. This remarkable difference is in this instance but a corroboration of a partial transformation, or rather of local alterations in brightness, which were pointed out as certain by STRUVE as early as 1862.* " LAMONT has hardly a recognizable trace of the two perfectly sure and thereby very characteristic configurations on the west side of Sinus Gentilii, round the stars 50 and 54 [558, 573] in my drawing, which are almost identical with those of BOND, and almost perfectly identical with those on ROSSE'S splendid map. Sir J. HERscHKLf remarked long ago, and no doubt justly, that the outline of LE GENTIL'S gulf was not, on the whole, correctly given in LAMONT'S diagram. I suppose that it was not at all LAMONT'S intention to give the outer parts of the nebula. "Huyghens 1 region of the nebula in Orion, apart from the mentioned, most prominent differences, is, on the whole, seen far more finished and with far finer particularities in my refractor, at Copenhagen, than might be presumed from the drawing made in Munich. LAMONT, I suppose, did not at that time make the large nebula an object of special study. We, therefore, need not attribute great weight to the really great difference existing between these two drawings, which, with an interval of so many years, have been made with equally excellent instruments. It is moreover to be remarked, that LAMONT fills the inner space of the trapezium with as dense a luminous matter as surrounds it on all sides. I see, on the contrary, the six trapezium stars always on a far feebler and almost dark background. HIERONYMUS SCHROETER{ saw it in his time as LAMONT has shown it. The empty space, in reality, is but apparent : of this part, however, 1 will speak at length in 14. "CHAPTER 12. " I shall in this chapter compare, in certain points, my own drawing of Huygliens' 1 region with other lately published representations. From such a comparison of con- temporary drawings made by the aid of different instruments, elements are obtained which will in future ages be of great value. However great the difference in the whole appearance of so complicated and difficult an object which climatical circum- stances may produce, the artistic representations over which the observers have but little power, may very easily happen to exercise a far greater and sometimes disastrous influence on the representation. Remarks in a negative direction may, from these reasons, be not without importance. ' * Observations dc la grange nebuleuse, etc., page 1 16. " t Results of Astron. Observations, 1847, 69- "{ Aphroditographische Fragmenle, Helrastedt. 1796. PI. II. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. I O I "A. The drawings which depict the nebula in its whole extent give occasion to the ollowing remarks: According to my general knowledge of the nebula in Orion, >SSELL'S steel engraving,* compared with the image in a ten and a half inch refractor, represents the single parts of the nebala much too loosely, and the portions around the trapezium are in particular anything but successful. ' The considerable nebulosity, for instance, which closes Sinus magnus to the west, and whose brightest part (according to ROSSE it is traversed by a curvilinear, narrow, and dark channel) extends toward the trapezium, is almost entirely wanting. There- from arises round the trapezium a vacuum of an extent which surely is not to be found in the sky. Neither is the north side of the * large gulf at all naturally given. Further, it is decidedly wrong that * nebula Mairanni' should, as LASSELL represents, surpass all other parts in brightness ; it has, and in reality never had, more than the third or fourth position in brightness. We may expect that these wants and disagree- ments in the engraving have been removed in the later drawing, which, after the return of the distinguished astronomer from his second sojourn in Malta, has been presented to the Astronomical Society in London. "B. In GEORGE BOND'S posthumous work on the great nebula is to be found that representation which in every respect reproduces both its general appearance and its finest particularities. It might seem desirable that the central part had been given apart; but although the longest diameter of Huyghens 1 region now is not longer than eleven lines in the picture, still all details are presented sufficiently distinctly ; while the characteristic appearance of the object, and especially the singular serenity which there reigns, is in perfect accordance with nature. My drawing of the main mass does not, in any important respect, deviate from BOND'S. I have, with our somewhat infe- rior refactor in point of light, found, after all, only the northern outline a little different. Less and separate masses in the pointed southeastern corner appear, too, more distinctly by BOND than I have ever been able to discern them. I consider, for instance, after repeated inspection and comparison, my own representation of the separate mass to the east of palus Bondii\ more conformable to the true form. EOSSE also finds it very nearly as I have. " C. I do not find ANGELO SECCHI'S large copper-plate of 1 868 quite successful. Of the disagreements, I shall only mention the following : The parts a and ft [E and I], of the main mass, whose real figures are, of course, for the present placed altogether beyond doubt, are hardly to be recognized, a. [E] has really four sides and is almost a square ; /? [I] has decidedly the form of a lengthened triangle ; but in the Roman repre- sentation they are both shown as round, almost circular. In the net of channels which traverse this region the courses are too broad. To the west of Sin its Gentilii all agreement with the sky is wanting. I find it is also difficult to understand how the " * Memoirs R. A. S., vol. xxiii, pi. i. " 1 1 have during a long time, used this Strnvian denomination (Observations, etc , page n channel (or, according to ROSSE, the deep inlet), which separates the luminous isles around stars 50 and 54 S. F. (558, S70 But it was perhaps STRUVE'S design to use the denomination ' pa I us Bondii' for th,- .a.sfn, .sh- ulom- : in tl case the words 'a narrow bridge' does not well correspond to existing relations. In my general view I have 1 the latter supposition. " 102 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. very remarkable radiations, especially those which emanate from the south side of the central part, can have escaped the notice of this experienced astronomer.* "No doubt much time, assiduity, and care have been spent in the Collegio Romano 'in the study of the nebula in Orion; the whole paper and some single observations of fine particularities testify this. The cause of the disturbance which apparently pre- vails in the drawing, and of the disagreement which exists on several points, I attribute to the circumstance that SECCHI made use of moonlight nights. The brighter and brightest parts have in consequence too much ascendancy over the fainter, and the general impression has thereby become somewhat different from that to which per- fectly dark nights have accustomed us. " D. As far as I, after all, can be in possession of a well-founded opinion on the importance of the large and splendid drawing which is the main result of the work of several astronomers during many years, with Lord ROSSE'S transcendant reflector, I shall express that I have, by degrees, arrived at the conviction that his representation approaches in every important respect most nearly to the true state of the nebula as it was about i862.f " I have generally, allowing for the very inferior instrument (in point of light) of this observatory, at least found the representation by Lord ROSSE to agree with the truth. This verification is not without considerable importance for, to be brief, the refractor surpasses HERSCHEL'S 2O-foot telescopes. Only in feebler extensions, in the very faint connections, and in the singularly intertwined bands which continue the nebula, especially to the west, I sometimes found it impossible to follow traces in ROSSE'S drawing. Furthermore, I share the opinion that the outlines, particularly in the sepa- rate plate of Huygliend region (Plate I) are sharper, and the dark furrows somewhat broader than they ought to be ; that the contrast between the brighter and fainter parts are very strongly marked, at least when we judge according to the image in the refractor of Copenhagen. "I shall add in particular, with respect to regio Huygheniana and parts surrounding it, the following remarks, after a comparison of the different drawings inter se, and with my own observations. " On the north side, I never saw the two large dark bays just below the tra- pezium [W 1 and W 3 of Index-Chart] appear as sharply and distinctly as on ROSSE'S drawing. In LIAPONOFF'S they are totally wanting, and in BOND'S and SECCHI'S but barely visible. They are, in reality, but darker intervals between two long, bent, tail- like areas, which, trailing off to the west and northwest, are lost far away between regio Derhamiana and regio Picardiana. "The east corner of the main mass [B in SECCHI'S diagram, Plate II, A in ROSSE'S zr Q] is-certainly bent somewhat upwards, and does not smoothly pass into the origin of "* I had some years ago occasion to lay stress upon certain disagreements of a similar nature (Astr. Nachr., vol. Ixx, No 1678, page 342), which were afterwards explained by ' che la figura litografica publicata henche esatto in generale, ha alcune inesattesse non trasciorabili.' (Sulla grantle ntbulosa, page 27.) The possibility of such an expla- nation is not open in this instance, as he remarks about the nebula in Orion, ' cosi siamo sicuri che 1'incisione rappre- senta la nebiilosa come vedesi da noi uel nostro strumento.' " 1 1 refer to the privately distributed plate which represents the object on a black ground, and which, no doubt, is also in technical respects, one of the most excellent productions of art. This 'plate is not annexed to the paper in Phil. Tram., but, on the contrary, another one, executed in the common way. Volume for 1868. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. i O " O proboscis major ; ROSSE'S words, ' curving slightly in opposite directions', convey the best idea of the way in which the transition takes place. " I have remarked above that I unconditionally adhere to my own conception of the parts beyond the west side of Sinus GentUii, which approaches essentially to ROSSE'S image, and is not contrary to BOND'S. But with this conception, the traces to which LIAPONOFF'S diagram on this point confines itself do not well agree. The bright and rather shapeless luminous masses which, according to SECCHI, between K. L. 12 [i. e,, between /* and 6 of Index-Chart], on the Roman plate, seem to rival in bright- ness the most apparent parts of HUYGHENS' region, did not exist during the time I observed. [These are probably the masses attached to the north shore of //.] "Sinus Lamontii I almost invariably found filled with rather dense nebulosity; here, however, the relative brightnesses seem so very inconstant that single objects are sometimes with difficulty recognized. In the adjacent Hemicyclium Liapunovii greater changes in brightness have doubtless occurred since O. STRUVE'S investigations in 1861 ; but on this point even later contemporary representations agree but badly. I should like to know if the luminous heap which, according to SECCHI, extends in m. % n. from 9^ to io>, may be considered as part of the Hemicyclium. "An agreement in this domain seldom to be found occurs with respect to the large, very deeply indented bay which 0. STRUVE has called ' Locus Lassellii? Its outline is still exactly as it was given for 1857: stars 76, 80, and 84 (652, 657, and 663), a little outside star 89 (669) somewhat inside the nebula.* SCHROETER observed this remarkable indentation, or rather intersection, as early as 1 795 and 1 799. Notwith- standing possible variations in brightness, it can be proved that here during the last seventy years no variation whatever in form has taken place.f "It is on the whole this constancy of form which I consider the most important result of the whole study which has been spent on the nebula in Orion. The observed variations in this extensive, gaseous mass seem solely and exclusively to end in tem- porary, luminous fluctuations, particularly in certain regions. Generally, perhaps always, the old forms reappear after a shorter or longer time. I have, during a lapse of years, seen instanses of this in pom Schroeteri, Sinus Lamontii, and Sinus Lassellii. Fluctuations of light, which, taken apart, are seemingly inconsiderable, produce here sometimes remarkable alterations. Several years ago, when examining a representa- tion dated 1779, in the 22d volume of ROZIER'S 'Observations de la physique,' I found the following, which leads to the same result : ' That we in the outline of the theta- nebula are able to point out an invariability in form which, the nature of the object considered, is plainly surprising.' At that time, spectroscopical investigations which have assured us in regard to the physical constitution, were yet unknown to me." "* STRUVE : Observations, etc., page 103. " tBeitriige z. den ueuesten Astr. Entdeck., Ill vol., Gottingen 1800, page 231, and figure 39, Table V, to COOL pare with Aphroditographische Fragraeute, page 247. SCHKOETKR'S star near (9) must be either 76 or 80 of STI; catalogue. The passage in vol. II of Melanges Math, et Astr., St. Petersburg, 1854, page 531, where i about this channel that "it never was represented by any other astronomer" is hardly correct. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. OBSERVATIONS OF WLNLOCK AND TROIJVELOT (1874). Fig. 37 is a copy of a pastel drawing made in 1875. It was published in NEWCOMB'S Popular Astronomy, page 446, Fig. 104. The electrotype kindly given me by the publishers of this work has been mislaid at the Naval Observatory, and, owing to my absence from Washington, it cannot now ( 1 882, January 3 1 ) be found. I refer the reader to the cut in Professor NEWCOMB'S work. FIG. 36. WINLOCK AND TROUVELOT, 1874. FIG. 37. TROUVELOT, 1875. The drawing of the central portion of the nebula of Orion given in Fig. 36 was made by M. L. TROUVELOT at the observatory of Harvard College in 1874, under the direction of the late Prof. JOSEPH WINLOCK. The Fig. 37 represents the sketch of the nebula of Orion made by M. TROU VELOT with the 26-inch refractor of the Naval Observatory of Washington. It is avowedly but a sketch, but is of value for comparison. A poor representation was published by the heliotype process in the Washington Observations for 1874, Appendix I. Fig. 37 is copied from Fig. 104, p. 446 of NEWCOMB'S Popular Astronomy. OBSERVATIONS OF DOBERCK (iSyy-'yS). In the Astronomische Nachrichten, .vol. 91, col. 335, Dr. DOBERCK, in an article entitled " Remarks on Nebulae," refers to the nebula of Orion, as follows: "Mr. COOPER made also a drawing of the OHow-nebula [this drawing has been previously described], and by comparing that with the image of the nebula seen last year (1877) in the indentical refractor, I had a rare opportunity of corroborating the changes which D' ARREST has pointed out." ***** Then follows the order of brightness of the various parts. The brightest part of the Orww-nebula is the northwest corner of the central part [D], then [A], then Q, then I and E. V as about as bright as /u. This is all that immediately relates to the Huyglienian region, but the complete article should be consulted, as it is a record of the order of brightness of the wlwle nebula. OBSERVATIONS OF TROUVELOT (1876?). M. TROUVELOT kindly undertook to make some observations on the nebula with different apertures and eye-pieces according to a scheme which I submitted to him. Unfortunately the notes made by him he has not been able to find, and the following memoranda of the intensity of the light in various portions of the nebula, as marked on .a copy of the Index-Chart, are all that now rSmain. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 1O5 k He noted on the Index-Chart the brightness of various portions on a scale of i to 4 1 was " brilliant." 2 was " half- tint." 3 was "quarter-tint." 4 was "black." In what follows I have given his results in my own words from the marks on the chart. rf (near frons), 3. Z (half-way from Sinus G. to 581), 3. /? (near the letter ft on chart), 2. K (near the letter K on chart), 2. 7, 4- 3 (near figure 3 on chart), 3. Channel between if> and 4 (half way from letter iff on chart to letter W), 2. [This is my mass W 2 .] Channel between if) and Xj o- W 3 , 3- W 4 , 2. A, i. D, i. q> (half way from star 2 to star 652), 2. Lacus Lassettii (near star 3), 3. r',4- f, 4 . ore (1/3 the way from o to TT\ 3. ff (y the way from o on chart to 681), 2. P, 3- follnwing p, 4. //, 2. Half way between 708 and 741, and 20" north of the line, 3. Dark band between x and A and near 666, 3. OBSERVATIONS OF LANGLEY (1879). Professor LANGLEY visited Mount Etna in January, 1879, taking with him a small CLARK equatorial of 3^ inches aperture, and making observations of various kinds for the purpose of determining the effect of high altitudes upon telescopic vision. This expedition was made under the auspices of the United States Coast Survey, and I am indebted to the Superintendent of the Survey and to Professor LANGLEY for the ready permission to publish the interesting and very valuable drawing shown in Fig. 38, together with the notes accompanying it. It is to be noticed that Professor LANGLEY APP v 14 fo6 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. has successfully applied to the nebula of Orion the method of contour lines imagined and first used by MASON in his drawing of the trifid nebula in 1837. FIG. 38. LANGLEY, 1879. "ALLEGHENY OBSERVATORY, "Allegheny, Pennsylvania, April 27, 1880. ****** * " I send by this two faithful copies of my original sketches of Orion (nebula) made on Etna last year with your Naval Observatory telescope of 3 ^ inches aperture. They were made in haste, in intervals of other work which took up nearly all the few clear hours. I think their value (if any) lies in the fact that the person who made them, while having some little experience in such sketches, was by chance almost absolutely ignorant of the aspect of the nebula in large instruments (I have not looked at it for many years), and did not at all know what he ought to see. There were two MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 1O ; or three rough sketches, of which the India ink one here [omitted] is the summary, so it does not very closely agree with the outline [Fig. 38], where the contours and inclosures are marked from i + (brightest) to 7 (faintest). This was chiefly done on one evening by beginning in the earliest twilight when only a little haziness was visible near 0, and sketching new contours every 10 minutes or so as the sky grew darker. I have scrupulously abstained, even in making the copies, from reference to any pub- lished drawing. * * * * * * * "S. P. LANGLEY." We may compare this photometric sketch of the nebula with the previous draw- ings, as follows: HUYGHENS (1656), Fig. i, differs largely from LANGLEY'S outline i ; the trapezium and 685, 708, 741 are involved in nebula in 1656, and not so later. The region near the north shore of Sinus magnus was seen much farther by HUYGHENS than light of the same intensity according to LANGLEY. The reverse of this is true just preceding the trapezium. HUYGHENS' drawing of 1694 (Fig. 2) is undoubtedly a better representation of the appearance of the nebula in his time than the earlier one. Comparing this with LANGLEY we find a very good agreement with his outlines 1 1 1 . . , with the same exceptions as before; i. e., HUYGHENS' region near a is brighter than LANGLEY'S and his trapezium is within the nebula. It is the same in MAIRAN'S Fig. 3 (1731), and also in LONG'S (1742) Fig. 5 PICAKD (1673) Fig. 4, agrees better with L \NGLEY when note is taken of the different kinds of shading than when the simple outline is taken, and this seems to be an important point LE GENTIL (1758), Fig. 6, seems to have seen out to LANGLEY'S 222 . . . and towards the north preceding portions even as far as 333 .... Here again the portion o is brighter in the older drawing than in LANGLEY'S. MESSIER (1771), Fig. 10, agrees much better with LANGLEY than any of the pre- ceding ; a comparison of the two figures should be made ; MESSIER'S o is very much as drawn by LANGLEY. He seems to have seen out to LANGLEY'S 444 . . . LEFEBVRE (i 779), Fig. 13, saw out to LANGLEY'S 333 . ... on the following side, but hardly so far just west of the trapezium. However, his figure is grossly misdrawn. In SCHROETEB (1794), Fig. 14, the neighborhood of the trapezium is very different, the southern end of E is, as before remarked, strangely so. Figs. 29 and 30 should be compared with Professor LANGLEY'S, as well as Fig. 12, and the description by DOBERCK in Astronomische Nachrichten, band xci, col. 336, No. 2 1 8. The earlier figures all seem to give a greater brightness to the region near 6 than Professor LANGLEY'S outlines warrant. On the whole I am inclined to regard this as accidental. An important paper, by my friend Mr. KNOBEL (Monthly Notice* R. A. S., vol. 41, p. 312), gives the results of his photometric measures on the relative brilliancy of three portions of this nebula. Unfortunately for my purpose different parts of the Huyglienian region are not compared in such a way as to assist in the photometry of the various masses laid down in the Index-Chart. IOS MONOGRAPH OF THE CENTRAL PARTS OE THE NEBULA OF ORION. PAET II. WASHINGTON OBSERVATIONS OP THE NEBULA OF ORION, IN CHRONO- LOGICAL ORDER.* 1874, JANUARY n. i i h 30 i2 h i5 m . Companion to 724 not seen. [This companion is laid down in LASSELL'S chart, Mem. R. A. S., vol. xxiii (1854), and was discovered independently by ALVAN G. CLARK, with the i8-inch refractor, at Chicago in 1862. It is not in G. P. BOND'S Catalogue. It is mentioned as a new star by TISSERAND ; Bull. Int. Obs. Paris, 1876, No. 1 19, and Comptes Eendus, Ixxxi, p. 891.] 1874, JANUARY 14. 9 h . Mag. power, 400 : very good seeing at times. Companion to 724 seen neatly. North of 635 and 641 saw at times quite plainly (i) [see Index-Chart]; suspected strongly a star at (2) ; saw twice or thrice a point of light at (3) ; saw a companion to 707, (4). I could see no stars inside the trapezium. 1874, JANUARY 16. 7 h to 9 h 30. Eye-piece 400 : seeing poor ; stars bright but unsteady. In tha neighborhood of 635, etc., I see only BOND'S stars. (641) [one of 0. 2,'s variables] very faint. 675 not seen, but in looking for (4) I could just occasionally see the compan- ion to 724, (a) obliquely, but never by direct vision, although I tried repeatedly. //. 78 (=. G. P. B. (654)) seen at 7 h and until 7 b 2O m (approximately), but not after 7 h 30. 9 h ~9 h 3 m > tne seeing is rather worse. 1874, JANUARY 17. 9 h . Eye-piece 400: seeing excellent. Nothing new near 635. 612 seen for the second time double [i. e., 618 also seen well]. (567) very faint. (642; not seen^ although its two neighbors [647 and 651] are well seen. 675 not seen ; I have never seen it. (a) [near 724] seen well. (575) seen. (602) seen faint. Besides the stars mentioned I see near the trapezium 612, 618, 601, 621, and 636, the last very faint. 1874, JANUARY 23. Many and thick clouds. (641) seen once. Suspected h. 78 (654), but too cloudy to verify. Seeing fair. End io h 30. 1874, JANUARY 24. 9 h 30. Prof. C. A. YOUNG examined the neighborhood of 635, 669, etc., with eye- piece 400 [negative]. He put in on a sketch-map my stars (i), (2), and (3) of Jan- uary 14 without knowing of my observation of that date. () he sees well. Neither of us see my (4) [near 707]. 675 seen by both of us. It is very faint. Seeing occa- sionally fine. Both observers say there are no stars inside trapezium. No trace of the resolvability spoken of by Lord OXMANTOWN [Phil. Trans., 1867]. *The observations are recorded as written. Additions and explanations are inserted in square brackets, []. The observer was HOLDEN, unless otherwise mentioned. The times are Washington mean times. The 26-inch telescope was employed with its full aperture, except in a few cases, which are noted. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 109 [The stars (i), (2), and (3) are about the smallest stars that can be seen if immersed in nebulosity.] 1874, JANUARY 25. (i) and (3) seen, faint, and seen only at times; suspected (2) and could not see (4). Seeing fair. h. 78 (654), (642), and 675 not seen, (a) seen. 1874, FEBRUARY 5. 8 h . Haze : stars steady. * * * 9*. During a short period of good seeing saw (641) and (i) and (3), also 675. This last is extremely difficult. Is 709 variable? It is quite faint this evening. 1874, FEBRUARY 14. Although seeing had not become good it was steady. * * * (654) and (602) seen. Star suspected n. f. the following star of the trapezium and not far distant. LAS- SEI L'S b [a double between 685 and 708] looked for especially and not found. 1874, FEBRUARY 17. 737 yellow and dull and nebulous; if I am not mistaken in the number (737) it has a small star preceding and north of it. 1875, JANUARY 22. [Made various experiments to obtain a method of drawing the nebula directly, i. e.j to throw its image on a surface by reflection or otherwise, and to trace it. Among other trials, placed a piece of finely ground glass in principal focus, and removing the eye-piece, I saw on the glass the trapezium, 635, 669, 685, 708, and 741, and other stars Made a sketch on the glass.] 1875, OCTOBER 27. i2 h .3 to i3 h .2. Very bad seeing. Win i. A star (575) exactly on preceding edge of A. which is very faint compared to following edge. The north end of A has dark channels in it of the shape figured by G. P. BOND [drawing omitted]. Order of brightness. 1. A (all the brighter streaks in it 1 . 2. D (following edge). 3. F = Gr (not including the region round X in F). 4. L 6. H = E. [I note here that the boundaries of N and Q are rather uncertain, under ordinary conditions, and that too great weight must not be given to comparisons of Q and N with O P K, etc.] 666 and 667 just seen. Suddenly much brighter, just north of 647. 647 > (640 J 6 47 = ( 6 70 ; 6 47 > (575) * Seeing very bad. [According to BOND, (647) is i2"\i ; (641) is i4 m 8; 671 is 1.1-5; (575) i > I "-9-] I 10 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 1875, OCTOBER 29. Begin i2 h , end i3 h . Mag. power, 175. Wt. 2. 647 = (575) > 671, but not much. (575) > 573- 651 and (654) about equal. (641) not seen. Reading of po- sition-circle for parallel =. 165. 2. Frons. Micrometer wire placed so as best to coincide with the frons (p. estimated n 40), i e , to the south edges of E, F, N, and Q. Position-circle readings : 24. 8, 24. 5, 2 7. 5, 28. 9, 28. 8 ; mean, 26. 9. Con- cluded position-angle, 48. 3 (5) (= 75. 2 26. 9). Occiput. Angle of position (est.) 135 ; this refers to preceding edges of masses E and I. Posi tion- circle : ii2.o, 112. 2, ii44, H4.4, 112. 3; mean, 113.! .'.p 142. i (5). The occiput is exactly parallel by measure to a line joining 506 with [570]. Position-angle of preceding edges of J and B. Position-circle: 67. 5, 65. 9, 69. 2 ; mean, 67^5 . - . p = 7. 7 (3). B points ex- actly to 575, which is at the very end of it. The reading 69. 2 above will serve to determine the angle of B from its base [i. e., north end] up to 575 . . p 6.o (i). Order of brightness. i. A. 2. D. 3. F and G. 4. I. 5. N and Q. 6. E. Perhaps the following is better: i. A. 2. F, G, D. 3. I. 4. H. 5. N. 6. Q = E. The general effect of each mass is taken, i. e., the brightest parts have most influence. 1875, NOVEMBER 5. i i h . 5 to i2 h .5. Mag. power, 175. Wt, = 2. Lacus Lassellii. 652, 657, 663 are on the preceding side of a dark space which ends a little to the south of 663 ; they are inclined across it from s.f. to n. p., 652 being nearest the^re- ceding side of the space. 647 > (575) = (671). V. The ground on which the trapezium stands is not totally black. r. The north part (n. two-thirds) of r is not black but filled with nebulosity ; the south one-third is certainly darker, but whether totally black the seeing is not good enough to determine. Pons Schroeteri In the middle of the bridge of SCHROETER the nucleus is not stellar, * * * at least it does not seem to be so to-night. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. T i i 6. Along the south edge of G it is notably brighter, and at the preceding end of this terminating bright streak I think I see a small star. a. The Index-Chart is right in making G concave towards the south. [This was marked to be examined again. It was found to be correct 1877, December 12.] On the following side of the pons it is quite dark. The line in the Index-Chart seems to limit this dark space properly. T". North of OTT it is quite dark. 4?. To the south of OTT it is filled with nebulosity. There is at least one dark streak in parallel to OTT and just south of it [con- trast?]: also in 4; I think I see a star half way between it and the Huygkenian region, on the same meridian as o. G. G much brighter on its south edge : then fainter and then all the preceding half is brighter than the following half (roughly speaking). c. Branch c extending to 523 is about right in Index-Chart; perhaps a little too much curved. It runs a little to the north of 523 ; but that star is nebulous, at least to-night. Order of brightness. i. A. 2. D. 3. F and G. 4. H and I. 5. N and Q. 6. E. In this, however, only the brightest parts of N arid Q are included. I do not think the stars 685, 708, 741 have cometic tails or brushes to them extending towards the south as BOND notices. The nebulosity seems darker between 685 and 708 and 708 and 741 (in the parallel nearly), but I take it this is the effect of contrast merely [drawing omitted]. It is darker between 685 and 708 than between 708 and 741. 1875, NOVEMBER 10. Begin n h 45"', end 13'' 20. Mag. power, 175. Wt. = 2. Measures of Ad with & l Orionis. Coincidence of fixed wire and micrometer wire = 64'. 2 5. Fixed wire on 6', mi- crometer wire on I 12 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. Object measured. Reading. A* Moving mic. wire south. Moving mi- crom. wire notth. Mean. Revolu- tions. Seconds of arc. r. r. r. 79-32 78.98 78.94 j- 79- 8 - 14-83 -I47-5 refr. o.i s A<5 147.6 74-64 74.16 63 34 74-46 _ 10.21 101.6 53 . refr. o.i A (5 101.7 Brightest part of F, on the same par- allel as 685. 74.16 74-62 .40 74.39 10.14 100.9 refr. o. I A'5 101 .0 70.63 70.83 70.73 6.48 fi/i e. U4 + 5 refr. o . o Brightest part of pans Schro't,rii (whose center is an elongated oval) 60.43 .25 60. 15 ( 60.28 + 3.97 + 39-5 on same parallel as bright star in .46 60. n ) D [=647]- refr. o.o Bright sharp n./. end of D ... 56.74 56.15 .44 "" ; T;y.5 [ 56.41 + 7-8i +77-7 refr. o.o S.f. sharp point of a = " Spitze" 56.57 29 56.40 23 [ 56.37 + 7-88 +78 4 refr. o . o Order of brightness. ist A! 2d A Dl D FG | IH GF | IH . . G = F. NQ NQ E. E. There are certainly no wisps or tails to 685, 708, and 741 (for a moment seeing good). Mag. power, 400. The star ROSSE 56 exists, and the line joining it and 581 is perpendicular to occiput. Sinus Gentilii. If there is any totally black inlet from the south into Sinus Gentilii it is very narrow. The Sinus is quite black in its north end. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 113 r. The north half of r is filled with light which join n to pons Schroeteri. W 1 = lacus Secchii. The distance from the hole (very black) W t , just north of W, to 685, is about equal to the distance from 635 to 669. W 2 is north of it, and is another remarkably black space. W l and W 2 wrong in position on Index-Chart. 1875, NOVEMBER 1 1. i i h 40 m to 1 2 h 30 m . Mag. power, 1 75. Wt. 4. Occiput. Position- circle : ii4.o, 114. 2, U5.9, 115. 5. Mean, 114. 9. . . p = i39.3. This is a measure of the preceding edges of E and I, and it cuts Y off entirely. Frons. Position-circle: 22.o, 22. 3, 22. i, 2i.8. Mean, 22. r. . '.^ 52. i. This measure is the best tangent to the whole line of light, but it cuts off some masses at s. p. corner of E, and some at Q. ' Q, P, R. Angle of position of n.f. sides of Q, P, and R (estimated 100) ; the line passes through 654 and O 4 , or nearly so. This cuts off some of R and most of T. Position-circle: 328. 2, 327. 2 (good), 328. 2, 329.8. Mean, 3284. .'.p io 5 .8 (4). o. Angle of position of south edge of G and general north shore of Sinus maynus up to D (to the north bright end of D). [This does not mean that D was one of the points of the line measured.] The measure is of the general trend of the shore. Angle (est), 95. Position-circle: 345-O, 344-8, 343-4, 344-7- Mean, 344-5- ' /' = 8 9-7 (4)- (676) = (654) = or > (641). (654) > 612 or 6 1 8. (654) 622 > 63 1. ? ? Query 62 1 1 (631 is in a black space) 622 on the Index-Chart is properly figured as to edges of A and U. P, S, M. The south edges of P and S right with respect to 67 1 and 676. The umih >////- iHf/ edge of M should be a little further off in the Index-Chart. PIHIS Srhrocfrri. The micrometer wire through the jww* passes through 685 and 669 nearly. Par- allel, 344-5- APP. V 15 114 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. Position-circle: 84.6, 82.3, 83.;. Mean, 83. 5. .' .p 170.; (3)- LASSELL'S star b [a double between 685 and 708] does not exist [as far as can be seen to-night]. 685, 708, 741. It might be said that there is a wisp or tail to the south of 685 ; between 685 and 708 is a darker space [drawing 1 omitted]. The shaded portions [of the sketch] repre- sent the dark spaces of which 685-708 is much the darker. This may be the effect of contrast, and probably is between 708 and 741, but not altogether so between 685 and 708. Messierian branch. The preceding edge of the Brachium Mess, follows 784 and 789 as in drawing [omitted] 675 not seen; indeed I have only seen it once or twice in i873,-'74,-'75. 686-688 not seen, but not carefully looked for. R. 56 plainly seen. * * * * A and L. In the south part of A and the north part of L the shapes are not caught in the lithograph. It looks like a rope with the strands untwisted so that you may see be- tween them There are at least two such openings. "Spitee" in (= 355 about 56.04 \ + 7-97 + 79-3 J Apex of E almost exactly south of 1 Mean 56.26 o o Middle of W 1 (Driving clock failed). Refraction cor- 3-74 3/ o o rections. Order of brightness. A, D, G, F, I = H, N, Q, E. The south part of F and the middle of G almost stellar in appearance. N and Q not very well defined. These masses, in the Index-Chart are perhaps not well drawn OTT. r . There are two brighter spots (elongated) in OTT, as in the Index-Chart. North of o it (the whole mass) it is quite dark, close up to the south edge of o. Sinus magnus. T' and V. The darkest space in the Sinus magnus is bounded by the curved line in Index- Chart, i. e., following t\\v pons Schroeteri Prolong OTT towards the west and the space south of this line [prolonged] and following the pons is blacker than any neighboring part. It is blacker than the ground on which the trapezium stands, for example. North of this line (o n prolonged) it is brighter, but still very faint. q> and 6. (lac us Lassellii.} Between g> and <7 there is a dark channel; just south of 663 it ceases to be very dark, and this darkest portion ends in a curve convex towards the south. There is a dark channel connecting this with the Sinus or nearly connecting it. I think the south end of this channel is wider than in the Index-Chart. r. South third is blacker than north two-thirds. The bay in which 647 is has never been seen so far. Messierian branch. BOND'S preceding edge of this branch is finely given. Its outline is very hard and I !6 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. sharp like water-color laid on and allowed to dry at the edge. The north edge of the' rostrum (/*) in BOND strikes me as too bright relatively, but I am not sure that it is so. MAIRAN'S nebulous star [No. 734] The shape in BOND is not quite right. The brightest part follows and is north (a little) of the star [737]. There are two dark spaces, one on the n. p. and one on the s. f. side, but not quite as figured; the bright portions do not seem to be exactly right. [Sketch omitted of a very faint star ^ = 355 5=24" (both estimated) from [734]- The very faint star above given is a very good test for light. 1875, NOVEMBER 24. Begin io h io m , end n h '30 m . Mag. power, 175. Wt. =4. 647 > (575) > (671) or (676) > 5 8 9 >(56;). The line through 685 and 708 passes through the brightest part of F (i. e , X [?]) and is parallel to the black channel between F and (G and H). [Such remarks as this are always founded on an observation of a micrometer wire laid through the stars.] OTT T TT is brighter than o / o is brighter than the middle of o TT. North of o ^ it is black ; preceding it is black ; south also black [a narrow channel]. After the channel south of OTT is crossed, the Sinus is filled with nebulosity up to its south border. Just following pons Schroeteri it is very black. Half way from o to south border of Sinus there is a nucleus, very faint and almost stellar, more nearly stellar than the nucleus in pons Schroeteri. G. Folloiving the " Spitee" in same parallel, there is a star whose distance from point of Spitze is equal, approximately, to the distance of the latter from 669. Order of brightness. A, D, G=H, F, N, Q, I, E, J. Sinus Lamontii. Although the Sinus Lamontii is plain, yet the Harvard College Observatory drawing of 1874 exaggerates the effect as seen now. Channel between cp and G. (lacus Lassellii.) This black channel certainly goes from 652, 663 through to the Sinus ma gnus. It is rounded south of 663, and black ; then faint and wider than the Index-Chart has it up to the Sinus. It is about as bright as north half of pons Schroeteri. 567. 567 is in a black space half way between B and i. 575 and 589 in two bright streaks, as in sketch of 1875, October 27. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. South shore of rj is about right in Index-Chart. Dark channel between E and F. p (estimated) 140. Position-circle: 293.5, 290.0. Mean,2qi.'j. .'.p This is the angle of position of the dark channel between E and F; its direction passes through the star 589. Hence the Index-Chart is wrong in this point. It passes in direction p 140, and then turns sharp off towards 63 1. The [vertex of the] angle is sharp, not rounded as in Index-Chart. 671, 676! [possibly 671-686! ?] prolonged intersect the from in a bright nodule. The pastel drawing of the Naval Observatory has the dark channels better than the Harvard College Observatory drawing of 1874 [both these are by M. TROUVELOT]. The channels are pretty wide and pretty well defined at edges. The dark channel following D does connect with Sinus maynus. r. The preceding and south edges of T are bordered by a very black stripe ; then the south third is black, its north two-thirds full of nebulosity. In the Harvard College Observatory print of 1874 the reverse is the case ; i. e., it is darker at the north end. 635, 7o8, 741. The same remarks as formerly made apply to these stars. It is dark between them, and brighter just south of them, but my previous sketches [omitted] are right, or nearly so. MAIRAN'S nebulous star [No. 734]. This nebulous star has certainly altered since BOND'S time. Between the principal star [734] and that one at the point of the comma [785] there is a broad dark streak extending quite across the nebulosity and dividing it into two parts. Its direction is s. f. to n. p. P> Following the " Spitze" there is a curious repetition of the prow-like shape of the Spitze itself; it is much fainter, and is close to ff t so that it looks like the shadow of it, a little distorted. 1876, JANUARY 3. Mag. power, 1 75 ; end i i h 45" Wt = 4- Seeing very good. Order of magnitude. i. (575)- 2 (671) or (676) [probably (671)]. 3. 622 or 625. 647 ? is brighter than any of these. It is in a dark space surrounded by bright nebulosity even towards the east, and this [nebulosity] is suddenly much brighter towards the north. r. The north half is filled with faint nebulosity; the south half is empty. Halfway between the foUowing edge of D and the preceding edge of pons Schm, t, rl there is cer- I 1 8 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. tairily a faint bright bridge of light, as sketched [in by me] on the Index-Chart, similar to SCHROETER'S second bridge. [See Beitrage zuden neuesten astronomischen Entdeck- ungen.~\ [Its base is connected on the north to v], and it extends towards the south as far as the parallel of the bright nucleus of the pons Schroeteri. Pons Schroeteri (g ). This nucleus is seen stellar beyond a doubt ; not the whole nucleus, but a point inside the central condensation. * T. The preceding side of r is the blacker. [There is a black stripe bordering D and T.] Order of blackness (not brightness). i. Space following pons Schroeteri and preceding [r']. 2. Space between 6 and o TT [r"]. 3. Space in which trapezium is situated [V]. 4. South half of T. 5. . N. B. This makes south half of T brighter than usual, but it is right. fji and 6. Rostrum >u is about as bright as 6. [I suppose this to mean, as bright as the main body of 6 and not as bright as the brighter southern edge of it. 1877.] Order of brightness. i. A. 2. D. 3. G, H. 4. N, Q, F, I. 5. E. (654) not seen, although looked for. 1876, JANUARY 4. io h 20 m . Mag. power, 175 Order of magnitude. i. (671) or (676). 2. (575) almost zz 671. 3. 622?. 4. 589. 5. 567. 612 = 618 zz: (676) ; 581 and R. 56 both seen; also 636, but not (654). Order of brightness. i. A. 2. D. 3. G, H, N. 4. F. 5. I. 6. E : and I = Q nearly. Rather hazy, and work unsatisfactory. At the same time the small stars are seen very well indeed. No signs of LASSELL'S b [between 685 and 708]. 709 seems rather fainter than usual ; I noticed this also last night, January 3. 1876, JANUARY 10. n h 3o m -i2 h . Mag. power, 175. Wt. = i. Order of brightness. i. A. 2. D. 3. G, H, N, Q. 4. F. 5. I, E. Not very good. Strong moon- light, and details faint. Sinus Lamontii appears very strongly marked to-night, much as in Harvard College Observatory drawing of 1874. This is simply on account of the moonlight, and not that it is really any stronger. [This remark is based on a great many observations in the years i873-'74-'75-'76.] The night is so bad that [much] further work is impossible. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. UQ A line in the Huyghenian region parallel to the frms and passing through 602 would pass through a region which is fainter than those on either side of it; that is, there is a fainter bar running through the whole Huyghenian region. T T' y. Even to-night I can see that the east side of T is not so black as'the west, and that the north is not so black as the south, r' is to-night of about the same blackness as r and as 7, but there is very little weight to be given to work done under such condi- tions. Nearly full moon, and seeing extremely bad. 1876, JANUARY 30. io h .^ Mag. power, 400 : seeing not good. Wt. = 2. (The original paper on which observations were recorded has been mislaid, and the following is from memory [and of course is only a part of work done], but I am certain of everything recorded.) Pons Schroeteri. Center distinctly concentrated ; almost stellar. on. and TT like nuclei, verging towards a stellar appearance but not so much as cen- ter of pom. T" and T. Quite black below [north of] OTT and on preceding side of T. A thin black streak edges all the west and north sides of T, but the north half of r is decidedly nebulous, while the south half is almost jet black. W 1 and V. W 1 =. lacus Secchii seemed blacker than V space around trapezium. ff. The south edge of G has a quite sharp bright termination ; i. e., comparatively much brighter than v, for example. The black space around 647 is not entirely black, and perhaps it is too well marked on Naval Observatory pastel drawing by TROUVELOT. V. 1 should say, also, that V was too black in that drawing. Two sequences of brightness of masses in Hnycjhenian region recorded which I cannot exactly remember and therefore do not record, but both agreed in making E quite faint. 1876, FEBRUARY. Begin 7 h 3O m , end 8 h 30. Power, 400. Wt. = 4 at first, then = i. I2O MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. Order of brightness. i. A. 2. D. 3. 0, H, F. 4. N. 5. I Q or Q I. 6. E. Order of blackness. i. T' y. 2. T (south half only). 3. V. 4. North half r. Again: i. T'. 2. T' 3. 7. 4. /SbtttfA half r. 5. V. 6. jVbr$ half r. Differences of declination. Object observed. Micrometer reading. A(5 in revo- lutions. A<5 in seconds. 71 .54 14. 3JJ 12.28 ** i (- '42. 7) I" 122 . 2 ) 7 . tfy 67.51 10.30 (- 122.3)} 102 <; ) v . J^* * *-** o r (- 102.6) ) This is about the same declination as north point of E. ' ( IO2.6) Through the center and brightest part of G. 64.21 \ 7.00 (- 69.6) North end of Sinus Gentilii * . 63.9 - 6.7- (- 66.7) South end of A near star 622 . 60.41 3-20 (- 31-8) North end of L on same parallel as 671 and the east point of Q. 6 } Orionis 59-78 57.21 - 2.57 o.oo (- 25.6) ( o o) Middle of break \npons Schroeteri . j / 55-0 + 2.21 \ ** v / (+ 22.0) Brightest part of fans Schroeteri . 53-0 + 4-21 (+ 41.9) All north of this in r is filled with nebulosity, except of course the black channel on its west edge ; nearly all T south of this is pretty black, though not so black as r 1 . 52.48 + 4-73 ( + 47-1) About through the center line of west half of OTT: this parallel passes through the break in pons Schroeteri north of its nucleus. 51.18 + 6.03 (+ 60.0) South side of a (approximately) . 48.8 + 8.4- + 83.6 ) (+ 83.7)1 South end of E ; seeing growing worse. 71.78 - 14-57 - 145-0 ) (- MS. oi South end of E ; seeing growing worse. 71.69 14.48 - 144.1 i (- 144. 2) y The J6's corrected for refraction are inclosed in brackets. Now put on mag. power, 175, all the measures having been made with 400. No tails or wisps to 708-741 to-night. G. P. BOND'S Eegio subnebulosa is right. The rostrum [/*] in Naval Observatory drawing of 1875 i too narrow. Made sketch-map of stars around and in MAIRAN'S nebula [omitted]. * [Drawing omitted.] The Sinus Gentilii is connected by a black channel running to the N. E., with space near trapezium. V is almost perfectly black * * [in sketch] is a little brighter [***... is a narrow space bordering V on its preceding side], and on its west edge is a little ridge [brighter], and west of this is the black channel, connecting with that just mentioned [i. e., that one running to the N. E.]. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 121 1876, MARCH 6. 8 h - 9 h . Mag. power, 175. Wt. = 2. Parallel, i5i.o. Occiput. Angle of position of preceding edges of E and I. Position-circle: 102, 101, 97.6, 964; mean, 99^3 . .p 142.;. Dark channel between F (on the south) and G and H (on the north). Its prolongation passes nearly through 685 and 708, whether exactly or not the night is not good enough to determine. Spiral formation of the nebula well seen OTTO v. STRUVE'S node of nebulosity, by O. 2. 126 (G. P. B. 793), is not seen; [referring to an observation communicated by letter]. (575) > 589 > (567). The last star is quite faint. 622. Angle of position of 622 from 0'. Position circle : 42.o . * . p i99.o. The line of 622 and 0' passes through (or nearly so) a bright star south of Huy- ghenian region 6' [570 G P. B.]. Dark channel between I (on west side) and X and G (on east side). This is a straight portion of some length whose direction goes through G. P. B. 5 70 (same star as noted just above). Position-circle: 213. 7, 2i5.7, 216. 8; mean, 2i5.4 . . p 25. 6. c. The direction of the spiral c (towards 523) is about right on Index-Chart. The seeing is not good on account of haze, and the weight of the measures is small. 1876, MARCH 14. S PRUVE'S new nucleus near 793. J6 micrometer wire on 793 - 89.91 wire B on new nucleus ----- 93.90 Again: 9 ai 3 93-90 - 3.77 = 3 J6 mean 38".S This nucleus precedes 793 = (O.2. 1 26), and is quite faint through the light clouds which cover the sky, still it can be steadily seen in a dark field, but not quite steadily APP. V 16 122 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. on the bright wire; [the p (estimated) of this object from 793 was about 190 to 200, and it is probably not 0. JS.'s nucleus]. [Sketch of 793 and vicinity omitted.] The small space [just following 793 and about 20" to 25" in diameter] seems to glisten with bright small points, a little like the Huyghenian region just south of trape- [Just north of 793 and immediately preceding the Messierian branch] there is a zium. dark channel which separates the branch from the diffused nebulosity of the Eegio sub- nebulosa. This channel is like BOND'S dark channels in nebula of Andromeda. Order of brightness. i. A. 2. D. 3. F, G, H. 4. N. 5. I. 6. Q, E, not a very satisfactory order in spite of good seeing. [Probably just on account of good seeing, so many details confuse a general judgment] i. A. 2. D. 3. F. 4. G, I. 5. H, E. 6. N, Q. This is better, but the mag. power 400 shows too many details to assign this order satisfactorily. V. - r'. V. is by no means as black as the Sinus Gentilii, but comparable with [in black- ness] and almost equal to the north half of r' '. Differences of right ascension. 575 is just exactly north of 573 [according to BOND 575 follows 573, 3"], and the line joining them skirts along the preceding shore of Smus Gentilii, and is the best tangent to this shore. On re-examination I find 573 preceding 575 by not more than o// -5 [3" according to BOND]. The line of shore between (/? and K) and y is curved, though not quite so strongly as it is drawn in Naval Observatory drawing, 1875 ; y, in that drawing, needs to be moved bodily towards the east to conform to the line 573-575- setting 241.!, wire A on 685, and at 64''. 1 8. Object measured. Micrometer reading. A a from 685 revolutions. A from 685 seconds. A a from 1 seconds. 82.62 r. 18.44 ii 183.4 i/ - 86.5 Preceding side of \Vi ..... 70.68 15.50 154.2 57.3 This declination-circle bisects the rounded apex of the mass I, and limits W 1 on its following side ; (W 1 narrows here on the following side to something like a canal.) 76.85 12.67 126.0 29.1 Bisects the rounded apex of E (not a very good observation) and passes through the northern star of t h e trapezium. 74-45 10.27 102.2 5-3 Tangent to the preceding sides of F and G and passes between 6 1 Orie- nts and the 6th star. 73- ^ 9.62 95-7 + 1.2 Tangent to brightest and following side of D ; tangent to following side of G and bisects F very nearly. 71.02 6.84 - 68.1 + 28.8 Nucleus of pans Schroeter.i, 66.24 2.06 2.5 + 76.4 N. B. Aa of 685 = + 96".g. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 123 End 8 h 3o m . Power 400, used throughout. Sky hazy, and images quite steady. 793- Looked at 793 again ; saw my star, but nothing else steadily, although the space /(Mowing is certainly full of bright points. 1876, MARCH 22. 7 h 40. m -8 h , Mag. power, 400. Wt. = 3. 793- I see s.p. 793, 5 = 40" the faint star previously measured (March 14); this is, of course, preceding the bright line of [west] nebulosity of the Messierian branch. Inside of the branch [and near 793] I am not sure of any point. There may be one 20 // -25 // off, a little following, but the night is not good enough to decide. Just preceding the Messierian branch from 793 north to 784 there is a black streak of varying width (not more than i') which extends from 793 towards the 'north. [Drawing omitted.] Order of brightness. i. A. 2. D. 3. F. 4. G, I, H. 5. N, Q, E. (Not a very careful observation.) \ 1876, APRIL i. 8 b . Very poor seeing. Mag. power, 1 75. 709 is a little harder to see than the 5th star. [Query, 6th star?] 709 = 671 or 647 about. It requires attention to see it. T. North half much brighter then south half. Seeing too poor to go on. 1876, NOVEMBER 5. s Mag. power, 400. End i4 h . Wt. ~ 3. 675 visible, arid well involved in nebulosity. Figure [omitted] shows two nuclei in N (denoted by a and b in this night's work), and the nuclei 686, 688 with stars 671 and 676. There is a dark space between 671 and 676. (671) >(6;6). South of the line 671-676 it is black, and 676 seems to be on the preceding edge of P or 0. The dark channel between and P not well seen (night not good). Q. The nort-h side of Q quite bright and sharp. T. The second bridge of SCHROETER is seen much as I have drawn it before. 124 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. a. &. 686, 688. i3 h 5o m - The line joining 676 and 685 passes a little east of the points a and I. These look nebulous, as do 686 and 688 of the figure and not like stars, a is on the following edge (the exact edge) of N and 688 is on the north edge of Q. 636 visible but not 654. The night is bad and work not satisfactory. 1876, NOVEMBER n. 1 2 h . Bad seeing. Order of brightness. H, G, F (about equal), I, E. Dr. C. S. HASTINGS sees the second bridge of SCHROETER. 1876, NOVEMBER 22. Begin io h 45, end i i h 15. Mag. power, 175. Wt. zr 2. Frons. Position-circle: 594, 59. 2, 59. 8, 6o.o; mean, 59 .6. Parallel 1 08. 9 p = 49.3 (4). Occiput. Position-circle: i52.9, i52.9, i524, i5i.2; mean, i52-3; P i3 6 - 6 (4)- Best tangent to south shore of Sinus magnus. (To the general direction of the shore, cutting off a little of the south end of pons Schroeteri.) Position-circle: i82.6, i8o.8, i86.o, i83.9; mean, i83.3; p iO5.6 (4). 1876, NOVEMBER 27. Begin io h 3O m , end n b o m . Mag. power, i75=Jb Wt. i. Moonlight and flying clouds, which finally prevent work. Preceding edges of J and B (through 575). Position-circle: 103. 8. Parallel, io8.9 ; p 5. i (i). 1876, DECEMBER 5. Begin i2 h o m , end i2 h 45. Mag. powers, 400 and 175. Wt. =. i. Parallel =. is -*. Differences of decimation with 9' and brightest part of G. 7 r -55> 7 r -78, 7o r .6i, 70^87, 70^69;- mean, 70^70 Zero, 64 r .i2 Poor measures. J6 6 r .58 65 /7 .5 (5). MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 125 Dark channel between I and E. Its prolongation precedes 671. Position-circle: 6o.9, 62.!, 64.o, 63.2 ; mean, 6 2. 6 (4). p 42 .6 (quite uncertain). First three measures with eye-piece 400; last with 175. 647 > (671), (575)- 68 1 > (676) > 651 ; not much difference in these ; 709 =. 663 about. Very cold and seeing bad. 1876, DECEMBER 13. Begin i2 h 40, end 13* o m . Mag. power, 175+- Order of brightness. i. A. 2. D. 3. G, H. F. 4. N, I, E. Mr. H. S. PRITCHETT puts D and A about equal, but thinks D a little the brighter. The space between 685 and 708 is blacker than that between 708 and 741. Clouds. 1876, DECEMBER 19. Begin 13* 15" end i 3 h 35. Wt. = 3. Order of brightness. (Mr. PRITCHETT.) i. DnR. 2. A. 3. I. 4. H and F. 5. B. SCHROETER'S second bridge. It extends to the south as far as the parallel of 647. T: T'. The south half very black; T' blacker than north half of T. Order of brightness. i. A. 2. D. 3. F. 4. Q. 5. N. 6. G. 7. H. 8. I. 9. E. B = F = W ; Y = (doubtful). Order of blackness. i. T'. 2. T". 3. South half of r. 4 W. 5. Sinus Gcntilli Sinus Gentilii brighter than W l . 685-708 in the prolongation of the dark channel between (F and G) and H. 7- As in Index-Chart. Pons Schroeteri. . Its direction passes through 685 nearly. Position-circle: 325-7, 323-5, 323-O; mean, 324. i. piJi Q .2. (3). 126 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. Preceding edges of J. and B. Position-circle: 301. 5, 303. 7, 3O4.o; wean, 303 .i. (3). pl2.2. (3). [Drawing made, omitted.] This drawing shows the portion of the Huyghenian region following the meridian of & ', as it appears in a general view. It was made particularly to show a darker band which rests on T, as a base, and extends towards the south, ending on the frons between H and N ; . This part is about as bright (in general) as the dark channels. It is apparent at first glance in a general view, and the drawing gives its general shape. It looks like a continuation of r. In this drawing the second bridge of SCHROETER extends south to the parallel of 647; 651 is precisely on the edge of T, \i. e., da of tangent to following edge of D 2 8' '.8] ; g is shown as a central nucleus, surrounded by an annulus, etc. T. The south third of T is black, but I seem to be aware of one or two bright stellar points in it, which I cannot fix, but which I believe to be real. SCHROETER'S second bridge. Its position angle is a little greater than that of pons Schroeteri. Lacus Lassellii. It is connected with Sinus magnus. Y. Quite bright and equal to 6 near k. 2. Contains a bright star [570]. B. Extends no farther than 575 as a bright mass, certainly not as much farther as is given by Lord ROSSE (1867). A. The convolutions in A on its following edge give the effect of LASSELL'S drawing of 1862 [unpublished, but most courteously communicated to me in a full size pencil- copy by Miss CAROLINE LASSELL], but some of the details are different now. 1876, DECEMBER 31. Begin io h 7, end n b . Mag. power, 150 (A x ). Wt. 2 at beginning. (Moon- light.) Sky very clear, and seeing improving a little toward the end. 709 < 657, 657 652 about-. 647 and 651 as in Index-Chart. (671) > 676; 575 589, although 575 is first caught by the eye on account of its situation, being more free of nebulosity. Both 575 and 589 less bright than 647 and 671, although 647, 671, and 575 are not very unequal. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. j 27 B, A. B runs exactly through 575, and if it extends beyond (south) of 575 as a distinct mass it is considerably fainter. 589 appears at the south end of a bright part of A, and between 575 and 589 is an oval dark gulf. [Drawing of convolutions in A omitted.] The branch (B) leading to 575 is brighter than that leading to 589. V and A. Dark channel between V and A, much as in Index-Chart. It leads around toward the west in a very regular curve, connecting with the dark space north of c. V con- tains 612 or 618 (the brighter of these two) [618]. Wi connects with the dark channel north of 2, which runs towards 524. W 4 . W 4 is larger and more toward the west than in Index-Chart. Wj blacker than W 4f but not much, and both much blacker than W 3 , which to-night is not dearly outlined. Telescopic meteor crossed lower half of the field of view (25' in diameter) from s. p. to n. f.j position-angle about 50 very rapidly, lasting about o 8 . i ; as bright as 724. Channel just north [south ?~\ of I is about parallel to frons. 685-708 prolonged is in direction of channel between (F and G) and H. Dark channel preceding M has a direction from 685 to a point about half way from 618 to 647. To : night it seems to extend and join with V. 686 seen well. It is probably a cluster of very small stars or a nebulous nucleus. The north half of the second bridge of SCHROETER seen. T is dark, in the same Jd as# . 1877, JANUARY 2. Begin 9 h 4O m , end io b 2O m . Temp. = 2. 5 F. Mag. power, 175 . Wt. n 2. Order of brightness. i. A. 2. D, G, F. 3. H, I, N. 4. E. E > M > o > /*. B, about half way from 575 to its north end, is about as bright as the general mass of I ; but this comparison is very hard to make. J very faint. J < space just south of W 3 and about W 4 , and J < E, but J > G. Order of blackness. T ' T" r" blacker than W 1? which is blacker than the south half of T. SCHROETER'S second bridge. It is seen more like ROSSE'S figure than before. Only the south* two-thirds seen. 558 > 709 > 524- * For south read north probably. 128 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. C. c is about as bright as B (roughly speaking) a little preceding 575, and it joins a little more smoothly into the preceding edge of A. It is uniformly filled with v. F. nebulosity. (575) = 671 =647, nearly. 6 and y as in" Index-Chart. The nuclei of F and Gr were stellar in appearance ; more so than usual, and in a less degree those of I and H. E is always nebulous and uniform in brilliancy, if we except the very small surfaces which give what HERSCHEL calls the "appearance of stippling" to the whole nebula, and which are not to be fixed in position. Y. Y is not a marked feature of E, as in LASSELL (1862), but requires a little atten- tion to see it. Y- y is pretty uniformly black, but, of course, not so black as r', etc., but is uniform. 77. 570 (in (s67), which is barely visible, i6 m of ARGELANDER'S scale. There is a channel from 635 to 663, as in ROSSE. 635 is just on the south border of a triangular mass. Dark channels in Regio Picardiana. O, , etc.] Beginning on the following side and naming the dark channels running approxi- mately north and south in order they are : i st. Lacus Lassellii. This is better laid down on Index-Chart than in ROSSE. 2d. One entering r on the preceding side of SCHROETER'S second bridge, and con- tinuing towards the north as in ROSSE. (Leaving this order for a moment W 4 con- nects with W 2 (this again verified); W 3 is just south of 635. W 4 and W 2 by no means so well marked as W 1 zr lacus Secchii.) 3d. The third channel in order starts from the channel connecting W 4 with W 2 and runs toward the north, forming the preceding boundary of the triangular mass (apex to the north), in which 635 is near the southern borders. (The channel con- necting W 4 and W 2 is tolerably black up to and including W 2 ; from thence it con- tinues to the west as in ROSSE, but is not so black after leaving W 2 .) W 1 is the origin of another (the fourth) channel towards the ivest as in ROSSE, and then there is a fifth to the north of c. These are the principal ones, and they are all nearly exact in ROSSE'S drawing. A channel goes from 657, 652 towards s. p. as in ROSSK, except that I doubt its crossing the northern end of the triangular mass just described (635 at south end of this mass). If it crosses this mass I do not see it so to-night, and certainly the rela- tive intensities near this point are not as in ROSSE. North of this (last described) channel is another across q> parallel to the one just described through 657 and 652, as laid down by ROSSE. This is outside the limits of the Index-Chart. 506 is a few seconds south of the dark channel, having its origin in W 1 lacus Secchii, and 516 is still in the same bright wisp. From 524, running towards the west and dividing the wisp just spoken of, is a dark streak as in sketch [omitted]. [Probably not due to contrast, 1877, April 3.] Palus Bondii. 567 is quite in the dark and quite faint =: 16 magnitude (ARGELANDER). From 575 south of 567 and across to the southern edge of c it is a very little brighter in a narrow wisp, so that the space bounded north and northwest by c, northeast by B, and south and southwest by this narrow wisp is quite dark. It is undoubtedly made darker by contrast near the junction of c with B. i. i is an irregular oval dark mass, separated from the darker space just described by the wisp from 575 to c and bounded on the south by an irregular line as in Index- Chart. This (i) is connected to the dark channel just north of K by a darker lane through G) as indicated on Index-Chart. (Sketches made of the parts described which are omitted.) Parallel 17. 8. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 131 Pons Schroeteri (angle of position). Position-circle: 293.;, 2 9 2.8, 28 9 .6; mean, 292 .o. ^=i75.8 (3); uncer- tain. Sketch of proboscis near 793 (omitted). 1877, JANUARY 24. Begin 8 h 30, end 9 h 40. Mag. power, 175. Seeing very bad. Moonlight. Using a pair of tourmalines, lent by Prof. S. P. L^NGLEY, back of the eye-piece (first removing the cap which contains the eye-hole). With the maximum light which passes through the tourmalines, I see all four stars of the trapezium, 685, 708, 724, 74 1) 669, etc. I can see the whole of the Huyghenian region and plainly trace the Sinus Lamontii between two bright nebulosities I and J. No dark channels seen well, but V is evident ; therefore V is darker than the bottom of the dark channels. The tourmalines were to-night held in the hand, but I find it will be necessary to make an adapter for them. They, however, indicate that in the Huyghenian region M, S, R, P, T, and between J and I it was the faintest; then E and the north part of I ; next G, F, H, and part of I, part of Q (near N, I think), are a degree fainter than A and D. The above results are approximate and tentative, and are not of much weight. W 1 , W 2 , W 3 , W 4 , W 5 . [Sketch omitted.] > = " blacker than" ; W, > W 4 > W 5 > W 2 . 6 1 2 in nebulosity, or very close to border. 618 inside V. (642) =: 654 each is just visible. 709 > (641) > (676) > (567) here > = "brighter than"; 671 622! 575 nearly, and 589 is a very little fainter than 671. 675 not visible. A. 589 and 622 are correct on Index-Chart in relation to A. 622 is in a dark space half way from V to A. 581 > 573. R 56 not seen to-night. L. L, from 621 to 60 1, and from thence to 595, that is, the north [shore], is very bright ; almost as bright as A near it. I. No nucleus (602) seen in I to-night. F. X and F seem to be almost separated by a fainter streak nearly in the parallel. The following end of F extends further east than in the Index-Chart, but [this part] is fainter than the rest of F. [Sketch omitted.] As often before remarked the channel between F and (G and H) is in the line 685-708. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. G. G seems elongated, and to-night resembles closely LASSELL'S oil-painting of it (made in 1854,) which is at the Royal Astronomical Society's rooms. M. M is elongated in the direction 622-741 (approximately); it is just below M, and to-night seems similar in shape to it. Frons. The frons is Convex to the east as ROSSE draws it. The greatest convexity is near 685. V. On the preceding border of U, about half way (?) from 622 to 628 (half the 4d of these stars), I twice saw a very faint star ir (642) as it is to-night. It is just on the very edge [of V]. Night transparent but'very unsteady. G. (See above.) Its shape was as in sketch (omitted) brightest at preceding side and brushing off to a fainter following point. 1877, JANUARY 27. C 2 Orionis has a small companion 1 5 magnitude. 1877, JANUARY 30. io h . Professor LANGLEY'S tourmalines mounted back of the eye-piece, new A. (Mag. power, 175.) [The tourmalines were gradually rotated, and at each stage the appearances through the tourmalines was noted.] i st. Totally dark. 2d. Three trapezium stars, 685 and 708 visible. 3d. Four trapezium stars, 685, 708, and 741 visible. Drawing made of the Huyghenian region at this stage. 4th. At this stage and through the tourmalines a measure of the position-angle of the north shore of the Sinus magnus with a bright wire was made. Position- circle: 259.5. (Parallel, 334.); ^ = 44 (i.) 5th. True outline of frons seen ; E fainter than G and H. Lacus Lassellii seen. This process was repeated several times and a careful crayon sketch made [omitted], 685 and 708 on s. f. edge of the frons. 741 free from nebulosity. 635 and 669 seen. 635 just plainly visible. A dark band penetrates the Huyghenian region from Sinus mag- FIG. 7. Drawing made through tour- nus ( w hi cn j s much blacker than this band) and divides maline plates. , , , into two, one goes south past 685, the other west near and then makes a little bay towards the north as in PICARD'S (1673) drawing [see Figure 4 of this text]. G, H, F, and E ? divide the two parts of the dark band. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 133 W 5 . (Without tourmalines.) W 5 is certainly not so black as W 4 and W 1 . Has it changed since Lord ROSSE'S drawing ? End io b 45 Eye much fatigued. 1877, FEBRUARY 3. Begin 8 h 2o m , end 9 h 25. Mag. power, 175. Wt. = 2. (654) and (675) just barely seen in moments of quieter seeing. 675 appears to be east of the line of the frons ; i. e., in the dark space outside of the Huyghenian region, but this is not absolutely certain [and it is quite different from all previous determinations]. 654 is only rarely visible. The air is exceedingly transparent. Parallel, 334. Coincidence, 64 r .i37. Difference of R A. of 9 l and following end of Q. Micrometer: 49 r .oo, 49 T '.6o, 49 r .7o; mean, 49 r -43 ; s rz i4 r -7i (3) n + i47"-3 (3) (this is rather uncertain and too small rather than too large) ; refraction =. o". Difference of R. A. of 0* and following end of G. Micrometer: 47 r -55, 47 r -55 47 r -5! 5 mean , 47 r -54l s i6 r .6o (3) -f 165".! (3) refraction, o" '. Using Professor LANGLEY'S tourmalines i st. (When the maximum light was transmitted) careful crayon sketch made [omitted]. 2(1. (Diminishing the transmitted light.) The portion [of the Huyghenian region south of the line 608-741 has vanished]. Along that line, or near it, it is brighter [than somewhat further to the north]. Order of brightness (through tourmalines). E < F, G, H, Q and I. 1877, FEBRUARY 6. Begin 8 h 45 m , end 9 h 50. Mag. power, 175. Wt. = 3. Measures of Ja with 6\ Wire put in the meridian of &. The meridian of 0' bisects as nearly as possible the apex of E. The north corner of E precedes this meridian a few seconds, meridian precedes nearly all of F. Only a little (fainter) part preceM.,,, X being cu 'V o m .) If the apex of E is not on the wire through Q' it does not precede it at least, and it may follow it 2 // -3 // . The preceding end of G comes nearly up to the wire through 9 . point of E to. & this wire is nearly entirely in darker portions except near the ing end of L, where it cuts off a little. Coincidence = 6 4 r .i i. 134 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. G (middle point). Micrometer: 62 T .6i >J 62 r .84, 62^67; mean, 62*.?!. aJ i r .4o, aJ = 13". 9 (3); all these measures somewhat uncertain. Point where s. p. edge of F intersects frons. (It is faintly nebulous here, not bright as in the middle of F.) Micrometer: 6i r .7o, 6i r 45, 6i r .78; mean, 6i r .64. Ja 2*47, Jot 24". 6 (3) ; all rather uncertain. Tangent to tlje following edge of D. The north point of D precedes this. The point of tarigency is near 651. All north of this D precedes the wire. 651 very faint. Micrometer: 6i r .i3, 6o r .98, 6i r .oo; mean, 6i r .o4. aJ 3 r .O7, aJ 30". 5 (3). ffo- ff is not well denned to-night. Micrometer: 56 r .25, 56^36, 56 r .39; mean, 56*. 33. da. 7^78, 4a ^" ^ (3) ; all quite uncertain. 675 seen only in the evening in the nebulosity. The low power always shows [small] stars the best, as often before noticed. [Probably this indicates that many of the smaller points of light are not true stars but nebulous nuclei or groups of small stars.] [See 1876, March 14 and March 22.] s nucleus near 793. I cannot see it. My former description [examined and] confirmed. The neigh- borhood of 793 is glistening [with minute but indefinable points of light], but no one point can be selected following it. I see (but just see) my former star preceding 793 and 40" (est.) south of it. 1877, FEBRUARY 7. Begin 9 h o m , end io h o m . Mag. power, 175. Wt. zz 1-2; parallel, 334; coin- cidence, 64 r .io8. Measures of JS with 9' . South edge of following point of G [sketch omitted]. Micrometer: 56 r .$i, 56 r 43, 56 r .6i, 56 r .66; mean, 56 r .5o. J6 f.6i, JS 75".7 (4) ; refraction, zero. From lacus Lassellii east to Spitze the south shore of o is concave to the south. Q (follotving point}. (g and D : The seeing is too bad to measure 4d of these points.) Micrometer: 67 r .i9, 67 r .39, 67 r 4o, 67 r .i8; mean, 6f.2g. Jd 3 r .i8, Jd 6 (4). MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. I35 Gr (midale point). Micrometer: 70^83, ;o r .66, 70^85, 7 o r .6 9 ; mean, 70^76. Jd - 6 r 6< Jd - 66".2 (4). Brightest part of F (ivhich is not X). Micrometer: 7 4 r . 4 o, 74 r -42, 74 r -32, 74 r -34l niean, 7^.37. Jd - io r . 2 6, J6' - I02 // .o7 (4); refraction -f o".o6, Ad 102". i (4). North point of Sinus Gentilii Micrometer: 70^77, 7o r . 9 i, 70^73, 7o r .8l ; mean, 7o r .8i. Jd 6*70, J6' 66". 65 (4) ; refraction -f o".O4, -^ = 66". 7 (4). All the above measures are somewhat more uncertain than usual on account of unsteady images. Frons. The from is convex towards the east. Its outline is furthest east near the par- allel of 685. The following side of E is nearly a straight line ; the following side of F is inclined somewhat to the prolongation of the following side of E. From the apex of the curve of the frons (near 685) the bounding line extends to about half-way between 685 and 708 (in R. A.) when v it meets the prolongation of the following side of E again and continues on this line to the termination at Q [see Index-Chart]. In spite of the unsteadiness SCHROETER'S second bridge is well seen to-night, best defined on the following side. 654 not seen. g not stellar in appearance. 1877, NOVEMBER 20. Begin u h 25 m , end n h 55"". Eye-pieces 175 and 400. Images. Wt 2 Measures of 4d. Lacus Secchii (center). ' 69". 74 (4) with 175. Kefr., 0.05 JS 69".8 (4) ff* =41 ".6 (2) with 400. D (north point). ' 80". 98 : : (2) This point is not well seen to-night; it appears to be curved Refr., 0.05 towards the preceding side. Jd 8i".o Q (following point). Jd 25". 67 (3) This appears to bisect the following point of Q. [36 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. G (brightest part). Jd' - 60". 88 Refr., o".O4 6o".09 Seeing very bad and satisfactory measures impossible. All the above are very poor. 1877, DECEMBER 2. i i b 30* to i3 h . Drew on chart. 1877, DECEMBER 3. D. Begin 9 h 30, end io h i5 m . Wt. = i. The shape of the following edge of D is correct in the Index-Chart. The brightest part of D is within this edge. There is no stellar appearance to this brightest part. 647 seems to be in a bay, whose shape is (to-night) more nearly circular than in Index-Chart. 651 appears (to-night) slightly preceding the edge of D. "W"i ..... W 5 as drawn on chart. [Drawing omitted.] 1877, DECEMBER 7. End i2 h 14. Mag. power, 400. Wt.= 3. E (south point). Measures of ' i3i"<3 (0- This is the extreme point towards the north which could Refr., .07 be taken [as the vertex of E] south of this nebulosity is fainter. Jd' - i 43 // .45 (4) Refr., .08 Jd = -i43"-5 1877, DECEMBER 7. A (south point). Jd 3 1 ".9 (2) not very certain. W 1 (lacus Secchii). Jd' = 6 7 ".6 5 (3) Refr., .03 Jd -}- 67.7 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 137 1877, DECEMBER 12. Beg-in i i h 30, end i 2 h 42. Eye-piece, 600 A, 400 A. Wt, 3. Measures of 4d with 0' . A (south point). ii h 30 m . Micrometer- 67.17 67.38 67.48 67.40 67.09 67.30 Zero - - - 64.16 A. The dark space which includes 622 and 625 on Index-Chart is quite black, blacker than the channel preceding A and separating it from L. 622 is in the black channel. L (north point}. ' ~ > This is in the same 4$ as 671 and as 622. 66 ' 64 I A little less Jd than 622. 66.70 $ 66.71 ^ = 2 r .55 = -25". 3 7(4) (602) not seen ; 589 > 567. Tangent to the north side of the curve in which A joins B. 6 1. 1 2, .10, .or, .14; mean, 61.09 ^ 8 3 r -7 = 3"-54 (4) The point of tangency is marked on the chart. [N. B Only on the MS. chart employed.] B. B is much fainter south of the parallel of & than north of it ; and it seems hardly to reach (as a bright mass) the star 575. 9* I2 h 7 m . 4dj 60.0 r, .19, .23, .01 ; mean, 60.11, Jd 4.05 =: 4o".29 (4) 1) (north point). The north point of Dis not well enough defined to measure; it is drawn on chart. APP. V 18 138 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. South shore of 6 near Spitze. 55.69:: (shore). 56.05 ^ 56.40 > south side of Spitze. 56.05 ) 56.12 56.16 Jd' -8.00 := + 79". 5 9 (4) The following point was not clearly seen Refr., .05 and these measures refer to the 46 of the brightest point, and are a .little greater than -f- 79-6 the Jd of the south edge, as it would be seen under the best conditions. This south shore is concave toward the south. Q (bisecting the following point}. 67.01, .30, .16; mean, 67.16 ^# 3.00= 29". 85 (3) E (extreme south point). 79.72, 79.86; mean, 79.79 48 15.63 = 15 5". 49 (2) South point of F (in frons). 75-93, 75-945 mean, 75.94 A# 1 1.7.8 = i i?".i9 ( 2 ) refr., .07 Extreme north point of F (X). 73-75, 73-63; mean > 73-69- ^ d 9-53= 94"-8i (2) refr., .05 -- 94 ". 9 (2) When the micrometer wire is set at 73.63 (that part of it following X) it is all in the dark channel. It just intersects the frons at the south end of H. Zero, 64". 155 (3) 1877, DECEMBER 14. Begin i i h o m , end i2 h 40. Eye-piece, 400. Wt. =z 2. n- Just south of 570 there is a part of 77 much darker than the rest. The line 570- 666 is approximately the south border of rj. On this line about 20" preceding 666 there is perhaps two or three, and pretty certainly one small star. This whole region preceding 666 seems occasionally to glisten with small separate points. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 139 ft and K. The line 573~575 is almost exactly the following boundary of ft and K. (602) not seen. Channel between I and E. The north border of the channel between I and E prolonged is very approxi- mately tangent to the n. p. end of F ; i. e. t X. I. The s. p. corner of I projects beyond (L e., preceding) the general line of the occiput. (See sketches.) L. The preceding edge of L near 60 1 (star not seen) prolonged would pass through the s. p. corner of I. Other notes placed on drawing. 1878, JANUARY 3. / A cap, reducing the aperture to 3.50 inches was put on the telescope and eye-piece 175 used. io b 30. The sky is very hazy, so that fourth magnitude stars are not visible to the naked eye. In spite of this I see four stars in the trapezium. 619 and 628 very easily, 624 and 640 easily, and 640 is very little easier to see than 624. io h 40. 685, 708, 741, and not 724, 570, and not 669. No other star seen except 737, and no definite outline to the nebula. The ques- tion of HOOKE'S observation of 1666 requires another arid a clear night to settle it. ANDERSON also saw the four stars. i i h . Sky extremely thick, and large stars have halos to the naked eye. 1878, JANUARY 5. 9 h . Aperture, 3.50 inches. Mag. power, 175. The sky is clear, but very unsteady. The following stars seen : 619, 624, 628, 640; all four stars of the trapezium. Also 685. 708, 741, 724; also 570 and 523. 669 and 635 well seen. [635 was not seen by HUYGHENS either in 1656 or 1694.] There appears to be a star between 635 and 0', too faint to fix in position, but probably 647 and 651 seen as one. 734, 781 (faint) and 848; 449 and 479, also. Comparing with HUYGHENS' drawing of the nebulosity. In fact, the frou* suul occiput are well seen and the angle at E. It is, however, faint. 685, 708, 741 an- seen free from nebula. Siuu* Gentilil and Sinus mu 618. 612 is just on the edge of the nebulosity, inside of U. 618 is in V. 602 exists, I think, and is a little out of position on my Index-Map. 654 not visible. 567 about as faint as it could be and still be seen. 686 and 688 not seen. 709 is on the following edge of the dark space, between 708 and 685. 675 again seen very close to the edge of from. Just south of E near 666 are certainly some very small stars [or points of nebulosity]. The small star h in the channel following D looked for and not seen. End n 1 '; seeing unsteady, but quite clear. 1878, JANUARY 6. Aperture, 3 in -5O; and mag. power, 175. Sky clear but not particularly steady. With 400 A I see easily the four stars [of the trapezium] and can fancy I see the 5th and 6th star at intervals, as I know exactly where to look for them. With 175 I also thought some traces of the 5th and 6th star were to be seen, but I am sure that these stars would never have been seen by HOOKE if only of their present brightness. 647 and 651 not seen. End i2 b . Windy. 1877, JANUARY 7. Photometer. Brightest part of D (see observations January 12). I. II. 9 h o m 7.20 6.40 6-85 6.55 6.70 7.20 Altered mirror. 6.95. 6.60 6.92 (3) 6.74 (5) MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 141 Brightest part of A; in parallel of 624. 5-55: 6.2O 545 io h 4o m . 6.00 5.80 (4) The brightness of D is proportional to i- and T^-y from the construction of the photometer. A oc-^L^. Hence the light of D expressed in units of A is ( 5 .8o) 2 (&*>) TZ~ Ng, etc. Hence from to-night's observations Dzr 0.70 A (3) D = 0.74 A (5) These measures are, as yet, only experimental. A = 1.42 D (3) A =i.35D (5) 1877, JANUARY 12. ^Photometer. Brightest part of D (about midway from 642 to 635 and following that line) io h 5o m 4.70 4.68 4.10 4.68 ii o 4-7 10 55 m. t. 4-57 (5) N B. A different combination of glasses was used from that employed January 7, and these were found to be too dark to measure A with. Brightest part of E (center of mass). n h io lu 5.17 4.90 4.27 4.80 4.60 ii 1-Sm.fc 4.8. (6) Lamp burned out. These measures are still experimental. All the adjustments are not thoroughly settled yet. 142 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 1878, JANUARY 16. 9 h ~9 b 3O m - Moonlight too strong to allow of measures with photometer. Several trials show this. V, 6 1 2 and 6 1 8 sketched. [Sketch omitted.] io h o m . Eye-piece, 175. Wt 4. Moonlight. 654 just suspected. 675 just surely seen following the line of the frons. Gr and H (to .the eye) both brighter than F. Occiput. Zero 123. 7. Position-circle: 343 c -5> 343- 2 > 344-2; mean, 343- 6 ; #=140.! (3) This is the best tangent from the south point of J [LIAPONOFF DJ to the south point of E. It cuts off Y. Frons. Position-circle: 72.o, 7i.9, 72. 6, 72.9; mean, 724; ^ 51. 3 (4) This is the best tangent to the whole line from E to Q. r. Length of frons, 45.05 84.00 45.10 83.68 84.21 45.08 83.97, 2 5. 38 r .89, 5. = i9 r .45, s. = 193" '.5 These measures were made for comparison with LIAPONOFF [HN. L -f- 3"-7]- J and B (preceding edges). Position-circle: n6.8, 115. 7, H54; mean, n6.o; p=7 .j (3) These measures make the edge pass through 575. 567 just visible and a little brighter than 709. V and 6 1 2 and 6 1 8 are right in Index-Chart. Dark channel between E and F. Position-circle: 342. 5, 338. 8 (half weight); 342. i, good; mean, 341. 6; _p-i42.i (3) Dark channel between I and E. Position-circle: 258^ 226 (i) uncertain. Pons Schroeteri. :i h . Position-circle: 3 14. 2, 315. 6, 318. 7; mean, 3 16.2 ; p = 167. 5 (3) uncer- tain. All three measures poor. (654) has not again been seen. D. The north point of D does not seem sharp to-night, and I believe I have noted this before. This deserves attention in a dark night. [See 1877, Nov. 20, Dec. 12.] MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. H3 1878, JANUARY 23. Set micrometer Mare on the parallel through & 628. From 628 to the R. A. of g this line is within the nebula. From 628 to the R. A. of 651 this line is in nebulosity fainter than the surrounding nebulosity. This line is very little south of the south border of T. It is immersed in the nebulosity of R ; following R it passes through a dark space (on the map) and is finally involved in //. Beyond (i. e., following] R it does not intersect the Huyghenian region proper. Preceding 628 this line passes a little north of 567. Half way from this line to 575 is about the point where B ceases to be quite bright, and where it begins to be nearly faint up to 575. [This is different from ROSSE, 1867.] W 1 . 48 of center. 57 r .i7, .37, .22, .30, .40, .55, .34; mean, 57^34 Zero, 64. 1 5 = 6 r .8i =6 7". 7 Refr., 0.3 + 68".o The parallel through 685 cuts off X, and passes (as exactly as I can see to-night) through 5. / corner of I. [. . J8 =>_ 95 ".'8, G. P. B.] Spitze. The s. f. point of G is about on same parallel with n. p. point of D and with center of W lt Near 793 in p =. i9O-2oo, s 3o"-4o" is a faint star which I have noted [18761 March 14 and 22, and 1880, January 3]. Sketch (omitted). F, G, H, > I or E. There are also two surfaces in P? and Q which are brighter than either I or E. End io h io m . 1878, JANUARY 24. n h 45 m . Eye-piece, 175. Wt='2. 709 not brighter than 666. 66 1 > 676. 1878, JANUARY 26. A. Photometer. Washington Sid. Time. Readings. Notes. h. m. 4 53 3-95 The part A measured was that following 619. Measures 4-85 not easy. Lamp flaming. 3-72 Altered mirror. 4-3=> Good. 5 7 3-90 t=s o Mean 4.14 144 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. In future I shall take D (that part previously used and defined by the line 635- 647) as the unit of brightness, as the position of these stars enables me to define the surface used for comparison readily. I notice that much of the uncertainty of readings is to be attributed to the difficulty of selecting the same surface for each comparison. N. B. In what follows the brightest part of each mass is used, unless otherwise mentioned. Washington Sid. Time. Readings. Notes. h. m. D. 5 7 4-30 5.10 4-45 4.72 5 15 4.28 5 li Mean 4.57 5 15 5-35 E. 5.08 4.70 4.80 5 20 5.08 5 18 Mean 5.00 F. 5 20 3-88 The lamp was flaming at first, so that much of the light 4-52 4-43 came from the burning wick. It gradually grew fainter until about constant. 5 27 4.42 5 24 Mean 4.31 5 28 4.08 G. 4-75 4-45 5 34 4.28 Good. 5 31 Mean 4.39 D, 5 36 5 4i 4.08 4-45 4-50 4-50 This measure of D satisfactory ; and it shows the lamp to be fainter than at first, since it has to be nearer to the mirror. The first D measure was also good, but not as good as this. 5 39 Mean 4.38 r. 5 43 6.55 The part used is on the line 708 to the point of Q and the width of the mirror [15"] south of Q. Eye very tired. 6.15 7-50 5 57 7.78 - 5 50 Mearfj.oo: : MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. Summary. '45 t Mass. Reading. h. m. 5 o A 4.14 ii D 4.57 18 E 5.00 24 F 4.31 3i G 4-39 39 D 4.38 5 50 fi 7.00 : : These measures are reduced for each value of D separately. I. D = 4 .57- II. D = 4-38. A . Mean I and II. A = 1.22 D A = 1. 12 D 0.10 A = 1.17 D:* E = 0.83 D E = 0.77 D .07 E = 0.84 D F = 1. 12 D F = 1.03 D .09 F = 1.07 D: G = i . 08 D G = i.ooD .08 G = i. oo D fi = 0.43 D fj. = 0.39 D 0.04 H 0.41 D : : 1878, JANUARY 28. gh ^m m { beg-in ; ; h 6 m sid. t. end. Mag. power, 400. Windy Wt. At 8 h 54 654 just visible = 618. 675 not seen. Measures of Jd (zero 64 r .i53). E (south point); Mic. 78^93, 78.94, 78.96; mean 78.94 (3) J6 f i4 r .7 9 zr 4. refr. .08 , sid. t. F (brightest part). Mic. 74 r .47, 74.62, 74.48, 74.41; mean, 74 r 495 (4) *# - io r .34= iO2 // .86, refr. .06; Jd -~ i02 // . 9 (4) G (south point). Mic. 7i r .6i, 71.59, 71.93, 7 I -9I refr., .04 (4) This part of A is not that previously used. It is to be remarked that the same zero for D (4.5: to-night and January 12. A PP. V 19 146 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. N (south point in frons). Mic. 7i r .6i, 71.59; mean, 7i r .6o; 46' 7*45 zz 74". n refr., .04 _ 7 4". 2 (2) I (bisecting the preceding angle of I, near A^). Mic. 7i r .93 46' zz 7.78 zz 77 // 4o; refr., ."04; 46 77^4 (O Gr (brightest parf). Mic. 7o r .67,' 70.94, 70.87; mean, 70^83; 46' 6.68 refr., .04 ~- '0 \O / Star 654. 46 Mic 63.356 4a Mic. 60.96 .29 star extremely faint. 61.10 63.29 60.82 63.31 60.96 0.84 4 a zz 3.19 46 = + 8". 3 6 (3) 4a = 3 i". 74 (3) G. P. BOND gives the co-ordinates for 1870.7 4 a. zz + 3 2 "-6 46 zz -f 9 /7 .2. HOLDEN-BOND (in a) zz o /7 .9 ; (in 6) zz o".8. D. D has to-night no sharp bright north point upon which to measure the 46. It has been so during this whole opposition, and I believe this is quite different from the appearances in i874-'75~ '76. The north part of D is uniform in tint, i. e., the extreme north point is not suddenly bright. The air very clear and images good. Drawing- made of D [omitted] in which the surface h (see Index-Chart), once called s star, is represented as about in the center of the triangle 635-669-651. It is really a little north of the center of this triangle. 647 is shown within a dark bay, but further within than formerly drawn. The darkness extends an equal distance from 647 towards the west, north, and south, h is as bright as the neighboring part of D, on the line joining h and 647 and two-fifths of the way from h. South shore of G. (Spitse.) 46. Mic. 55 r .88, 56.06, 56.16, 56.06; mean, 56.04 . 46' zz 8 r .n - 8o".68 refr., .05 = + So". 7 (4) MONOGRAPH OF THE CENTRAL F*ARTS OF THE NEBULA OF ORION. 147 Measures of 4 a folloiv. Spitze (G). Mic. 47 r .83, 47.74, 47.80; mean, 47 r -79; 4<*? = = +i62". 75 refr., -f .01 = + i62".8 (3) Q (following point). Mic. 48 r .62, 48.92, 49.02, 48.68; mean, 48.81; Ja i5 r -34 4 a? = +152". 60 (4-) refr., -f - 2 #<, (MO nucleus visible, brightest part taken). Mic. 56 r .37, 56.50, 56.48; mean, 56 r 45l ^' = 7-7 Ja = + 76".6 (3) This is in same 4 ex. as 676, which is according to Gr. P. BOND for 1877.0 v JOL -{- 77". 9 HOLDEN-BOND, I ".3 F (brightest part). Mic. 6i r .32, 61.60; meaw, 61.46; z/ x = 2.69 N.B. G > F. G (brightest part). Mic. 62 r .46, 62,78, 62.41; wzertw, 62.55; ^ a> l - 6 = i5".92 ^ =+i5 y/ -9 (3) D (following edge). 7 b 8 m sid. t. Mic. 6o r .i7, 60.20, 60.18; mean, 6o r .i8. 2.97, 4a = + 29 7/ .55 (3). [Error of i revolution; mean, 61.18.] On G. P. BOND'S MS. (unpublished) drawing of 1861, Mnirh n, SCHROETER'S second bridge is indicated. 148 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 709. In my drawings and notes I have placed 709 exactly on the following edge of the dark space between 685 and 708, or exactly on the preceding edge of what G. B. BOND described as the cometic tail to 708. In Lord ROSSE'S drawing it is included within this cometic tail, and on several of G. P. BOND'S elaborate drawings (lent me by the kindness of Prof. E. C. PICKEKING, director of Harvard College Observatory), it is in exactly the same position as ROSSE, 1867. I am confident that it is not so now, but I will again examine it. Night very clear and images good. Zero, 64^153 (3) 1878, FEBRUARY 4. Photometer. E (brightest part). Sid. t. 6 h 43 m ; 6 in .oo, 5.70, 6.00, 6.30; mean, 6.00. [Flame too high.] D. [I intended to choose the same part of D as formerly used, i. e., on line 647-635 and about 10" north of 647, but I was not as successful as I desired. The third reading particularly was on a surface from 15" to 20" preceding the part used before] 6 h 5i m - 4-35, 4-55 [5-oo], 4-50; mean, 4.47 (5) Flame still too high, but a little lower than for E. D. t 7 h 3 m ; 3-9 3-9) 3-8?; mean, 3.89. Lamp lower. The part of D used here (and above) was in the line of 642 and 647. G. 7 h n m ; 3.78, 3.68; mean, 3.73. Lamp lower a very little. The bright mirror of the photometer is a semi-circle with a diameter of about 1 5' on the edge, i. e., about 0.027 inches. Removed photometer. Eye-piece 1 75 put on. Air beautifully clear, but unsteady. 675 just seen, about on edge of the frons with this power. V?7 LASSELL'S 1. Something is certainly in this place. I should say it was a double star. 654 and 602 looked for, and neither seen. (New) small star between a and 709. I am not thoroughly certain of it, as I do not see a satisfactorily. ANDERSON sees it. End u b . [The reduction of the photometer measures gives E 0.55 D. This is doubtful, for the reasons given above. G 1.09 D. This again is not thoroughly satisfactory.] MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 149 1878, FEBRUARY 5. Photometer. E. 6 h 28 ra sid. t. 9.10, 9.08, 8.83; mean, 9.00. The photometer has been slightly changed, so that more rays fall on the screen. The change consisted in making the holes through the body of the tube a little larger. D. 6 h 3i m ; 7.77, 8.08, 8.50, 8.35; mean, 8.18. The nebula is so far beyond the meridian that the part of D which I am forced to use is not exactly the same as that chosen for the unit surface, although it varies very little from it. G. 611 34 m ; 7-5 2 > 7- 6 3> 7-85; mean, 7- 6 7- F. 6 b 3 8 m ; 7.98, 8.57, 7.60, 7.73; mean, 7.97. The difficulty with these measures is to fix upon the brightest part of the mass F [The same difficulty has been always experienced in the measures of Ja and Jd of this brightest part.] 6 11 45 m ; 948, 9.05, 9.95, 9.40, 9.41 ; mean, 9.46. The measure 9.95 was taken too near to the following side of I. D. 6 h 4 9 m ; 8.53, 7.79, 8.70, 8.70; mean, 8.43. Q. 6 h 56 m ; 8.86, 8.88, 9.23, 8.90; mean, 8.97. The surface used has one edge in the frons in the line joining the Spitze with 605. A. 7 h i2 m ; 5.28, 5.27, [4.60::], 5.31 ; mean, 5.29. A is a difficult mass to measure, as it is hard to fix upon the surface to be measured. E. 7 b i8 m ; 7.80, 8.12, 8.25; mean, 8.06. D. 7 h 22 m ; 7.00, 6.90, 6.72, 7.06; mean, 6.92. G. 7 h 28 m ; 7.05, 7.04, 7.27; mean, 7.12. 150 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. The images are quite tremulous ; but the photometer is working very well, as I have a more certain adjustment for making the beam of light from the lamp move along the axis of the rod than that formerly used. The lamp was satisfactory through out, but continuously diminished. It was certainly lower to the eye after f\ The rough reduction of the photometer measures gives : Sid. Time. Mass. D = 8.18. = 8.43- h. m. 6 28 E 0.83 0.88 31 D 34 G 1.14 1.21 38 F 1.05 I. 12 45 I 0-75 0.79 49 D 6 56 Q 0.83 0.88 D = 6.92. 7 12 A i./i 18 E 0.74 22 D 7 28 G 0.94 1878, FEBRUARY 20. 7 h 6 m sid. t. Eye-piece, 1 75 A. Wt. 2. (The images are too unsteady for 400.) Mic. 76.27; zero, 64.15. south point of F. 12.12 120". 5 7 * refr., .07 (i) F extends to the south beyond this, but it is so faint and uncertain a boundary to-night that measures should not be made on it. It is easily seen, however, in the absence of bright wires. south point of F. M 1 11.54 = ii4".8o refr., ,07 . 75.77, 75.54, 75.75; mean, 75.69. =n 4 ".9 (3) These measures refer to the well-defined and brighter southern edge, which is, however, within the mass of F. 654 suspected; 675 invisible; 709 barely visible. Then drew on large chart [omitted] End io h io ra m. t. Work very unsatisfactory. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 151 1878, FEBRUARY 25. Drew on chart; clouds 8 h 15. 1878, FEBRUARY 26. [Drew V, H, D on chart.] F to the eye seems brighter than usual. Seeing very poor ; images unsteady and ill-defined. Photometer. F (brightest part). 6 h o m sid. t. 5.25 : wt. i . 4.60 wt. 2 4.65 wt. 2 4.45 wt. 2 Good. 4.66 mean by weights. D. 6 h io m ; 4.80, 4.20, 4 50 (good), 4.25 ; mean, 4.44 ; eye estimated F > G, but not much ; also G > H, ditto. These estimates very difficult [G]. (These estimates belong, not to the brightest part of G, but to a point marked on chart.) 6 h i5 m ; 4.10, 4.70, 4.40; mean, 4.40. This point is towards the following end of G. At first this was chosen as brightest part, and the error was only discovered at 6 h 40 on remeasuring G. This shows me that a chief source of error is certainly the difficulty of settling on the point to be compared. Some means must be adopted for fixing on the- same point night after night. If this is not done the measures will not be strictly comparable. I > E (eye). E. 6 h 22 m ; 4.30 (wt. i), 4.80 (good), wt. 2, 4.65, wt. 2 ; mean, by weights, 4.64. I. 6 h 29 m ; 4.45, 4.55 (very good), 4.40; mean, 4.47. D (brightest part ?). 6 h 29 m ; 3.96; this is a little s. and / of the proper unit. Taken by mistake. D (unit surface'). 4.40, 4.05, 4.20; mean, 4.22. H. 6 b 35 m ; 4.10, 4.20, 4.00; mean, 4.10. 152 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. [G]. 6 h 40; 4.0. This is the following part, same as at 6 h i5 m . G (brightest). 3.90, 4.00; mean, 395. A not well enough seen to measure. Eye-pieces, 175 and 400. a near 724 seen ; distance quarter (709-724) r= 1 2" . 709 is on the following edge of a darker part of the dark space between 685 and 708, but the darkest and most obvious space between the two has its following edge west of 709, so that G. P. BOND appears to be right in his position of this star relative to the cometic tail to 708. This requires examination on a better night. U. Prolong the lines 640, 625, and 624, 6(9 to the following side of A; between these lines the channel between U and A is darkest; north of the last line (624, 619) it is brighter for about 10", measured, along the axis of the channel, and then darker again. 654 not looked for. 675 just barely seen. h well seen as before. LIAPONOFF'S points F and c [near 651 and 654] are neither of them well enough defined to measure. Reduction of Photometer measures. Mass. I. D = 4.44. 11. D = 4,22. F 0.91 0.82 [G] 1.02 0.92 E 0.92 0.83 I 0.99 0.89 H 1*17 1. 06 [G] 0.97 0.88 G 1.26 I.I? Brightest D 1.30 I.I? 1878, FEBRUARY 28. A>D; F > G = H ; I>Eby eye. Photometer. D. 6 h 23 m ; 4.90, 5.30, 4.80, 5.15; mean, 5.04 (4) E. 6 h 27"'; 5.95, 5.75, 5.35, 5.25, 5.40; mean, 5.54 (5) F. 6 b 35 m l 4-70, 4-70, 4-30, 4-55 (good); mean, 4.56 (4) MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 153 H. 6 h 40 m ; 5.25, 4.90, 5.25, 4 70; mean, 5.03 (4) D. 6 h 46; 5.65, 4.60 (good), 5.20, 5.50, 4.30; mean, 5.05 (5) I. 6 h 54 m ; 5.10, 5.40, 5.25, 535; mean, 5.28 (4) D (brighter). 6 h 68 m ; 4.65, 4.75, 4.55; mean, 4.65 (3) This is on the line 645-663. 1878, MARCH 4. Eye-piece, 175. Wt. i. 8 h 654, seen; 675 and 602, not seen. Drew all the black channels on large chart, and all the details that could be seen [omitted]. Images too poor for measures. Too windy for photometer. 8 h 30. Order of brightness. i. A. 2. D. 3. G, H, F. 4. I, E. M quite bright ; brighter than usual. M I, El! 1878, MARCH 5. Begin 7 h io ra , sid. t. Eye-piece, 400 A. Measures of Jd ; zero = 64^13. Wt = 3. F (south point preceding frons) f That is, these measures refer rather to the s. p. edge, which is, perhaps, not so far south as the part of F in the frons. Mic. 75 r -55> 7543i 75- 6l 5 mean > 75-53- ^d' n-40 = "3"4i refr., 0.07 H (south edge). This, again, is preceding the frons, and is the limit of the brighter parts of F about in the meridian of 666. Mic. 71.81, 72.00, 71.94; mean > 7 I -9 2 - 4& = 7-79 = 77"-49 refr., 0.05 JS 77"-5 This parallel is only a very little south of the south point of N. All of G is nnrtl, of -77"- APP. V 20 154 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. This parallel is almost the axis of the curve of the following edge of I, s. f. 602. It is, however, a little north of the axis of that curve. N (south point). Mic. 71.66. Jd' 7.53 = 74".9i (0 refr., = 0.04 AS =-75"-o (i) F. A faint channel is visible through the following end of F, as before. Q. Drawing of masses in and near Q. N, n t q, etc. [Omitted.] n (north end). Mic. 67.70; ^ = 3-57 = 35"-5 2 (0 q (middle). Mic. 66.48 ; Jd 2.35 23".38 (i). Stopped ; h 35'", sid. t. y (north end). Mic. 69.6.2,* 69.94,* 70.17, 70 22 ; mean, 69.99. ^8 5.86 = 58". 3 (4) * Recorded, 7o r ; if it really is 7o r , as is possible, the borders being here very uncertain, then 48 =- 6 3 ". 3 7 (3) refr., ^r 0.04 = 63"3 (4) 6 (south end). Mic. 73.67, 74.27, 74.34; mean, 74.09. Jd 99".o8 refr., .07 = -99"-' (3) The last two measures pass nearly through the brightest part of F. . . JS' ioi".i7; refr. o".o7, and Jd brightest part of F =. ioi".2 (2). I (s. f. corner). Mic. 73.93 ; 4d' 9.80 97".49 ; refr. = o // .o7. . . 46 97 /x .6. I (corner near A^}. 8 h i5 m , mic.; 72,22, 72.36; mean, 72.29. Jd' 8.16 8i".28 refr., 0.06 =-8l".2 (2) MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 155 This parallel 81" is still a little north of the axis of the curve of the folloiving edge of I near 602 ; the 4d Si". 2 for I near A! is uncertain, as the point is not well defined. V (south point) or M (north edge). Mic. 66.30, 66.48, 66.44; mean, 66.41 ; J6 2.28 22".68 (3) refr., o.oi Jd =-22". 7 ( 3 ) Images unsteady towards the last. Apparently some haze. 1878, MARCH 9. 675 seen by Professor HALL outside from by 2" - The seeing is very steady. 1} seen by Professor HALL, but not double. Very faint, a seen, a > 675. 647 is on the following and north side of a channel. Photometer. E. 6.50, 6.80, 6.80; mean, 6.70 (3) D. 5.90, 6.00, 5.80; mean, 5.90 (3) [G]. (On line through 647 and tangent to the preceding side of F.) 6.50 (i) G. 5.80, 5.70, 5.90; mean, 5.80 (3) I. 5.60, 5.70, 5.95; mean, 5.75 (3) Lamp lower I This note put in because the reading for I appears so large. On looking at I with the eye-piece it certainly appears much brighter than E. I > E (eye). 1878, MARCH 9 [F]. (On line through 676 and apex of F.) 6.30, 6.60, 6.40; mean, 6.43 (3) F. 5.60, 5.40; mean, 5:50 (2) I. (On line from 628 to apex ) 5.90, 5.95 ; mean, 5.93 (2) I returned to tins mass to verify the former measures. D. 5.60, 5.40, 5.50 (good); mean, 5.50 (3) E. 6.20, 6.30 [5.40: : rejected], 6.10 (good); mean, 6.20 (3). End 7 h 37"' sid. t. 1878, MARCH 20. J (preceding point). Eye-piece 400. Wt. = 2. 7 h 35 m sid. t. 4a! 73^69, 74 r .i6, 73^96, 73^92 ; mean, ^3 r -92 zero, 64 . 1 5 Ja' 9.78 = 97"- 2 9 refr., .01 Ja- 97"-3 (4) 68.86, 68.98 : :; mean, 68.92. 4a 4 r -77 = 47"4 156 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 1878, DECEMBER 6. Begin 4 b 25, sid. t, end 5 b 35, sid. t. Eye-piece, 400 A. Wt. := i. Measures of da.; zero of micrometer 64 r .O9. I. West point LIAPONOFF'S A, approximately. The point on which I endeavored to measure is in the straight line of the occiput itself, and therefore it is inside the edges of I which extends west of this point some seconds with tolerably bright nebu- losity. The bright nebulosity ends about at this point, which is about the Jd of 685. I. 4 b 35 m ; ^a' = 70 r .6i, 70.31, 7025, 70.43; mean, 70.40. ' 6 r .3i 62 x/ .77 (4); refraction, -f o".o i. 62".8 (4). 4 b 50 ; J a! =. 68.91, 68.8 1, 69.00, 69.00; mean, 68.93. Ja' da 4 r .84r= 48". 15 (4). D. 5 b io m ; tangent to the following side of D. Jo! 6i. r .26, 61.2 1, 61.09, 61.10; mean, 61.15. 2 r .94 = + 29'\25 (4). Second bridge of Schroeter. About 4 h 29; I could see this very well in spite of the moonlight; preceding it as far as D the channel was very black (the blackest part of T) following it to ff the channel way not so well defined. LASSELL'S b (near 685). At about 4 b 25 this object was suspected just as on previous occasions. I am by no means sure of its existence. I am certain that there is some objective cause for so many suspicions. At 5 b it was not to be seen. Neither 654 nor 675 are visible. 9* 5 b 20 m ; 4a! 56 r .37, 56.28, 56.48, 56.39; mean, 5638. 4a! da =7 r .7i =+76".7o (4). 6 (following point. 5 h 3 o m ; Ja' = 47 r -95, 47 r -7i, 47 r ,83, 4^.02; mean, 4 7 r .88. Ja' Ja -}- i6 r .2i + 162". 26 (4). Images very unsteady. Moonlight. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 157 1878, DECEMBER u. Begin [i h i5 m , end I2 h 5 m . (Cloudy.) Mag. power, 175. Wt= i. Moonlight. Images too unsteady for measures. E. E is nearly uniform in brightness all over ; the n. f. corner appears, however, a little brighter. [This may be erroneous.] Y. Y is very faint ; [the outlines are barely seen] it is fainter than the neighboring parts of E. Z. Z, up to and including 581 and 573 is quite marked by brighter than the masses n. and e. of it. The contrast seems more striking than I remembered it from former years. F, G, H. All of about the same brightness, but F is, perhaps, the brighter. This is doubtful. F. The brightest part of F is south of X. H. The axis of H appears to be nearly in the line 685-622. H and M. The line of the following edges of H and M (prolonged) passes between 640 and 624. P. 676 is in the dark channel; but very close to the preceding edge of P. i2 h io m ; sky all cloudy. 1878, DECEMBER 23. i i h ; 67 1 > 709 ; 709 > 647, but not much brighter. 709 not much more easily seen than 666. Images very bad. 1 2 h 45 no better. 1879, JANUARY 10. 8 h -io b . Mag. power, 400. Wt. := 5. Prof. A. HALL, observer. "At about 8 h 22 m I looked at the nebula of Orion with power 400 A. The images were very good. I could see no star inside the trapezium where Professor Boss reports a companion. On examining the region near [685, 708, 741] I saw two faint stars near the places indicated in the sketch [omitted]. The faint stars are denoted by a and 6. On looking again at 9 l1 40 I could not see either, the seeing not being s good, a and 6 were about of same brightness." [Note by E. S. H. a is in the position of LASSELL'S double, "6" (see Index-Map) 1) is from 708 in ^ 45 8 = 48" est. from sketch]. 158 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 1879, JANUARY 18. End n h 2o m . Eye-piece 200. Wt. = i. . Occiput. p = 33o.6, 331.8, 330.5, 333.2; mean, 33i c .5 4747 P- 143-2 (4) Frons. p 244.;, 244.6, 245.1, 247.5; e an > 245-5 294.7 p = 49.2 (4) None of the faint stars are visible; 709 is just barely seen; 633 and 617 not seen. Images very bad. 1879, JANUARY 20. 9 b 55. Mag. power, 400 A. Wt. 3. Measures of JS ; zero 64^13. Brightest part of F. Mic. 74 r 47, 74 r s8, 74*40, 74.37; mean, 74^46 48' 10.33 iO2".76 refr., 0.06 I02".8 (4) N. point of E. Mic. 73 r .92 ; clouds; 73 r -76; mean, 73 r .84 9 h 55 m J6' 9.71 n 96". 60 refr , 0.05 JS - 9 6".7 (2) The n. point of X (F) is further north still. Brightest part of G. Mic. 7o r .77, 70.64, 70.55, 70.64; mean, 70.65 J6' 6.52 = 64".86 refr., 0.04 -64.9 (4) Clouds constantly passing, hiding 42 (c) Orionis to the naked eye. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 159 1879, FEBRUARY 23. Photometer. (Very windy.) D, brightest part: 5.5, 5.0, 4.8; mean, 5.10 (3) E, brightest part: 5.1, 53, 5.5; mean, 5.30 (3) E 0.98 D 1879, MARCH i. 8 h 40. Eye-piece, 400. Wt. = 3. 654 visible; also LASSELL'S b (double??). h visible; 651 visible; 642? visible; not quite sure of it. Schroeter's second bridge. Visible as far south as 647 ; south of this r is black. No real stellar nucleus g , but a condensation of nebulous matter io"-2o" large is there, separated from the rest of the nebula by dark channels n. and s. of it. That channel to the south is the blacker. D. It is not quite certain if the following edge of D extends further south than 647 as a continuous edge. All the attention J. could give will not decide to-night. 1879, MARCH 7. Photometer. D, 6.4, 5.6, 6.5, 6.9 ; mean, 6.4 (4) This part of D is defined thus: join 669 and 641 ; bisect this line, join its middle point with 647. The mirror is 10" north of this line. E, 7.1, 7-3, 6 -5, 7-o; mean, 7.0 (4) D (brightest), 6.3, 6.3, 6.1 ; mean, 6.2 (3) A, 4.1 :, 4.2:, both doubtful; mean, 4.2 (2) F, 54, 5-9, 54 ; mean, 5-6 (3) In the reductions I have used D = 6.3. 1879, DECEMBER 26. Power, 175. Wt. zr i. Strong moonlight, but sky very clear. 61 2 and 6 1 8 both seen ; h near the point of D seen like a nebulous mass and not like a star; a clear dark space is between h and the folktving side of D. D. The bright following edge of D does not (seem to) extend further south than 647 as very bright. South near T it is definite but fainter. Order of brightness. i. A, D. 2. G, H, F, I, Q, E. G and H notably brighter than F. Moonlight too strong to examine the neighborhood of 793. l6o MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 1880, JANUARY 3. Beg-in io h , end i2 h . Power, 175. Wt. 4, and very clear. 675 seen just outside of frons. LASSELL'S b seen. 612 > 618. 618 > 567- 567 636 very faint. 647 in a dark space. 651 just on the edge of D [with 175; with 400 A it was slightly within D]. h visible; nebulous and not stellar. io h 5 m , 654 just barely visible. Neighborhood of 793. The apex of the curve of the Messierian branch near 793 is probably n. p. that star a little, but the shapes of the edges north and south of this star are such that if a drawing were made by continuing the outline of the edge from the south of 793 north- wards, the Cape would be put south of 793, while, if the drawing were begun north of 793 and continued toward the south the Cape would be put north of 793. In fact there are really two capes like this one [the southern and fainter one ends in an exceedingly minute star, , of the drawing (omitted), which is in p 2oo-2io, s =. 30" from 793. This star , even if not seen, gives a point for the eye to rest on in approaching 793 from the south, and would cause the apex of the Messierian branch to be put at t; approaching 793 from the north the apex would be put at that bright star (793)]. I am satisfied that the nature of the object is such that no evidence of a change can be based on the position of this Cape in relation to 793. [See observations of 1876, March 14 and March 22. These drawings (omitted) confirm and agree with to- night's.] On an unpublished drawing af Gr. P. BOND'S I find the statement that the nebula reaches exactly to 793 (about March n, 1861). Near 793 are three or four bright points which could probably be located by measures if it were worth while. They may be very minute stars. One of these is at , in the figure (omitted), i. e., p = 2oo-2io, s = 30" from 793. D. The following edge of D up to 647 is brighter considerably than the following edge of T just south of 647. 1880, JANUARY 3. Photometer. io h 50. The flame of the lamp does not seem to burn satisfactorily. E. 4.6 4.2 4.1 4-30 (3) I. Southern end. 4-50 (0 The above will do for a comparison of E and I. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 161 Flame too low and wick changed ; then D. 5-0 4.8 5-oo (3) Clock not running regularly. E. 4.9 5-o 4.8 4-90 (3) Measures of D rejected, not in proper part of D I. 5-3 5-6 5-3 5-40 (3) . 4.9 5-o 4-7 5-0 4.90 (4) G. 4.7 4.8 4-5 4-67 (3) D. 5-' 5-2 4-9 507 (3) End 1 2 h . The clock stopped several times and the measures not satisfactory. 1880, JANUARY ic. Photometer. Images very unsteady. The mirror was put in D on the line joining 641 and 647, and with its south end at 647 (its north end was therefore about 15" north of 647). APP. V - 21 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. Begin g\ D. 6.1 This is not by any means 6.3 the brightest part of D ; the 6.3 brightest of D is farther east. 6-23 (3) E. 7.3 7.0 7-13 (3) F. 5-9 5-53 (3) a 4-9 5-2 4.8 4.97 (3) F. 5.0 5-5 5-3 5-27 (3) D. 4.6 5-i 54 54 5-i2 (4) G. 4-9 4-7 5-o 4.87 (3) F. 4.8 5-2 5-2 5-07 (3) MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 163 E. 6.3 6.1 -. 6.23 ( 3 ) D. 4.6 5-2 5-0 4-93 (3) Summary. Do. i. D. 6.23 6. 20 2. E. 7-i3 6.10 3- F. 5-53 5.80 4 . a 4-97 5-50 o* * 5-27 5-30 6. D. 5- 12 5.10 7 . a. 4.87 5.10 8. F.x 5-07 5.00 9. E. 6.23 5.00 10. D. 4-93 4.90 As the lamp was perpetually growing fainter, I have used interpolated values of the readings on D, as in the second column. 1880, JANUARY 10. Looked with the finder and with low power (175) for the star LE GENTIL puts furthest north in his drawing. It does not exist. I can only explain it at all by sup- posing it may be 570 G-. P. B.. put north of & in d instead otsoiith, but in the same Ja. 1880, JANUARY 13. Photometer. D (same part as before observed). 7-8 7-9 7.6 7-77 (3) E (bri{/M<*t). 7-3 7-3 7.2 7-27 (3) 164 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. I (brightest). 7-7 7.6 7.0 7-5 745 (4) F (brightest). , 57 5-7 54 5-6o (3) G- (brightest). 6-5 5-5 6.0 6.4 6.10 (4) D. 6.7 6-3 6-3 6.43 (3) Then set the lamp at 7.9, the same reading as at first, and found the mirror just barely illuminated ; so that the lamp certainly has grown fainter. The above observations seem to me very good, the best this year, and compar- able with those of last winter. Summary. D n . E. 7.27 7.50 I. 7.45 7-20 F. 5.60 6.90 Gr. 6.10 6.60 [As before, I have used interpolated values for the readings on D, as I experi- mentally proved that the lamp grew fainter.] MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION, 165 1880, JANUARY 14. Photometer. D; 5.8, 5.6 6.0, 6.1 ; mean, 5.88 (4) E; 5.9, 6.0, 6.4, 6.8, 6.0; mean, 6.22 (5) I; 5.2, 5.4, 4.8; mean, 5.13 (3) D ; 5-o, 5-9, 5-5 5-5 5 mean, 5.48 (4) F; 4.8, 4.8, 5.0; mean, 4.87 (3) G; 4.6, 4.9, 4.2, 4.1 ; mean, 4.45 (4) E ; 5A 5.2, 4.8 ; mean, 5.00 (3) D ; 5-2, 5- 2 , 5-i I mean, 5.17 (3) H; 54< 5-7, 541 mean, 5.50 (3) Summary (using interpolated values of D). D; 5.88, - E; [5.80], 6.22 I; [5.60], 5.13 D; 5.48,- F; [5.40], 4.87 Gh [5-30], 445 E; [5.20], 5.00 H; [5-20], 5.50 1880, JANUARY 16. Photometer. D; 5-6, 5-8, 55; mean, 5.6 (3) E ; 5.3, 5-2 ; mean, 5.3 (2) I; 5-o, 5-5, 6.0, 5.4; mean, 5.5 (4) F; 4.8, 5.1, 5.0; mean, 5.0 (3) G; 4.5, 5.0, 5.2; mean, 4.9 (3) D; 5A 5-7, 5-5; mean, 5.6 (3) E ; 54, 5-5 5 mean, 5.5 (2) I; 5-5, 5-o; mean, 5.3 (2) [E] 15" south of the brightest parts; 6.4, 6.6, 6.9; mean, 6.6 (3) 1880, JANUARY 21. Eye-pieces 400, 600 A. D. The nebulous patch 7? , near the n. f. point of D, is 2 // -3 // on a side; it has dark spaces all round it. The Index-Chart compared with the sky as follows : E. The n. f. angle of E in the chart is about 90 ; in fact, it is less than 90. F. The 11. point of F is correctly drawn ; that is, it is -north of the north point of E considerably. 1 66 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. Channel between X and F. This (although barely seen to-night) is not correct on the chart ; it is not perpen- dicular to the channel separating E and F. I. The south side of I (chart) is too long compared to the north side of E. F, G, H. The channel between F and (G and H) is right in the chart at the east and west ends, but it is rhomboidal in shape ; and the width, as given in the Index-Chart, just north of the letter F, is not more than one-third of the true width there. In fact, the whole mass H (in the chart) should be moved north and east a considerable distance as my measures show. Otherwise, Gr and H are about right, i. e., as to shape. Q. The details of Q cannot be made out in this bright sky, but the shaded portions of the chart near stars 676 and 686, while they are right enough in themselves, give a wrong impression. The whole space directly south of T should be shaded far enough to join with the space ij. The Sinus Lamontii is not satisfactorily laid down. A. This mass is not well drawn, but the figure, with the explanations as given in my various measures, will explain it. A good figure of this mass would be very complex \Vi (lacus Secchii). Should be more nearly circular, and its position in the chart as to dot and dS is not correct. D. is tolerable well drawn as to shape. T. should be faint. W 2 , "Wa, etc., cannot be well seen in this bright sky, nor can any of the fainter and outlying parts. 1880, JANUARY 29. Eye-piece, 400 A. Wt. 3. 612 and 618 visible ; north and east of them is a dark space, which is only partly shown on the Index-Chart. W 1 . is not well drawn as to shape, and W 4 is correct ; W 2 and W 5 not well seen on account of moonlight. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 167 B. 575 is the southern limit of B [this is different from Lord ROSSE'S drawing]. C. The dark curved channel just north of C is darker than that south of it. T. The south edge of T is near the place of 654 (not visible to-night), and it is tolerably sharply denned, more so than is shown in the Index-Chart ; it seems as if the Sinus Lamontii should be continued past 622, 625, 648, and 654 in a nearly straight line to join with T near [c]. T. The south edge of T from [c] to the bridge of SCHROETER, is much fainter than the edge of T]. The moonlight is too strong for the finer details. SYNOPSIS or THE PRECEDING DETAILED OBSERVATIONS. In the pages immediately following I have collected all or nearly all the single observations relating to each particular bright mass, dark channel, etc., and have arranged them chronologically for coavenience of reference. The measures in this sec- tion are corrected for differential refraction. In general, the essential portions of the original observations are alone given, but occasionally where the description is too long or too unimportant to be repeated, I have added, after the date, "see observations." By this arrangement it will be easy for any one to examine into the accuracy of the original observations, for under each head each statement can be examined in detail and verified or disproved. The various masses, etc., are here arranged nearly in the order of right ascension. In the following pages are given the observations of supposed variable stars, the examination of various drawings, the order of brightness of the various masses, etc. Frons. 1875, Oc. 29. p = ^-3 (5) 1875, Nov. ii. p = 5 2. i (4). This is the best tangent to the wlwk line of light, but it cuts off some masses at 5. p. corner of E and some at Q. 1876, Nov. 22. p 49-3 (4) 1877, Jan. 24. The frons is convex to the east, as remarked by Lord ROSSE. greatest convexity is near 685. 1877, Feb. 7. Convex as above. 1878, Jan. 1 6. p = 5* -3 (4),* = '93".5 (3)- The measures of * [length of fron were made to compare with LIAPONOFF ; but they are very uncertain from the i of the case. HOLDEN-LIAPONOFF = + 3". 7. 1879, Jan. 1 8. # = 49-2 (4)- Wt= i. Occiput. 1875, Oct. 29. #=142.! (5). Occiput exactly parallel to line 506-570. 1 68 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 1875, Nov. 10. The line joining R 56 and 581 is perpendicular to occiput. 1875, Nov. ii. p= 139. 3 (4). This measure cuts Y off entirely. 1876, Mar. 6. p= 142. 7 (4) 1876, Nov. 22. p =. 1 36. 6 (4). This measure cuts off a little of E, and is hardly satisfactory. 1877, Jan. 5. p 135. 5 (3). This measure cuts off Y. Wt. i. N. B. The last two observations are not comparable with the preceding- three. 1877, Jan. 5. s (from 9') io6".5 (3); HOLDEN-LAMONT = -f- io".2; HOLDEN-LIA- PONOFF + io".i." 1878, Jan. 1 6. |; I39,6 (7). This is the best tangent from south point of J to south point of E. It cuts off Y. 1879, Jan. 18. j0 143.2 (4). Wt. i. Preceding edges of E and I. Sinus Gentilii (/?, K, and y). 1875, Nov. 10. If there is any totally black inlet from the south in the Sinus it is very narrow. 1876, Feb.! The Sinus Gentilii is connected with V by a channel running NE. (See observations.) 1876, Feb. ? 4S north end 66". 7. 1876, Mar. 14. The line joining 573 and 575 is the best tangent to the preceding shore of y. 1877, Jan 2. As in Index-Chart. It is pretty uniformly black, but not so black as r', etc. 1877, Feb. 7. North point of Sinus Gentilii Jd = -\-66".7 (4) 1877, Dec. 12. The line 573-575 as on 1876, March 14. 1878, Mar. 5. Northendof y; Jd ^".^ (4) or 63". 3 (4). (See observations.) 1876, Feb.? North point; Jd = io2".6. 1877, Jan. 2. As in Index-Chart. 1878, Mar. 5. North end; Jd = 99 ".2 (3) J. 1877, Jan. 2. See observations. 1878, Mar. 9. Preceding point ; ^# 97"-3 (4) c. 1875, Nov. 5. The branch is about right in the Index-Chart, perhaps a little too much curved. It runs a little to the north of 523, but that star is involved in nebu- losity to-night. 1876, Mar. 6. About right on Index-Chart. 1877, Jan. 2. c about as bright as B (roughly speaking). MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 169 J AND B. 1875, Oct. 29. p 7. 7 (3) angle of preceding edges. 1876, Nov. 27. p 5. i (i) angle of preceding edges, which pass through 575 in this measure. 1876, Dec. 19. p=z 1 2. 2 (3) 1878, Jan. 16. jM-rz 7. 7 (3) this measure passes through 575. B. 1875, Oct. 29. B points exactly to 575, which is at the very end of it. Position angle of B from its north end to 575 6.o (i) 1875, Nov. 24. 567 half way from B to i, in a dark space. 1876, Dec. 19. B extends no further than 575 as a bright mass, certainly not as much further as is given by Lord ROSSE, 1867. 1876, Dec. 31. B runs exactly through 575, and if it extends south of 575 as a distinct mass it is considerably fainter. The branch (B) leading to 575 is brighter than the branch (in A?) leading to 589. 1877, Jan. 2. About half way from 575 to north end, B is about as bright as the general mass of I. 1877, Dec. 12. Tangent to the north side of the curve in which B joins A, 46 = + 3"-5 (4)- B i g m uch fainter south, of the parallel of & than north of it; it seems hardly to reach 575, as a bright mass. 1878, Jan. 23. See observations on brightness of B. 1880, Jan. 29. 575 is the southern limit of B. This is very different from Lord ROSSE'S drawing. W 1 , W 2 , W 3 , W 4 , W 5 . W 1 lacus Secchii (discovered by SCHROETER, 1 794). 1875, Nov. 10. The distance from [the center of] W 1 to 685 is about equal to the distance 635-669. W 2 remarkably black. 1875, Nov. 17. Middle of W 1 , ^ = -37":(i) 1876, Jan. 30 W 1 blacker than V. 1876, Mar. 14. Preceding side of W 1 , 4 a 57".3 ; following side of W 1 , Jot 29".!. Middle of W 1 , 4a == 43".2, according to above, but the preceding end of W 1 is very narrow, almost a canal. 1876, Dec. 19. W 1 brighter than Sinus Gentilii. 1876, Dec. 31. W 1 connects with dark channel north of 2, which runs towards 524. W 1 blacker than W 4 , and both blacker than W 3 , which is not clearly outlined to-night. 1877, Jan. 10. W 4 connects with W 2 (verified) ; W 3 is just south of 635. 1877, Nov. 20. 4d of middle + 69".8 (4), not very certain. 1877, Dec. 7. 48 of middle = + 67 /7 .7 (3), much better than observation of November 20. 1878, Jan. 23. Jd of middle + 68".o (7) 1878, Mar. 9. Ja of center 47"-4 : ( 2 ) 1878, Dec. 6. J . _ 1875, Nov. 10. North extremity ; Jd rr ioi".7 ) 1875, Nov. 17. Apex of E almost exactly south of & ; i. e., Ja o".o. 1876, Feb. 1 South point of E, Jd 142"^, 145".!, i44".2; weaw, I44".o. 1876, Feb. I -ZVortf/fc point of E, z/ io2".6 (approximately). 1876, Mar. 14. Apex of E, Ja = $".$ (poor images). 1877, Jan. 2. E nebulous [no nuclei] and uniform in brilliancy. 1877, Feb. 6. The apex of E certainly does not precede 0'. It appears to be in same R. A. (9 h .o). It may follow it 2"-$". 1877, Feb. 7. The following side of E is nearly a straight line. 1877, Dec. 7. 4d of south point i43"-5 (4); the smallest J6 which smaller telescopes could give is not less than i3i // 4 (i) 1877, Dec. 12. Jd of south point 155". 6 (2) [extreme]! 1878, Jan. 28. Jd of south point i47 // -2 (3) 1878, Feb. 28. The brightest part is the center of the mass. 1878, Dec. n. E uniform in brightness all over; the n. f. corner appears, pos- sibly, a little brighter ! 1880, Jan. 21. The n.f. angle of E is less than 90. Z. 1878, DECEMBER n (see observations). Dark channel between I and (X and Gf). 1876, Mar. 6. A straight portion of some length, whose direction prolonged passes through 570, p =1 25.6 (3). Dark channel between I and E. 1876, Dec. 5. Its prolongation precedes 671. p = 42.6 (4) ; quite uncertain. 1876, Dec. 31. It is about parallel to frons [p = 50 ?]. 1877, Dec. 12. Its north border prolonged is very approximately tangent to the n. p. end of F ; i. e., X. 1878, Jan. 1 6. jp = 46 (i); uncertain. Dark channel between E and F. 1875, Nov. 24. j?rri39.6 (2). Its direction passes through the star 589. For further description, see observations. 1878, Jan. 1 6. p= i4 l0 -7 (3) 172 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. Dark channel between F and (G and H). 1875, Nov. 17. See under F, synopsis. 1876, Mar. 6. Its prolongation is in the line 685-708. 1876, Dec. 19. Same as March 6. 1876, Dec. 31. Same as March 6. 1877, Jan. 5. Same as March 6. 1877, Jan. 24. Same as March 6. V. 1875, Nov. 5. The ground on which the trapezium stands is not totally black. 1875, Nov. 17. V not so black as r' . 1876, Jan. 30. V not so black as W l 1876, Mar. 14. V not so black as Sinus Gentilii, but comparable with and almost as black as north half of r'. 1877, Jan. 24. 618 inside V; 61*2 in nebulosity or very close to border. The extent of V towards the north is therefore (612) ; 46 -f 24".o. 1878, Mar. 5. South point; Jd 22".^ (3) F. 1875, Nov. 10. Brightest part, ^ ioi".o ( LIAPONOFF'S a ) ; his measures give 97".4- 1875, Nov. 17. The south part of F almost stellar in appearance. 1875, Nov. 24. The line through 685 and 708 passes through the brightest part of F (i. e., X [?]), and is parallel to the black channel between F and (G and H). 1876, Feb.? South point, JS- 122".$. 1876, Mar. 14. Tangent to preceding sides of F and G, da -f- \" .2. Mem. X must be too far east in Index-Chart. 1876, Mar. 14. F is bisected very nearly in Ja -f- i8".8 (28 // .8). 1877, Feb. 3. Intersection s. p. edge with frons, da 19". 2 : (3) 1877, Feb. 7. (Brightest part which is not X), Jd = 102". i (4) 1877, Jan. 2. Nucleus stellar to-night. 1877, Jan. 24. X and F seem to be almost separated by a fainter streak nearly in the parallel ; following end of F fainter than the rest of the mass. 1877, Dec. 12. South point of Fin frons, JS = ii7".3 (2) Extreme north point (X), Jd 94". 9 (2) 1878, Jan. 28. Brightest part, JS 102". 9 (4) ; da + 26".8 (2). G > F. 1 878, Mar. 5. J6 south point preceding the frons, 1 1 3". 5 (3). See observations. 1878, Mar. 5. A faint channel visible through the following end of F. 1878, Mar. 5. Brightest part, Jd 101" .2 (2) 1878, Dec. ii. F, G, H about equal in brightness, and perhaps F is the brightest. This is doubtful. The brightest part of F is south of X. 1879, Jan. 20. Brightest part of F, J6- 102". 8 (4) 1880, Jan. 21. The north point of F is north of the north point of E. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 173 G. 1875, Nov. 10. Brightest part, Jd 64".$ ( LIAPONOFF'S 6 ); he gives 63".$. 1875, Nov. 17. The middle part almost stellar. 1876, Feb.! Center and brightest part, J6- 6g".6. 1876, Mar. 14. Tangent to preceding edges of F and G, Ja -f \" ,2. 1876, Mar. 14. Tangent to following side of G, J6 -f- 28".8. 1876, Dec. 5. 4d (brightest part), 65".5 (5) ; poor measures. 1877, Jan. 2. Nucleus stellar. 1877, Jan. 24. See observations. 1877, Feb. 3. G (middle point), Ja -f- i^'.g : (3) 1877, Feb. 6. The preceding side of G comes nearly to the R A. of & '. 1877, Feb. 7. G (middle point), Jd 66". i (4) 1877, Nov. 20. 4d (brightest part), = 6o".9 (3). Images very poor. 1878, Jan. 28. 46 (brightest part), 66".5 (3) ; Jd (south point) 75". 7 (4) 1878, Jan. 28. Ja (brightest part), -f- i$".g (3) 1878, Mar. 5. All of G is north of -- 77". 1879, Jan. 20. Brightest part of G, Jd 64.". 9 (4) H. 1877, Dec. 12. JS south end of JI in frons=. 94". 21 (i) 1878, Mar. 5. Jd south edge = 77".5 (3). [This is, of course, within the line of fronsJ] 1878, Dec. ii. The axis of H is nearly in the line 685-622. The line of the following edges of H and M (prolonged) passes between 640-624. M. 1878, Mar. 5. North edge of M, Jd 22". 7 (3) D. 1875, Oct. 27. It is suddenly much brighter just north of 647. 1875, Nov. 10. Bright, sharp n. f. end, 46 = -f 77"-7 (3) 1876, Mar. 14. Tangent to brightest and folloiving side, Ja = + 28".8. 1876, Dec. 19. 651 is precisely on the edge of D; i. e., Ja = + 28".8. 1877, Jan. 12. The brightest part follows the line 635-647 a little. 1877, Feb. 3. Tangent to folloiving edge, Ja=3o".$ (3) 1877, Nov. 20. [JS Si". : :] See observations. 1877! Dec. 12. Jd of north point not seen well enough to measure. 1878' Jan. 1 6. The north point is not sharp nor bright to-night. 1878, Jan. 28. Ditto. See observations in detail. 1878, Jan. 28. Ja (following edge) = -j- 29^.6 (3) 1878, Dec. 6. 4a (following edge) z=+ 29 /7 .3 (4) 1879, Mar. i. See observations. 1878, Feb. 28. The brightest part is on the line 647-663. 1878* Mar. 9. 647 is on the following and north side of a channel. I 74 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 1879, Dec. 26. The bright following edge of D does not (seem to) extend farther south than 647 as very bright; south near T it is definite but fainter. See observa- tions, 1880, Jan. 3. 1880, Jan. 3. 651 just on the edge of D (with 175) with 400 A it was slightly within D. 1880, Jan. 21. The nebulous patch h is 2"-$" on a side. Dark channel following D and preceding v. 1875, Nov. 24. .It certainly does connect with r. r. 1875, Nov. 5. South third is darker than north two-thirds. 1875, .Nov. 10. North half is filled with light which joins on to pons Schroeteri. 1875, Nov. 1 1. North third is too bright in G. P. BOND'S drawing. 1875, Nov. 17. South third is blacker than north two-thirds. 1875, Nov. 24. The preceding and south edges of r are bordered by a very black stripe. (Contrast! 1 ?) The south third is black, the north two-thirds full of nebulosity. 1876, Jan. 3. The north half is filled with faint nebulosity; the south half is empty. Half way between the following edges of D and the preceding edge of pons Schroeteri, there is certainly a faint bright bridge of light as sketched to-night on the Index-Chart, similar to SCHROETER'S second bridge. Its base is connected on the north to v, and it extends towards the south as far as the parallel of the bright nucleus of pons Schroeteri, [46 39" approximate]. 1876, Jan. 3. The preceding side of r is blacker; i. e., there is a black stripe bor- dering D and T. 1876, Jan. 3. South half of T brighter than usual. See observations. 1876, Jan. 4. The east side of r not so black as the west; the north part not so black as the south. 1876, Jan. 30. Quite black on preceding side; a thin black streak edges all the west and north sides of T ; the north half of r is decidedly nebulous, while the south half is almost jet black. 1876, Feb.? All north of ^tfzz-f 47".! in r is filled with nebulosity, except, of course, the black channel on its west edge ; nearly all south of this is pretty black, though not so black as r'. 1876, April i. North half much brighter than south half. 1876, Dec. 19. South half very black. 651 is precisely on the edge of r. Schroeter's second bridge (in T). 1876, Jan. 3. See under T. 1876, Nov. 5. It is much as previously drawn. 1876, Dec. 19. It extends to the south as far as the parallel of 647 [i. e., 46-=. + 38".o. G. P. B.]. Its position angle is a little greater than that of pons Schroeteri. 1876, Dec. 31. Does not extend south of g Q . 1877, Jan. 2. Only north two-thirds seen. Its general appearance like Lord ROSSE'S figure. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 175 1877, Feb. 7. It is best defined on the following side. 1878, Dec. 6. See observations. 1879, Mar. i. Visible as far south as parallel of 647 ; south of this r is black. Pons Schroeteri (0 ). 1875, Nov. 5. The nucleus does not seem to be stellar. 1875, Nov. 10. Brightest part, Jd = -\-^ 9 ".^ (5) on same parallel as 647 '[+38".o, G. P. B.]. 1875, Nov. ii. The micrometer wire through the pons passes through 685 and 669 nearly. p = 170. 7 (3) 1875, Nov. 17. The nucleus is almost stellar. It is very bright, almost as much so as the south edge of ff. 1875, Nov. 17. Brightest part, Ja + 79".$ (4). The north part precedes this a little, ^ 175 about (estimated). 1876, Jan 3. Nucleus stellar beyond a doubt, not the whole nucleus but a cen- tral point. 1876, Jan. 30. Center distinctly concentrated, almost stellar. 1876, Feb. 1 Middle of the break, in the pons Schroeteri, which is south of the nucleus, 46-=: -f 22".o (i) 1876, FebJ Brightest part, JS -f 4i".9. 1876, Feb.! Middle of the break in the pons Schroeteri, which is north of the nucleus, Jd + 6o".o (i) 1876, Mar. 14. Nucleus, Ja = + 76". 4. 1876, Dec. 19. p 171. 2 (3) 1877, Jan. 5. p = 165. 8 (2). Wt. i- This is rather to be considered as the p of a line joining 669 and , and is a poor measure. 1877, Jan. 10. j?rr 175.8 (3). Wt. rr i. Images unsteady but brilliant. 1876, Dec. 19. 0o is in a central nucleus surrounded by an annul us. 1877, Feb. 3. , ^=77"-5 : (3) 1877, Feb. 7. g not stellar in appearance. 1877, Nov. 20. 0o, J6 = -{-4i".6 (2). Images poor. 1877, Dec. 12. 0o, ^ see observations. Not stellar. ff. 1875, Nov. 5. Along the south edge it is notably brighter. 1875, Nov. 5. Much brighter on south edge, then fainter. Roughly speaking, the preceding half is brighter than the following half; the north shore is concur, rewards the south?? 1875, Nov. 10. 8. f. sharp point of ff = Spitze ; 4d : f- 78' .4 (4) 1875, Nov. 1 1. P 89-7 (4) ; general trend of south shore. i;6 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 1875, Nov. ii. In G. P. BOND'S drawing the Spitze is relatively too bright, but not much. 1875, Nov. 17. The south edge of ff for the east two-thirds of its length is notice- ably and suddenly brighter than the rest of G. 1875, Nov. 17. Extreme following end of G- 4a=.-\- 169". 9 (3). 1876, Jan. 30. South edge terminates in a sharp and bright edge ; i. e., compara- tively mnch brighter than r, for example. 1876, Feb.? South edge of G; Jd = -f- 83". 7 near pons Schroeteri f (approximately). 1877, Jan. 30.* P of south shore (through tourmalines to compare with LE GENTIL, etc.); p = 44 (i) 1877, Feb. 3. 4 a. following point of G -j- 165".! (3) ; good. 1877, Feb. 7. South edge of following point; Jd =. 75"-7 (4)- From lacus Las- sellii east to Spitze the south shore of o is concave to the south. 1877, Dec. 12. South shore concave toward south ; Jd of Spitze 79".6 (4) ? G and Spitze. 1878, Jan. 23. The sow^ folloiving point of tf is about on same parallel with n. p. point of D, and with center of W 1 ; i. e., Jd -f- 68" approximately. 1878, Jan. 28. South shore of Spitze; ^d -\- 8o".7 (4) ; 4a + i62"-8 (3) 1878, Dec. 6. ^<* + i6i".3 (4) Lacus Lassellii. This is laid down in SCHROETER'S map of 1 794. 1875, Nov. 5. 652, 657, 663 precede its axial line. 1875, Nov. 17. See observations. 1876, Dec. 19. It connects with Sinus magnus. T r . 1875, Nov. 5. On the folloiving side of pons Schroeteri it is quite dark; the line in Index-Chart seems to limit this dark space properly. 1875, Nov. ii. G. P. BOND'S drawing gives its north third too bright relatively. It is now quite dark there. 1875, Nov. 17. T' blacker than any neighboring part. North of on prolonged (towards the west} it is brighter, but still very faint. T' blacker than V. 1875, Nov. 24. Just following pons Schroeteri it is very black. P, S, M. 1875, Nov. n. See observations. 1878, Dec. ii. 676 is in the dark channel, but very close to the preceding edge of P. Dark channels preceding and following M. 1876, Dec. 31. See observations. 1877, Jan. 2. See observations. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. ! 77 N. 1876, Nov. 5. Two nuclei seen in N (see observations for details). 1878, Jan. 28. South point (in /rows) / Jd ^".2 (2) 1878, Mar. 5. South point ; J6 7 5 ". o (i) Q, P, R. 1875, Nov. 1 1. Angle of position of n. f. sides of Q, P, and R. The line passes through 654 and 624, or nearly so. This cuts off some of R and most of T p = io 5 .8 (4) 1876, Nov. 22. p i05.6 (4). This cuts off some of R. Q. 1875, Nov. 17. Following end of Q in the same R. A. as 708 [+ 150". 6, G. P. BOND]. 1876, Feb.? East point of Q, Jd 25" .6. 1 876, Nov. 5. North side of Q quite bright and sharp; 688 is on the north edge of Q. .1877, Feb. 3. Following end of Q a little preceding 708, ^/a + 147". 3 (3) Wt. z= 2. The extreme point is undefined, and these measures place the point rather too far west than too far east. 1877, Feb. 7. Following point, ^ 31 ".6 (4) 1877, Nov. 20. Folloiving point, Jd r= 25 /7 .7 (3). This line bisects the fottoicing point. 1877, Dec. 12. Following point, .^# 29". 9 (3) 1878, Jan. 28. Following point, Ja=. -f- 152". 6 (4) ( north end of n, Jd 35". 5. 1878, Mar. 15. Nuclei in Q, < .-.it f >* 1 ( middle of g, ^ (575) > (602) > (567) and (642) not seen. 1874, Jan. 17. 675 not seen. 1874, Jan. 23. (641) seen; clouds. 654 suspected. (Cloudy.) 1874, Jan. 24. No stars inside trapezium. 1874, Jan. 25. (642) not seen; 654 not seen; 675 not seen. 1874, Feb. 5. (641) seen. "Is 709 variable? it is quite faint." 1874, Feb. 5. 675 seen. 1874, Feb. 14. LASSELL'S b not seen. 1875, Oct. 27. 666 and 667 just seen. 1875, Oct. 27. 647 > (641), 647 - (671), 647 > (575). 1875, Oct. 29. 647 = (575) > (671), but the inequality is not great. 1875, Oct. 29. (575) > (573), 651 =(654), (641) not seen. 1875, Nov. 5. 647 > (575) - (671). 1875, Nov. ii. (676) (654), (654) > (641), but not much. 1875, Nov. ii. (654) > 612 or 618. LASSELL'S b not seen. 1875, Nov. Ir - (654) =622 > $3 K Query 62 1? 1875, Nov. 1 1. 686 and 688 not seen, although not particularly looked for. 1875, Nov. ii. 675 not seen. 1875, Nov. 24. 647 > (575) > (671) or (676) > 589 > (567). 1876, Jan. 3. 647 ;> (575) > (671) > 622 qr 625, 676? 647! See observations. 1876, Jaii. 4. (671) or (676) > 575 almost=:67i > 622? [or 625] > 589 > 567. 1876, Jan. 4. 612 = 618 = (676), 581 and R. 56 both seen; also 636, not (654). 1876, Jan. 4. 709 rather fainter than usual. LASSELL'S b not seen. 1876, Mar. 6. (575) > 589 > (567), 622. See observations. 1876, April i. 709 is a little harder to see than the 5th star of the trapezium. [Query, 6th star ? it is probably the 6th star.] 1876, April T. 709 = 671 or 647 about. It requires attention to see it, 1876, Nov. 5. 675 visible. (671) > 676; 636 visible but not (654). 1876, Dec. 5. 647X671), (575). 1876, Dec. 5. 68 1 > (676) > (651) ; not much difference in these. 1876, Dec. 5. 709 = 663 about. 1876, Dec. 31. 709 <657, 657 = 652 about. 1876, Dec. 31. (671) > 676, (575) =(589). 1876, Dec. 31. 647 and 671 > (575) and (589) although 647, 671, and (575) are not very unequal. 686 seen well. 1877, Jan. 2. 558 > 709 > 524, (575) = 671 =647 nearly. 1877, Jan. 2. 654 and 675 have not been seen since Nov. 5. 1877, Jan. 10. (575) > (641) [just seen ] > (5 6 7) [barely visible]. 1877, Jan. 10. 567 = i6th mag., ARGELANDER. 1877, Jan. 24. (642) = (654) each is just visible. l82 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 1877, Jan. 24. 709 > (641) > (676) > (567), 671 =622? = (575) nearly. 1877, Jan. 24. 671 > (589), but not much. (675) not visible; 581 > 573. 1877, Feb. 3. (654) and (675) just barely visible. 1877, Feb. 6. 651 very faint. 675 seen. 1877, Dec. 7. (602) riot seen, 589 > 567. 1877, Dec. 14. (602) not seen. 1878, Jan. 3, Jan. 5, Jan. 6. See observations with aperture of 3.50 inches. 1878, Jan. 5. 6.75 seen following frons ; L A SSELL'S b suspected; 6i2>6i8; 602 seen ? ; 654 not seen ; 567 16.3 mag. ; 686 and 688 not visible. It not seen. 1878, Jan. 16. io h ; 654 seen!; 675 just visible following the frons ; 567 > 709; 567 just visible. 654 not visible at n h . 1878, Jan. 24. 709 not brighter than 666. 771 > 676. 1878, Jan. 28. 8 h 54 m ; 654 just visible 618. 676 not seen. 1878, Jan. 28. Position of 654, ^a = + 8".4 (3), 4d = + 3 i".7 (4) 1878, Jan. 28. 709. See observations. 1878, Feb. 4. 675 seen about on edge of. frons with 175. LASSELL'S b seen! 1878, Feb. 4. 654 and 602 not seen; new star near 709? a not well seen. 1878, Mar. 9. 675, b ; a seen, Professor HILL. (See observations.) a > 647. 1878, Dec. 6. LASSELL'S b suspected at 4 b 25 sid. t. ; not visible at 5 h . 1878, Dec. 6. 654 and 675 not to be seen. 1879, Jan. 10. No star inside the trapezium (HALL and HOLDEN) ; LASSELL'S b seen (HALL) ; also a star in _p 45, s 48" from 708 (HALL). 1879, Mar. i. 654 seen (8 h 40) ; b seen; h visible; 651 visible. 1879, Dec. 26. 612 and 618 seen; h seen like a nebulous mass and not like a star ; a clear dark space between h and the following side of D. 1880, Jan. 3, io h ; 675 seen just outside of pons Scliroeteri ; b seen; 612 > 618 ; 6i8> 567; 567 = 636 very faint; 647 in a dark space; k visible, nebulous, and not stellar. io h 5, 654 just barely visible. ' h 78 = 654. 1874, Jan. 1 6. 7 h ~7 h 2o m ; seen, but not after 7 b 30. 1874, Jan. 17. 9 h ; not seen; (a) seen well; 636 seen faint. 1874, Jan. 23. io h ; suspected, but too cloudy to verify. 1874, Jan. 24. 9 h 30; (a) seen; 654 not mentioned?! 1874,' Jan. 25. Not seen; (a) seen. 1874, Feb. 5. 8 h ; 675 seen; 654 not mentioned"?? 1874, Feb. 14. Seen; (602) also seen. 1875, Oct. 27. I2 h -i3 h ; not seen. 1875, Oct. 29. I2 b -i3 h ; (654) = 651 nearly. 1875, Nov. 5. n b .5-i2 h .5 ; not seen, although no mention is made. 1875, Nov. 10. i i h .7~i3 h .3 ; not seen although no mention is made. 1875, Nov. ii. n h .7-i2 h .5 ; (654) > 612 or 618, (654) 622 > 621. 1875, Nov. 17. I2 h ; not seen, although no mention is made. 1875, Nov. 24. io h .2-n h .5 ; not seen, although no mention is made. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 183 1876, Jan. 3. i i h .7 ; not seen, although no mention is made. 1876, Jan. 4. io h .3 ; not seen; 612, 618, and 636 seen. 1876, Jan. 10. i i h .5-i2 h ; it is doubtful whether 654 was looked for. 1876, Jan. 30. io h ; not seen although no mention is made. 1876, Feb. 30. 7 h .5-8 h .5 ; it is doubtful whether 654 was looked for. 1876, Mar. 6. 8 h ~9 b ; not seen, although no mention is made. 1876, Mar. 14. Not seen, although no mention is made. 1876, Mar. 22. 7 h .7-8 h . It is doubtful if 654 was looked for. 1876, Nov. 5. 654 not visible, although no mention is made. 1876, Dec. 5. 654 not visible, although no mention is made. 1876, Dec. 31. 654 not visible, although no mention is made. 1877, J an - 2 - 654 no * visible, although no mention is made. ^1877, Jan. 10. 654 not visible, although no mention is made. 1877, Jan. 24. 654 642. 1877, Feb. 3. 654 just visible. 1877, Dec. 7- 654 not visible, although no mention is made. 1877, Dec. 14. 654 not visible, although no mention is made. 1878, Jan. 5. 654 not visible. 1878, Jan. 1 6. io h ; 654 seen? not seen at n h . 1878, Jan. 24. 654 not visible, although no mention is made. 1878, Jan. 28. 8 h 54 m ; 6 54 -6i8. Ja + 8". 4 (3) Jd = + 31".; (4) 1878, Feb. 4. 654 not seen. 1878, Dec. 6. 654 not seen. 1879, Jan. 10. 654 not seen, although no mention is made. 1879, Mar. i. 8 h 40; 654 seen. 1879, Dec. 26. 8 h 40 m ; 654 seen. 1880, Jan. 3. io h 5 m ; 654 just barely visible. With regard to the small nebulous mass k (at first supposed to be a star, and so marked with a ? on my observing copy of the Index-Chart) I have to say that I am absolutely certain it did not exist in its present form from 1874, January, till 1878, January. Since this time it has been constantly seen, and is growing brighter.* Also, it may be recorded here that the nucleus to pons Sckroeteri, which others have called stellar, and which seemed so to me during the first of the work, is not so at present (1879- 1 880). COOPER'S star c', near 516, I have never seen. LASSELL'S double star 6, near 685, certainly exists. It is very faint. have never seen his star g (near 676), and I doubt its existence. These stars are laid down in his Plate III (and p. 56) of Mem. R. A. S., xxiii. The others I identify as follows: a = 675; k = 671 and 676; / = 1 = 612; fc = 6i8; c = 62i; ^ = 625; e = 595 ; /= 6o8 - With regard to 675, it seems to me that this extremely faint star is not proved l & inch CLAHK telescope of the Wa8hburn Observa - tory, although I have repeatedly looked for it under the best conditions. 1 84 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. be variable. Atmospheric changes will account for all the variations I have observed. It is very close to the edge of the regio Huygheniana, perhaps even within it. 709 is certainly variable between the magnitudes of about 1 1.5 and 13.0 on Gr. P. BOND'S scale. The few new stars I have myself added to those laid down by BOND, I regard as of no special importance. The list could be slightly extended if it were desirable, but not more than five or six stars above 16.0, ARGELANDER, exist in the limits of the Index-Chart which were not laid down by BOND. The minimum 'visibile of BOND'S telescope is 15.1 magnitude (ARGELANDER'S scale extended), and for the Washington refractor it is i6 m .3. It seems to me that the fact that so few additional stars have been seen is of great importance in throwing light on the question of the depth of the stellar universe in this direction. It will be interesting to know if other large telescopes (as Mr. COMMON'S new reflector) can detect any stars fainter than 675, my i, 2, 3 (near 663) or LASSELL'S b. Lord ROSSE has, I believe, but one star (R. 56) on the Index-Chart, not laid down by BOND. SYNOPSIS OF THE MORE IMPORTANT MEASURES. From the preceding synopsis I collect the most important measures for purposes of comparison: Frons. Date, P No. of obs. Remarks. 1875, Oct. 29 48.3 5 Length of Frons ; 1878, January 16 ; s = 193". 5 (3). Nov. ii 52.1 4 r876, Nov. 22 49-3 4 . 1878, Jan. 16 51.3 4 Adopted /= 50.3 (17) ' Occiput. Date. P No. of obs. Remarks. 1875, Oct. 29 142.1 5 Perpendicular distance of Occiput from 1 = 105". 5 (3) Nov. ii 139-3 4 1877, Jan. 5. 1876, Mar. 6 142.7 4 (Nov. 22 1877,5 T 1878. Jan. 28 corner near [A,], Ad = 77".4 (i) C Adopted Ad = 79 .3 (3) 1878, Mar. 5 corner near [Ai], Ad = 81 .2(2)) 1878, Dec. 6 corner near [Ai], A = 62''. 8 (4) L (north end]. Date. Ad No. Remarks. 1876, Feb. ? 1877, Dec. 12 25.6 - 25.4 , 4 On same parallel as 671. Near parallel of 617 and 622. (See observations.) Adopted Ad = - 25.5 (5) (See observations.) E. Date. South point. A<5 No. North point. A '5 No. Apex. Aa No. 1875, Nov. 10 - 147-6 3 101.7 5 " Nov. 17 . . . 0.0 1876, Feb. ? 144.0 *j IO2.6 .1 . Mar. 14 . 5-3* i 1877, Feb. 6 . . + 2", + 3" (est.) . Dec. 7 - 143-5 4 . . Dec. 7 - I3i-4t I . Dec. \2\ - 155- 6 2 . . - . 1878, Jan. 28 - 147-2 3 . . . 1879, Jan. 20 - 96.7 2 Adopted Ad = - 144.6 (16) 100.3 (8) From + o" to +i" (est.) * Poor observations. fThis is the extreme point towards the north which could be taken as the vertex. S This is the extreme limit of E towards the south. The mean of these is about the true Ad. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. Dark channel between I and (X and G). 1876, Mar. 6. ^ = 25. 6 (3) Dark channel between I and E. 187 Date. 1876, Dec. 5 42.6 Dec. 31 [50] 1878, Jan. 16 46.0 Adopted /=44.o No. 4 est. i (5) Remarks. Uncertain. Dark channel between E and F Date. No. Remarks. i875,Nov. 24 / = I39.6 2 It passes through the star 580^ 1875, Nov. 24. 1878, Jan. 16 . ^ = 142.1 3 Adopted / = 140.9 (5)- Dark channel between F and (G and H). Its prolongation is in the line 685-708, from many measures. V. Extreme north point in same J& as 612; i. e., Jd -\- 24". 6; J6 south edge of n. e of M, JS 22". 7 (3) ; good. F. Date. Brightest part. North point. South point. Middle point. A No. A<5 No. A'5 No. A 1 * No. A ( No. 122.3 i ... . . . + 28.8 i 1875, Nov.^io 1876, Feb. ? Mar. 14 IOI.O . 3 1877, Feb. 7 Dec. 12 102.1 4 - 94-9 2 i - II7-3* 2 1878, Jan. 28 Feb. 20' + 26.8 2 - 102.9 4 120.6 I . Mar. 5 1879, Jan. 20 Adopted + 26.8 IOI.2 102.8 2 . . 4 (I?) - 94-9 . [ 113.5]! . ... - 120. I (4) + 28.8 (I) (2) - 102.0 (2) * In Frons. fThis is the s.p, corner ; see observations. I 88 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 1876, Mar. 14. Tangent to preceding sides of F and G, z/# -f i"-2. 1877, Feb. 6. Point where s. p. edge intersects frons, J6 = 24". 6 (3) 1878, Feb. 20. The bright part of F first begins to be fainter at Jd 1 14." 9. 1879, Jan. 20. The north point of X (F) is further north than JS 97". G. Date. Brightest part. Middle point. Remarks. A a No. Ad No. 2 Aa No. Ad No. 1875, Nov. 10 . . . . -64.5 " " Tangent to following side of G, 1876, March 14; A" = + 28".8. 1876, Feb. ? . . . -69.6 I 69.6 I Tangent to preceding side of G, 1876, Mar.. 14; Ao=-t-i".2; 1877, Feb. 6, about o" . Mar. 14 . + 14-2 Dec. 5 . -65.5 5 1877, Feb. 3 . -M3-9: 3 Feb. 7 66.2 4 South point; A] 4- 18.5 - 63.3 4- 15-9 - 65.3 2.6 2.O Brightest point. [F] 4-26.5 4- 69.6 4- 29.4 4- 2.9 . . [o] 4- 28.6 - 97-4 4- 26.8 IO2.O 1.8 4.6 Brightest part. l>o] + 71-9 + 42.3 + 77-4 4- 40.8 4- 5-5 1-5 [K] -t- 143.6 4- 71.5 : 4- 164.8 4- 78.6 4- 21.2 4- 7-1 [B] 4- 146.5 22. O 4- 150.2 28.2 4- 3.7 6.2 NOTE. The letters in [ ] give LIAPONOFF'S nomenclature. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 191 REDUCTION OF PHOTOMETRIC OBSERVATIONS MADE AT WASHINGTON. i. Results from eye observations of order of brightness. In making the eye estimates of the order of brightness of the various bright masses of the Huyyhenian region, it was the object simply to give the order, without attempting to assign how much brighter one was than another. Again, as these masses have a sensible magnitude, it was impossible to estimate the effect of the mass as a whole, and to compare it with the total effect of another mass, perhaps three minutes of arc distant. Hence, these estimates must be understood to refer to the brightest parts only of each mass, and they give the facts as faithfully as was possible. "F is brighter than H," for example, means that after carefully examining these two masses I believed that there was a considerable portion of the surface of F which was brighter than any considerable portion of the surface of H. Again, 1876, January 4, I esti- mated the order of brightness as follows : . i. A. 2. D. 3. G, H, N. 4. F. 5. I, Q. 6. E. The numbers i 6 simply indicate the order, and it must not be supposed that the ratio of A to D is the same as that of D to G, or that the steps are equal. The difficulty in obtaining the relative brightness of the various masses from the observed sequences follows chiefly from the want of a numerical ratio between the different steps, and also from the fact that the possibility remains that different portions of a certain mass are taken at different times, as representing the mass. These two objections are inseparable from the method, and perfectly definite results can only be obtained from some form of photometer. If a drawing could be made each night which would represent the various portions of the nebula as they appeared on that night, and if such drawings could be continued without bias or prejudice from night to night, then it is possible that increased accuracy might result, as something like a numerical ratio of the brilliancy of the various masses might be obtained. But this process is impracticable, first, because the nebula is too large to sketch carefully in one night; and secondly, because after becoming familiar with its parts it is extremely difficult to avoid (in drawings) inserting the results of past observations instead of present appearances. The published drawings of various astronomers, in so far as they relate to the order of brightness of the various masses, are, in general, but imperfect evidence. The final drawing, which has probably required weeks to make, may be said in general to give the average forms of masses and parts which are well seen, with accuracy, and the forms of the fainter masses will also approach to correctness. With the bright of these masses it is different; the completed drawing gives the average Appearance to the observer so far as his artistic skill enables him to render it, together with a number of imperfect or even false representations due to the imperfections ,,f the telescope, etc., the state of the atmosphere, the bias or prejudice of the obsen -.-, (which mostly is due to his good remembrance of previous views of the object in question), and above all to the shortcomings of the engraver. In Part III, I have more particularly considered the value of the evidence which may be had from drawings, and I have suggested the use of a terrestrial eye-piece M a check upon this kind of work. 192 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. In the sequences of brightness here given, I endeavored to avoid, as far as pos- sible, all bias or prejudice by not collecting and examining the results of observations until some two years had elapsed since beginning the work. The examination of the sequences that follow raised a strong suspicion of varia- bility among some of the bright masses of the Huyghenian region, and to get further light upon this the photometric observations of 2 were made. The eye observations, however, are independent and may be considered by themselves. Table shouting order of brightness of the various masses (by eye estimates). "o Date. Decimal a year. I 2 3 4 5 678 ' 9 Remarks. 1875. Oct. 27 .82 A D F = G I N=Q H=E See observations. 29 S3 A D Fand G I N and O E 29 83 A F, G, D I H N Q = E Perhaps better than last preceding. Nov. 5 .85 A D FandG H and I N, Q E Only brightest parts of N and O included. 10 10 .86 .86 A D A D F,G G, F I, H N,Q E I, H N, Q E t . . G = F. 17 .88 A D G F I = H N Q E 24 .90 A D G=H F N I E&J 1876. Jan. 3 .01 A D G,H N,O, F, I E 4 .01 A D G, H.N F I,Q E . I = Q nearly. 10 03 A D G, H,N,Q F I, E .... Poor obs'n ; moonlight. 30 .08 . . .... E is the faintest mass of the principal ones. Feb. ? .09 A D G, H. F N 1,0 E .. 5- I. Q or Q, I. Mar. 6 .18 [A D F, G, H N I O, L] " Not verv satisfactory" ; rejected. 6 .18 A D F G,I H, E N,Q This is better. 22 .22 A D F G, I, H N.Q.E . . Approximately. Nov. ii .86 . H.G.F* I E *About equal. Dec. 13 95 A D G. H, F N, I. E . . . . " Mr. H. S. Pritchstt thinks D > A a little." 19 97 D = R A I H,F B . Mr. Pritchett. 19 97 A D F Q N G H I E B=F=W; y = o; y = a near K. 1877- Jan. 2 .00 A D G, F H,I,N E [M a //] . . J < E, J XT 1878. Ian. 28 .08 G > F. For observations of relative brightness by means of tourmaline plates see obser- vations of 1877, J an - 2 4 J an - 3 an d Feb. 3. 1878, Jan. 23. F, G, H, and two surfaces in P ? and Q are brighter than I or K. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 193 1878, Dec. ii. F G - H ; perhaps F is a little brighter than G or H, but this is doubtful. 1879, Dec. 26. i. A, I). 2. G, H. F, I, Q, E, G, and H notably brighter than F. Discussion of the Results from the preceding Table. It will be convenient to consider the masses in pairs, and the following nomen- clature is used. G =: F means that these masses were considered equal. G > F that one was decidedly brighter than the other, and that they are in different compart- ments of the table. G 1 = F means that they are in the same compartment of the table, but that G is written first and was brighter, though not greatly so. This nomencla- ture was used during the observations themselves. A and D. From my own observations from 1875 to 1877, January, the result is that A > D. By this is meant that the bright strip on the following edge of A is brighter than any considerable part of D. It will be noticed that Mr. PRITCHETT makes D > A. There is little doubt but that this is erroneous. F and G. \ The following are the separate inequalities, the last line showing the means: Date : Ratio. Date : Ratio. Date : Ratio. Date : Ratio. Date : Ratio. Date : Ratio. Date : Ratio. i875.82,F=G 7 5 .8 3) F=G 7 5 .8 5 ,F=G 1875.86, F=G 1875.88, G>F 75.90, G>F 76.01, G>F 76.01, G>F i876.03,G>F 1876.09, G! = F *i876.i8, F'=G *Unsatisfactory. 1876.18, F>G 1876.22, F>G 1876.86, G' = F 1876.95. G' = F 1876.97, F > G 1877.00,0' = F 1878.08,6 > F 1875.84, F=G i875.g6,G>F 1876.09. G' = F i876.20,F>G i876.9i.G' = F 1876.97, F> G 1877.00, G l = F 1878.08,6 > F Following the last line of this table, it appears that 1875.84 F and G were equal; G was brighter than F a little later, then passed through equality with F to G < F. A similar consistent fluctuation is shown from 1876.91 to 1878.08. It should be noted that in the above table there are no cases like this : F > G, F = G, and then F > G ; and, again, there are no cases like F > G, and then G > F without passing through F G. These results, taken alone, appear to indicate fluc- tuations in the relative brilliancy of F and G. Whether these are irregular or periodic is not to be decided by observations of this nature, but it will require a careful and long-continued series of photometric experiments conducted with every possible pre- caution against systematic errors and against preconceived opinions and bias. 25 I 9 4 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. I and H. 1875.82,! > H 1875.83,! > H 1875.83,! > H 1 875. 85, H =1 1875.86.11 =H 1875.88,! = H 1875.86, H = 1875.90, H > I 1876.01, H > I 1876.01, H > I 1876.03, H > I 1876.09, H > I 1876.18, I > H 1876.01, H > I 1876.18, I > H 1876.22, I 1 = H 1876.86, H > I I 1876.97, I>H 1877.00, H'= I 1876.95, H > I 1876.22, H = H 1876.91, H > I 1876.97, H > I 18.6.97, H = I? 1877.00, H'= I This table is not so clear as the preceding one. The comparison was more diffi- cult. It seems to me to indicate changes, however. N and Q. i8 7 5.82,N = Q 1875.83. N Q 1875.83, N > Q 1875.85, N' = Q 1875.86, N 1 O 1875.88, N > Q 1875 90 N > Q 1876.01, N' = Q 1875.86, N' = Q i8 7 5.8 3> N = Q 1875.83, N > Q 1875.86, N! = Q 1875.89, N> Q 1876.01, N! = Q 1876.01, N > Q 1876.03, N 1 = Q 1876.09, N > Q i876.i8.N = Q 1876.22, N' = Q 1876.97, Q > N 1876. 01, N> Q 1876.03, N' = Q 1876.09, N > Q 1876.20, N 1 = Q 1876.97, Q > N As the difference between Q and N in any event is but small, the above com- parisons, taken alone, indicate that N Q, and that the variations of the above table are accidental. I and Q. Date: Ratio. Date : Ratio, Date: Ratio. Date : Ratio. 1875.82, I > Q 1875.90, I < Q 1876.18, I > Q 1876.97, I < Q 1875.83, I > Q 1876.01, I =Q 1876.22, 1 > Q 1875.83, I > Q 1876.03, I O 1876.09, I = Q 1875.86, I > Q 1875.86, I > Q 1875.88, I > Q 1875.85,! > Q 1876.0 I?=Q? 1876.20, I > Q 1876.97, I < Q It is possible, almost probable, that variations in brightness are here indicated, taking these observations alone. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. I and N. 195 Date: Ratio. Date: Ratio. Date: Ratio. Date: Ratio. Date: Ratio. 1875.82, I > N 1875.83, I > N 1875.83, I > N 1874.90, I < N 1876.01, I = N 1876.01, I < N 1876.18, I > N 1876.95, I < N 1876.22, I > N 1876.97, I < N 1877 oo I N 1875.85, I > N 1875.86, I > N 1875.86, I > N 1875.88, I > N 1876.03, I < N 1876.09, I < N 1875.85, I > N 1876.01, I < N 1876.20, I > N 1876.96, I < N 1877.00. I = N? Taking the tables N and Q, I and Q, and I and N together, it would seem pretty certainly that Q N, and that variations in I place it sometimes brighter and sometimes fainter than Q and N. These may vary also, but if they do they vary together. E. E is pretty uniformly the faintest of the principal masses (A, D, E, F, Gr, H, I, N, and Q). It is to be noted that the position thus assigned to E in order of brightness is different from that of the older drawings, 1837-67. My observations since 1878.0 indicate that E was brighter than before this date. E and J. 1875.90. W J -, 1877.00, J < E. 1876.97- Y ff, and Y o , . Y zz a. 1877.00. E > J > ff, and M 1 zz ff. 1877.00. ff 1 zz y". Probably the foregoing observations are not in themselves sufficiently exact to merit any better reduction. Without giving them any great weight they still seem to show that several of the masses of the central region are subject to variations of bright- ness. The periods of these and the amount of change cannot be deduced, add to the above recorded observations my strong impression that in the years 1878-1879 the mass D was fainter relatively than formerly, and the mass 2. OBSERVATIONS WITH HASTINGS'S PHOTOMETER. In view of the foregoing results, I requested Dr. CHARLES S. HASTINGS, of tin- Johns Hopkins University, to devise for me some form of photometer for mea the suspected changes, which he kindly did. The conditions were somewhat roi as the filar micrometer could not be removed to insert a photometer without angr the zero of position, which was undesirable, and therefore some form had which could be inserted beyond the micrometer box. 196 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. The form finally chosen was as follows (see Fig. 39, which is drawn to halt the natural size): FIG. 39. Dr. C. H. HASTINGS'S nebula photometer. A terrestrial eye-piece I, II, III, IV was screwed in the micrometer box in the same thread as the ordinary micrometer eye -piece, which was removed. The principal focus of the objective is at the 4. in the figure. A and B are diaphragms. C D is the side of the eye-piece tube. In the focus of IV was placed a silver mirror, m, which was illuminated by a movable lamp, m was semi-circular in shape, and about 15" on the straight edge (diameter). A screen was fixed in position outside the tube C D. This screen was covered with a bit of paper cut from BOND'S engraving of the nebula (see frontispiece). It was illuminated by the. (movable) lamp. In looking into the photometer a semi-circle (m) of precisely the same color as the nebula, was seen illumi- nated by the lamp. This semi-circle was projected upon the nebula, and to measure the brightness of any part, as E, for example, the m was placed next to the brightest part of .E, and the brightness of m changed by moving the lamp until it matched E or until the line of separation vanished. This was done sometimes by increasing, sometimes by diminishing the brightness of m. The position of the lamp relative to the screen was read off on the graduated rod that bore it. These are the essential features of the apparatus. The observations were never easy. The chief difficulties with this apparatus were first, that the illumi- nation from the small lamp was never constant, and I had continually to return to the mass D, which I used as a zero mass; and second, there was a difficulty in being sure of the exact -portion of the surface of D used as a zero. That is, there was a possibility of using different parts of D for zeros at different hour-angles. Both these difficulties were purely mechanical and could easily be overcome, but it was impracticable to do this under the actual circumstances. The results are somewhat less consistent on these accounts, but after all, systematic errors are more to be feared here than comparatively small accidental ones. If d is the reading of the graduated rod for the zero mass D, and if e is the read- ing for any other mass, as E, then i i d 2 brightness of E; brightness of D zr -* -: , 9 or E =: D. e 2 d 2 e 2 ' MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. '97 Following I give the reducti6ns of all the photometer measures made. The reductions previously given in the observations of each night were prelimi- nary, though only slightly different from the ones here set down. Collection of single results of photometer observations. Mass. Date. A. F. G. [G]. H. I. E. Q. f- 1878. Jan, 7] I.42*/ T 1C*f \ 12 26 26 Feb. 4 1 O5 > I. 12 1.03 i. 08 1. 00 1.09 - o.gof 0.84 0.77 [0.55] see obs. 0-43 0.39 5 1.71 1.05 1. 14 . . 0.75 0.83 0.83 . 5 1. 12 I. 21 . 0.79 0.88 0.88 . 5 0.94 0.74 [D]f 26 . . O.gi 1.26 i. 02 I.I? 0.99 0.92 1.30 . 26 . . 0.82 i. A 0.92 i. 06 0.89 0.83 I.I? . 26 . . . 4 . . . . 26 (By eye) F>G G>H . I > E . . . 28 (By eye) A> D F > G G = H; I>E . . . . . . 28 . 1.23 . I.OI 0.91 0.83 i.iS . Mar. 9 [I.I5] 1.04 [o.82] . 1.05 0.78 . . 9 1.90 0.90 [0.71] . . 0.91 0.67 . . 9 . . [0.84] part. . . . . . . 0.99 0.91 . . 9 [0.73] part. 0.86 0.79 1879. Feb. 23 > 0.98 Mar. 7 1880. Jan. 3 2.25 :: see obs. 1.27 Vj [I = o. 91 E] [I = o.82 E] 0.81 I 3 . . 1.18 . . 1.07 0.88 1.07 10 . I. 10 1.23 . . . . o.73 10 . i. or 1.07 . 0.62 10 . 0.97 . . . . . 13 14 14 1.52 1.23 1.42 0.90 0.93 i.ig i. 06 0.87 i. 08 16 . 1.28 I-3I 1.04 1. 12 16 ' 1.14 1.04 * Preliminary observations. fThe brightest part of D compared with the standard part of D. \ This is not the same part of A as Jan. 7. [G] is a part of G defined in obs. Feb. 26. q. v.; D] is the brightest par t of D, not the part used as the standard of comparison. Not the same part as Feb. 26. 198 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. In the preceding table I have given the separate results of each set on each mass. The numbers in the table are those by which the brightness of the standard part of the mass D must be multiplied to give the brightness of the mass measured. The sources of error were two: First, all masses were compared with a part of D, and it was intended to compare them with the same part of D; this may not have always been done. Second, the lamp did not maintain a constant brilliancy, and in spite of precau- tions taken to eliminate the bad effects of this change, errors still remain due to this cause. Taking the difference between the greatest and least readings on each mass for each night when more than one determination was made, we have the following table. The range here given is in per cent, of relative brightness. Mean. On the mass A ; range = -.07, 10 - - - - 0.09 F; .09, .07, .09, .15, .11, .13 - - o.n G; .08, .27, .12, .14, .16 - 0.15 [G] ; .10, .11 - - - - - - - o.i i - - o. 1 1 - - o. i o .08 - 0.13 - - 0.05 - - 0.04 - - 0.13 Mean - - o. 10 The probable accidental error is thus much below 10 per cent. It should also be noticed that for every night where there is more than one deter- mination for the same mass, and where the second of such ratios is smaller than the first, it is the same for every other mass on that night. This shows that much of the error is in assuming the zero for the lamp, but that the relative brightness of two masses F and G, for example, is still to be depended on. Having regard to what goes before, I conclude that the mass A is certainly brighter than D, though, probably, not very much brighter. F and G. Collecting the results of these measures, we see that 1878, Jan. 26. F > G 1878, Feb. 5. G > F (probably). 1878, Feb. 26. G> F (this differs from the eye observation of this date). 1878, Mar. 9. F >G 1880, Jan. 10. G> F 1880, Jan. 13. F >G 1880, Jan. 14. G>F 1880, Jan. 1 6. G> F It seems to me clear, after making allowance for all sources of error, that F and G change in relative brightness, as was suggested by eye observations alone. H; .1 I - - - - - - - I; .04, . 10, .19, . 09, .10 - - E; .07, 1 4, .09, . 24, .11, .21, Q; 05 - - - - - - - f, .04 - - - - - - - [D]; -is MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 199 G and H. 1878, Feb. 26. G > H (this agrees with eye estimate). 1878, Feb. 28. G H (by eye estimate). 1880, Jan. 3. G> H 1880, Jan. 14. G> H From these I conclude G to be a brighter mass than H. H, I, and E. 1878, Feb. 5. I I, H > E, I > E (also by eye). 1878, Feb. 28. H > I, H > E, I > E. 1880, Jan. 3. H > I, H =z E, I< E. 1880, Jan. 14. HE, I > E. The observation of Jan. 3 gives H > I and H E, therefore E > I, and in fact it was so observed. On other occasions I was certainly brighter than E, whence it is concluded that I and E vary in relative brightness, as do also H and I. E. \ In general the photometer results corroborate my strong impression previously and independently recorded, that E became brighter relative to D in the last years of observation. D has also grown fainter according to my eye observations, and the photometric observations I have made agree with this conclusion, since the brightness of all the masses relative to D is greater in 1880 than in 1878. The conclusions which I am disposed to draw from the above observations are these: A has been throughout my observations the brightest mass of the Huyglienian region. E has grown brighter from 1874 to 1880, while D has grown fainter in the same time. The masses F, G, H, and I change in absolute brightness. Although at first sight it might seem that such observations as I have made should have yielded definite numer ical results, yet, I think, that a consideration of the difficulties and uncertainties will excuse me from attempting to draw any more exact conclusions from this part of the work. I conceive that I have demonstrated the existence of certain changes of bright- ness, and I am disposed to leave, for the present, the question as to the exact amount of these, open. It will be noticed that the results independently obtained from the eye observa- tions of 1875-1878 and from the photometric observations of 1878-1880 agree. 2OO MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. PART III. SUMMARY OF ALL THE OBSERVATIONS (1656-1880) WHICH HAVE BEEN PRE- VIOUSLY GIVEN IN DETAIL. In what follows I have collected all the observations which have been given in detail in the preceding- pages, and have separated them into divisions. Each division treats of the observations which have been made on some separate mass or channel or point, and the various observations are arranged in chronological order under each head. In this way it will be easy to see if there are evidences of progressive and regular change in any of the masses considered. Irregular changes can hardly be detected in this (or any other) way, since the observations and drawings considered are the work of so many different observers so variously equipped and in so many different stations. I desire to call attention to the way in which the following summary has been made. The evidence presented is derived from the recorded observations and from the drawings of all the observers since 1826. Blank forms were prepared, one for each mass, as A, E, etc., and on the edge the names of the various observers, witli the dates of observation, were written. As the various memoirs came into my hands I read them carefully and entered any important remarks under their appropriate headings. In this way these sheets were filled up in an irregular order (not chronologically), and my judgment was left entirely unbiassed. The drawings were treated in the same way. The original engravings, and in many cases the original pencil drawings, or photographs of them were consulted, and all the evidence I obtained from them was entered in its appro- priate place. After the sheets were full or nearly so (in 1880) each sheet was con- sidered by itself and my final conclusion reached. This conclusion can be tested by any one at a glance. The testing of the summary itself will require a reference to the original paper, which is not always easy. To facilitate this I have in many cases added here a reference to the page from which the statement was taken. I trust I .have made it plain that not only was the summary made without any preconceived notions of what was to be expected, but that such ideas, even if present, could not have been recorded in these tables, owing to the way in which they were constructed. The final discussion of these sheets was put off till the last moment, when all the evidence was in. With regard to the nature of the drawings themselves it may not be out of place to say a few words. I am acquainted with but one drawing of the nebula which is entirely above criticism, that of the late Gr. P. BOND. (See frontis- piece.) He was himself a skilled artist, and he had been familiar with the nebula for fifteen or twenty years. He made scores of drawings in white and black and the reverse, in colors, etc. Iach of these was revised and re-revised many times. The final drawing in water-color was copied by Mr. WATTS, a skillful engraver, who him- self was extremely familiar with the nebula from repeated views and studies of it through the Harvard refractor. The revisions of the original plate lasted many months, and I have myself exam- ined from fifteen to twenty final revises of the plate. Color, form, and relative brill- iancy were all successively and exhaustively criticised, and Professor BOND expressed himself as fully satisfied with the plate in every essential feature. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 2 OI Add to this, that with the exception of a few points, elsewhere considered, this engraving has been constantly satisfactory to me in my very frequent comparisons of it with the nebula even under the severest criticism which I could apply, and it is safe to say that this nebula as it appears in a refractor is satisfactorily represented. The drawing (white on black) of Lord ROS.SE appears to deserve almost equally high praise, but as I have never seen the nebula through a reflector, and as the pictorial effect of the nebula, as seen in Washington, is somewhat different from that presented by Lord ROSSE'S large engraving, I cannot speak with proper certainty upon this point. As a map of the nebula the drawing of Lord ROSSE (black on white) is, as has often been said, almost perfect, but I presume that it was not intended in any way as a pictorial representation, but rather as a scheme for representing by conventional signs (as in topographical color-drawing) the features examined. As pictorial repre- sentations the original pastel drawings of M. TROUVELOT, with the Harvard College refractor and with that of the Naval Observatory, are extremely fine, but, as in these cases, sufficient time was not available for the study of the various objects they are lacking in minute accuracy of detail. Most other drawings of the Orion nebula, except those mentioned, fall into the class of maps, which give by conventional signs the features examined. The nature of these signs is not perfectly fixed, and has in each case to be determined from the drawing itself on the supposition that some features, at least, are now as they were at the^time of the drawing. . Their examination is rendered more difficult by the shortcomings of the engraver or of the process of delineation adopted, and this is why the examination of so many original drawings has been made. I have expressed elsewhere my thanks to the numerous gentlemen who have aided me in obtaining access to the various originals, and particularly to Miss CAROLINE LASSELL, who has made for my use a fine fac simile drawing of the (unpublished) Malta drawing of 1864. There is only one way, of which I am aware, for avoiding undue bias in the drawing of nebulae, and that is in making one drawing in the ordinary way with an inverting eye-piece, and others through reflecting prisms, or erecting eye-pieces which will so alter the usual appearances, without changing the real brilliancy and shape, that an unbiased judgment can be made by the artist; I should rather say, from trials by myself, so that it is scarcely possible for him to have a hurtful bias. This method I confidently commend to those possessed of more artistic skill than myself. In my own case 1 am certain from repeated trials that a definite amount of time can be more advantageously expended in measures and verbal descriptions than in sketches, and this is why I have not attempted to add another drawing to the many excellent ones already available, but have confined myself to the outline Index-Map. Even this is by no means correct even as to form. The results of my work must be looked for in the preceding and following summaries of Washington observations. LAMONT (Ucler die Nebelflecken, Munich, 1837, p. 22) gives an account of obvious reasons for the different appearances of the same nebula in various telescopes, which is at the same time so brief and so clear that I quote it in full : ' eine Flache vor, die aus kleinen Abtheilungen von verschiedenen Lu-htstarke 1 durch serimre Zwischenriiume von einander o-etrennt, so werden die schwacheren Abtheilungen in kleineren Fernrohren unsichtbar bleiben: die Starkeren a APP. V 20 202 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. sie nalie an einander liegen, durch Verschwinden der Zwischenniume in erne gleich- formig erleuchtete Fliiche iibergehen. Diess 1st nun gerade der Erfolg, den die Beo- bachtung an den Nebeln nacliweist. Ein schwaches Fernrohr zeigt den mittlereii Theil des Orion-Nebels gleichformig erlenchtet, ein stiirkeres giebt ihm ein flockenartiges Ansehen, und der hiesige Refractor lost ihm in einzelne messbare Abtheilungen auf, wie in Fig. XI (our Fig. 20) zeigt. Eben so muss ein Nebel, der gegen die Grenze immer schwaeher wird bis er sich ganzlich verliert, in einem grossen Fernrohre viel ausgedelmter erscheinen, als in einem kleineren Fernrohre, Avelches dieselbe vergros- serung hat." The evidence to be obtained from all the drawings of the nebula is presented and discussed in the tables that follow. If two or more drawings are made cotempora- neously, the points in which they agree may be taken to be correct. Points of differ- ence must be examined in the light of the conditions under which the drawings are made. The presumption is always against differences of delineation corresponding to real differences in the surfaces and features drawn. With this preface I proceed to the detailed consideration of the history of each particular mass. This is contained in the pages immediately following, where the masses are arranged in order of right ascension (nearly). Reference should be made to the Index-Chart and to my detailed observations in doubtful points. Connection between the nebula of Orion and its contained stars. A physical connection between the nebula of Orion and its contained stars is suggested by several circumstances. One of the most striking indications is found in a comparison between the number of stars in the same area in different parts of the nebula, with similar instruments, such as the Pulkova and Harvard College refractors, for example.* If the stars observed by BOND in the whole nebula had been uniformly distributed through it, we should have expected to find about 72 in the central portion worked over by STRUVE; as a matter of fact STRUVE has 155 (and BOND even more), which alone constitutes a strong argument for a connection between brightness in the nebulous matter and number of stars. As has been pointed out (V. J S. der Ast. Gesell, 1868, p. 31), it is necessary to this argument that BOND should have given equal atten- tion to all parts of the nebula. Accurate information as to this point is not attainable, but from what is known of BOND'S method of working, and of the exceeding patience and minuteness with which this research was prosecuted, there can be no doubt that much force attaches to this consideration. Order of brightness of the various masses. This order has usually been derived from the drawings published, but sometimes from original pencil drawings or from the notes of observation. The order of bright- ness is usually given in three grades i, 2, 3 ; i being the brightest. The interval between these grades is by no means equal. Where a letter is inclosed in a paren- thesis, as (A), only a part of the designated mass is referred to. The letters are arranged under each head in the order of brightness, so far as it can" be made out. * See also Lord ROSSE on this subject in the extracts from his memoir previously given, p. 87. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 203 HERSCHEL, 1824 (published engraving). There are no separate masses laid down, and the comparison is somewhat doubtful from this cause. It appears to be, i. E, F, G, H, N, M, 0, Q, D. 2. A, g, B, W. The last not much brighter than <7. HEESCHEL, 1837 (published engraving), i. E, I, G, N, Q. 2. F, H, S, M, R, P (all equally bright). 3. D = <7 (this is very different from other drawings). 4. A. LAMONT, 1837 (published engraving), i. (W) I, E, (A), (A), G, F, (Q). 2. D, S, or R, e = Z, K (W) ; part of A? E and I brighter than all the rest, D Q. LAMONT, 1837 (from an original pencil-drawing kindly lent me by Dr. DOBERCK). 1. I, E, F, G, A, Q, D, H. 2. C, B, W, J, and all the rest about equal. DE Vico, 1839 (published engraving). No separate masses; and therefore the order here set down is somewhat doubtful, i. D, W, A, h. 2. Q, H, G, F, E, I. LASSELL, 1847 (engraving), i. E ? - - I, D, ( F, but not much; Q D (p. 77) ; I H (p. 75). G>H>EandI>E;A very bright, and W very faint (p. 75). SECCHI says that LIAPONOFF gives B F the maximum light. LASSELL, 1854. Some copies of the published engraving are poor on account of the plate having worn, but I have made these estimates from a beautiful fresh copy kindly given me by Mr. LASSELL. i. F, G, I. 2. D, g (B!). 3. H, N (Of). A, totally wanting. SCHMIDT, 1861 (from original drawing kindly given to me by Dr. SCHMIDT). R, Q, I, L, G, H, E, F, A and B faint. LASSELL, 1862 (a careful copy of his original drawing), i. A, B, D. 2. W, I, G, R, (P), E, and Y. Perhaps this is not very certain, although I had the advantage of a copy of Mr. LASSELL'S original drawing made by Miss LASSELL. G. P. BOND, 1865 (published engraving), i. A. 2. D, I. 3. F, G, H, E. All of classes 2 and 3 nearly equal. G. P. BOND, 1863. Completed drawing based on many studies, and "compared with the heavens about Feb., 1863." i. A, part of B! 2. D, I, Q, F, G, E, H ( A. E nearly equal to to G, F, and I. ROSSE, 1867. See extracts from the memoir ante, where the order of brightness is care- fully given. From the drawing it is about as follows: N, G, F, I, E, A, (B), Q, N, P, etc. 2O4 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. SECCHI, 1868. i. E, I, F, H, N, D. 2. (W), (B), and? A. E > G, F. B and J < north shore of K. The line of maximum light is G F of LIAPONOFF (that is [G] [F] of our Index-Chart). The description by SECCHI gives the following results: E>GandF; GandH>M? andS? SECCHI'S a Washington F; b G; c=H; d zz I. G is one of the brightest masses (p. 1 4) ; [D] > E ; D, E , B > K. Accord- ing to LIAPONOFF the line of maximum brightness is along the line 647 to point of Q; according to SECCHI from 647 to W 1 . W? and D? are the brightest masses. D'ARREST, 1 872 (published engraving), i. W,D,Q,P,R,O. 2. J, I,E, (Hand G), F. 3. A brightest part of IJL. WINLOCK and TROUVELOT, 1874 (published engravingX i. A. 2. D, ( 1874 142.0 TEMPEL .... HOLDEN .... 1876 134-0? 1877 141-1 p _ I;}6 i O f t he occiput exclusive of Y (see LASSEI.L, 1862). Probably constant. It should be remarked that the angle between the frons and orciput is 116, according to HUYGHENS (1694); 129, according to LE GENTIL present time (1824 to 1880). 2O6 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. ./'arallel through 0' (628) towards the east. The numbers in the column 4a indicate the length of that portion of the above line which is involved in nebulosity. Only so much of the line is considered as lies between 628 and 708. This line is about 150" in length. Observer. Date. M Remarks. HERSCHEL . 1826 all It is all involved in the bright nebulosity. HERSCHEL . 1^37 all It is all involved in the bright nebulosity. LAMONT .... 1837 [4i"l [About three-elevenths of the distance.] DE Vico 1839 50": About one-half involved. LASSELL .... 1847 ? Only a portion involved. W. C. BOND . . 1848 150" All involved. LlAPONOFF . 1851 60" LASSELL .... ,1854 ? SCHMIDT. . . . 1861 all LASSELL .... 1862 all G. P. BOND . . . 1865 37". 5 I ROSSE .... 1867 42" SECCHI .... 1868 150" All involved. D' ARREST . 1872 100" WlNLOCK . . 1 TROUVELOT . . > 1874 150" ? All involved? TEMPEL . . 1876 50 Only about one-third of the line 628-708. 1878 Set wire on parallel through ff = 628. From 628 to go this wire is im- mersed in nebula (77")- From 628 to the R. A. of 651 (29"] this line is in a darker space. It is only a little south of the south border of r. It is immersed in R ; following R it passes through a dark space, and finally is involved in /*, North point of Sinus Gentilii. Observer. Date. A Remarks. HERSCHEL . . . 1826 122. 6 HERSCHEL . . . 1837 87.5 LAMONT .... 1837 . - [Not laid down.] DE Vico. . . . 1839 55-8? LASSELL .... 1847 - - W. C. BOND. . . 1848 77. c LlAPONOFF . . . 1851 / . ' J 58? SCHMIDT 1861 40.8? LASSELL .... 1862 109.4 WEBB .... 1863 See text, p. 78, et seq. G. P. BOND . . . 1865 57-5 On original drawing from which engraving was made, a few seconds south of this. WEBB 1866 In same Ad as south point of G. ROSSE . . . 1867 70 SECCHI . . . . 1868 97-3 D'ARREST . . . 1872 60 WlNLOCK . . ) TROUVELOT . . ) 1874 77-5 H OLDEN. . - 1877 66.7 Probably constant. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. C. 2O7 With regard to c it should be remarked that D'ARREST makes it concave towards the north. This is different from all other authorities. Z. ROSSK, 1867, JS of north point 120". SECCHI, 1868, JS of north point 140". d. BOND, 1865, J6 of north point 105", Ja = 6o". ROSSE, 1867, J6 of north point 94". D'ARRE&T, 1872, Jd of north point 134". (Very probably this is a part of Z, seen differently from the other drawings). HOLDEN, 1877, Jd of north point ioo".g. Probably constant. L. W. C. BOND, 1848, Jd north point = o" ? ? ROSSE, 1867, J6 north point ^o" ? HOLDEN, 1877, ^ north point 25". 5. It is difficult to say if this corresponds to any real difference ; most probably not. Angle of preceding edges of J and B. Observer. Date. P HERSCHEI. . - 1826 . . HERSCHEL . 1837 LAMONT. . . . i837 [24] DE Vico. . . . '1839 174? LASSELL .... ;8 4 7 W. C. BOND. . . 1848 39? LIAPONOFF . . . | 1851 10.2 LASSELI 1854 - - SCHMIDT . . - 1861 172.5 LASSELL .... 1862 i8o G. P. BOND . . - 1865 6.1 ROSSE .... 1867 O. 2 SECCHI .... 1868 13 D'ARREST . - . 1872 165 WISLOCK \ 1874 7-2 TROUVELOT . . > TEMPKI 1876 3 0?? HOLDKN. . - - 1877 7-7 Remarks. [J tolerably bright.] [ 8 does not point towards 575, but to the west of it. At least the maximum brightness (20" to the north of 575) is on a meridian halfway between 575 and 589.] For position of 575 see "Occiput". B forms with the following edge of A and part of I the hcmicyclnim Liafomn-h. The south part of J and the north part of I have sonu'tuncs appeared to L. " perfectly black," text p. 78. These masses are further towards the east than in other drawings. "1864, Apr. 16. BOND says, 'This edge of nebula in meridian precisely through 575.'" This is the angle of the preceding edges of the entire two masses. For position of 575 see "Ocdput". Do. This includes only that part south of 0'. The shape of these edgesjiuite different from those in any mddern figure. This refers only to the part south oft)'. 208 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. Probably constant in position. There may have been changes in brightness of B and J. See B following. B. Observer. G. P. BOND. . ROSSE . . . Date. / Remarks. 1865 1867 - - About as seen in the heavens by me (1874-' 79). B extends one-third of its length beyond (.>-.) of [575]. Quite different from BOND, 1861. SECCHI . . . D' ARREST . . WINLOCK . . TROUVELOT . . 1868 1872 \ 1874 - - B begins to curve to the west immediately north of the parallel of 6'. Quite different from other drawings; see his figure. Different in form and brightness from other drawings. Consult the figure. HOLDEN . . 1877 - - About as in BOND'S drawings, published and unpublished. The brightness in Lord ROSSE'S drawing is different from that in preceding and following ones. W, W 1 , W 2 , W 3 , W 4 , W 5 . W'. Observer. Remarks. SCHROETER . 1794 W 1 = lacus Seccliii discovered by SCHROETER. HERSCHEI, . . . 1824 W ' not laid down ; W = I, E, etc., in brightness. HERSCHEI, . . . 1837 W 1 not laid down ; W = I in brightness. LAMONT .... 1837 \V ' not laid down ; W = T in brightness. DE Vico . . . 1839 W not laid down; W almost as bright as any mass. RON DON i LASSELL .... 1841 1847 W 1 not laid down ; \V almost missing. W. C. BOND . . 1848 W not laid down. G. P. BOND ... 1865 45 +59 Center of W 1 . ROSSE .... SECCHI .... 1867 1868 . . +68 A '5 of center of W (brighter than) ; 62 +82 Center of W 1 . '; diameter in (5 24"; W 1 and W- are black; W 1 > r w X; v>w. 1 D'ARREST . . . 1872 \V not laid down ; W = D in brightness. WINLOCK . . } TROUVELOT . . ) 74 _ 5 o -|-74 Center of W; W 1 > - ; c > W ; W 1 > V, /. ,-., different from 1867. HOLDEN .... I8 77 - 45 + 68 Probably constant. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. A. 209 Observer. Date. A HERSCHEL . 1826 // HKRSCHEL . . . 1837 - - LAMONT .... 1837 - - De Vico .... 1839 . LASSELL .... W. C. BOND . . 1847 1848 - - LlAPONOFF . LASSELL .... 1847 1854 - - SCHMIDT . . 1861 . LASSELL . . . 1862 . WEBK .... G. P. BOND . . . 1863 1865 . WEBB .... 1866 . ROSSE .... SECCHI .... 1867 1868 3 to 4 D'ARREST . . . WINLOCK . . TROUVELOT. . \ 1872 1874 * TEMPEI HOLDEN .... 1876 1879 ;; Remarks. 31.6 A is not laid down as a separate mass, but it is of the second order of brightness ; by no means one of the brighter parts. A ; the following portion is very faint. There is no bright nebulosity pre- ceding the star 619, except a portion of I. [The axis of the brightest parts of A begins near star 608 (not laid down), and is further towards the west than now, though generally similar in shape.] 622 is on its following edge. A < I, E, F, G. There is no separate mass laid down, but the space preceding 0> is of the first order of brightness. There is no brighter mass A. The bright following part of A is not laid down at all. On the contrary, this portion is very faint. From a MS. drawing (no date) I find both D and E brighter than A. Very bright ; text, p. 77. A is totally wanting, and its absence and that of the adjacent parts is one of the most striking features of thisdrawing. A is rather of the second order of brightness ; A = Q, etc. A full of singular convolutions and very bright. A is by no means the brightest mass. A < G or F. The brightest mass, according to a sketch 1864, April 16. A terminates pretty exactly at 608. Same as WEBB, 1863. South point. A is much as in G. P. BOND'S drawing. Not so bright as other masses. Probably not so bright as other masses. A and the base of B the brightest parts. A, }T\S\. preceding the trapezium (U ?), is perhaps the brightest mass. A is the brightest mass. Up to 1839 the part A is not conspicuously bright. In i847~'48, two authorities make A faint, the other very, bright In 1854 ir is totally wanting- (LASSELL), while the same authority in 1862 makes it the hrighn-sr mass, and later authorities in the main confirm this. It certainly seems to me that A about i847~'48, and up to 1861 was not of its present brilliancy. Compare LASM-:LL 1847, J 854, and 1862. LIAPONOFF (1847) is opposed to 'this, and his observations an- entitled to the greatest weight. In spite of this I can reconcile the observations in no way but by supposing sonic change of brilliancy, and I believe any careful examination of the originals will lead to the same conclusion. App. V 27 2IO MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. E. Observer. Of line join- ing 628 and south point. Date. South point. North point. Diam. in dec. Remarks. / J 46 Ja M ." / // a n n // It has a nucleus (c) which SCHROETER calls SCHROETER i 794 - - a star. HERSCHEI. . . 1826 . 10.2 i7t 2 Not a separate mass. */j' * HERSCHEL . . 1837 . + 15- 140.0 - . 92.5 46. 5 Uniformly bright oval. LAMOMT 1837 1 80 27 precedes 152.6 - 119-9 3 2 -7 [E is inferior in brightness only to I. Its center is brighter than its borders. It is more sharply terminated on the side towards I than towards F.] DE Vico . . 1839 T$Z,1 T - ' 0.0 161 7 Not a separate mass. 1U O' / RON DON i . 1041 T ( 90 > The south half; then it turns at right angles. LAMONT LAMONT . . . i*37 1837 [44] [Inclined to fro as ; X not laid down. The preceding end of this channel is perpendicular to occiput. ] LASSEI.I 1854 60? - ' LASSELI 1862 53? Parallel to from '' G. P. BOND . . . 1865 29? On original drawing from which engraving was made / seems rather larger than this. ROOSE .... 1867 28 SECCHI .... 1868 90 D'ARREST . . . 1872 57 The west portion only. WlNLOCK . . 5 TROUVELOT. . > 1874 42.5?? HOLDEN. - - - 1877 25.6 \ . __ Constant since 1865, and probably the earlier observations differ only on account of errors. But compare HERSCHEL (1837). Dark channel letwcen E and F. Observer. Date. / Remarks. ' o HERSCHEL . . '837 LAMONT . - J 837 160?? 134?? [F alone is sharply terminated; E fades off towards F.] W. C. BOND . . SCHMIDT LASSEI.I WEBB . . - - 1848 1861 1862 1863 143 ? About parallel to occiput. 130? Neaily perpendicular to from : it is best terminated on the following >idc; G. P. BOND . . - WEBB .... 1865 1866 132.5? Its prolongation passes nearly through 575 on original drawing from which engraving was made. Same as WEBB, 1863. ROSSE .... SECCHI .... 1867 1868 I34-I 135 This channel is much s-horter than in ROSSE. D'ARUEST . - - 1872 142 WlNLOCKi - - ) TROUVELOT. . ) TF.MPEL . - - 1874 1876 144-5 134? Parallel to occiput. 1 HOLDEN . . I 1877 140.9 Probably constant. 212 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. V. [The mark a ^> b signifies here a brighter than l>.~\ Observer. Date. Remarks. HERSCHEL . HERSCHEI. . LAMONT . . DE Vico. . W. C. BOND LIAPONOFF . LASSELL . . SCHMIDT STRUVE . . LASSELL . WEBB . . G. P. BOND- WEBB ROSSK . . SECCHI . . D'ARREST - WIXLOCK . TROUVELOT. HOLDEN . 1826 1827 1837 1839 1848 1851 1854 1861 1861 1862 1863 1865 1866 1867 1868 1872 1874 1877 V is a marked feature ; see text. V ^> | ; V ^> south half of T. V is a marked feature ; it is totally black. See text. [V is not laid down on his original pencil drawing.] V very much brighter than T. V is as bright as any portion. V > r. V < W. V < Sinus Genii Hi. " V perfectly black;" text, p. 78. V perfectly black. V ^> r ; V ^> Sinus Genii lit. V is in general quite dark. See (a) text, p. 115. V > r ; V = W 1 ?; V much brighter than Sinus Gentilii. V is not a marked feature. V is nearly symmetrically disposed about the trapezium. V ^> r ; V ^> W; V ^> Sinus Gent. V better marked than in WEBB, 1863. V > r ; V = W 1 = W 2 . V > r ; V > Sinus Gentilii. V ^> r, but not much so. V = r ; V = Sinus Gentilii ; V < W 1 . V < r 1 ; V < Sinus Gentilii; V < W 1 . Compare the relation of V with Sinus Gentilii: 1861-1868, V>S. G. ; 1874, V S. G. ; 1877, V I. The preceding edges of F and G in same meridian. ROSSE .... 1867 - - 72 i 83" south point : JS. [1877, south point : Jf> 76 , HOI.DK.N.] ( 61" north point: J8. SECCHI .... 1868 . . . G quite unlike BOND and ROSSE, and present appearance. D'ARREST . 1872 ; . . . G and H are one mu--. WlNLOCK . . ? TROUVELOT. . ) ' 1874 72.5 TEMPEL . . . . 1876 . . Brightest point fnreJcs 628 (a TV./yV;//). HOLDEN. . 1 . 1877 15.9 65.3 One of the brightest masses. Probably no change in form. 214 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. H. ROSSE, 1867, Jd south point 83'"! ROSSE, 1867, 48 brightest point 78" ? HOLUEN, 1877, 46 south point 94". Probably no change. D. . f. point. Observer. Date. * Remarks. Ao: AS HERSCHEL . . - 1826 " a 27.7 II 74-5 The point of D plainly laid down. B is concave towards the east. SMYTH . . . - 1834 . . - In a coarse sketch in the celestial cycle, ii, p. 132, D is concave to the east and the brightest mass. HERSCHEL . . - 1837 . . . There is no bright sharp following point, 647 and 651 as in Naval Observatory drawing. D is fainter than E, F, G, H, Q, etc., and concave to the east. LAMONT .... 1837 . 17.4? 98.1 647 and 65 1 as in Naval Observatory drawing. [651 on exact edge of D. D has no north sharp point, but its north end is rounded.] [D brighter than H? but fainter than I. E, F, G, (A), Q, just as in HERSCHEL, 1837.] DfiVico. . - . 1839 . - - - STONEY .... 1851 . . . D is partly concave, partly convex to the east, in an unpublished drawing made at Lord ROSSE'S Observatory. W. C. BOND . . . 1848 . 16.6 56.8 647 and 651 as in Naval Observatory drawing. See text. LIAPONOFF . . . 1851 , 26.1* 55-5* A 5 = 69". 6 and A = 26". 5 of point of intersection of follow- ing edge of D with north shore of Sinus magnus. [* Co-or- dinates of s .] T A CCTTT T 181:4 Following side quite bright. .LjAootL..!-. - - *^JT- 1861 Tangent to bright following side, A = i3"-6. There is no O C I i M IDT . - sharp northern point ; the mass is oval. QTB TTVT? 1861 D seemed to be subject to considerable variation both in form and o 1 K. U V r. brilliancy. LASSELL .... 1862 . 11.4 66.1 641 and 657 in Naval Observatory drawing. 651 as exactly on edge. WFHR 1863 D is not so bright as F ; no sharp north point laid down. G. P. BOND ? . . 1865 ? 36.2 69.8 Measured 1864, April 15, following edge of D, A (measured) = 26". 6 same date. 647 and 651 as in Naval Observatory drawing except that 647 is not in a bay. " 647 far within neb- ulosity," 1864, April 15. WEBB .... 1866 D is the brightest mass ; no sharp . /. point laid down. ROSSE .... 1867 - . 26 69 Same as Naval Observatory drawing. SECCHI .... 1868 . 44-9 - . - A of tangent to brightest of following side of D. D has no bright sharp following point, and is clearly different from its present appearance. P'ARREST . 1872 - - 32? 44? D quite bright, but outlines are not so sharp as in Lord ROSSE'S or Naval Observatory drawing. WlNLOCK . . . 1 1874 "3 92-5 As in Naval Observatory drawing. TROUVELOT . . > HOLDEN .... 1877 . 7-8 A of tangent to brightest part of following side 29". 4. D is convex towards the east. The form seems to have varied from concave to the east (1826 and subsequently), to convex to the east (1865 and subsequently). If the mass had been always of its MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 215 present brilliancy, the question of convexity or concavity could not have taken a moment to decide it. I, however, agree with 0. STBUVE in believing this mass to have varied in brilliancy, and I am inclined to believe that the (possible) changes of form are due to this fact. The star 651 seems to have remained in a fixed position relative to the following edge, and no change of form is shown there. The extreme north point, however, has certainly varied in brightness during the period 1826-1877; it has even so varied during my own observations 1874-1880. r. Observer. Date. Remarks. HERSCHEL . . . 1826 North half: Streaks of nebulosity. South half: Black. HERSCHEL . . . 1837 r uniformly completely black. LAMONT .... 1837 Do. DEVico. . . . 1839 Do. RONDONI 1841 LASSEU 1847 Notth half: Partially nebulous. South half: Black. W. C. BOND . . . 1848 T uniformly completely black. LASSELL .... 1854 North half brighter than south half. SCHMIDT .. - - 1861 Pretty uniformly black. LASSEI.I 1862 Pretty uniformly black, except for SCHROETER'S second bridge. WEBB . ". - - 1863 All filled with nebula. G. P. BOND . . . 1865 The north preceding part is brighter than the south following part. Black streak near fvns Schroctcrii. The south shore of T reaches to the declination of 6' ; i. e., AS = .0". ROSSE . . . - 1867 Two black streaks in it on its west and east sides ; the north half brighter than south half. WEBB .... SECCHI . . . - 1866 1868 Completely filled with nebula. On the whole, n. ^ > s. *4, t although somewhat doubtful on account of absence of definite outline to potts Schroeteni. The preceding edge is brighter than the other portions; and T extends to the south of 6'. T much darker in 1867 than in 1857. D'ARREST . - - 1872 Pretty uniformly filled with faint nebulosity. WINLOCK . . ; TROUVELOT . . 1874 Pretty uniformly black. TEMPEL . . - HOLDEX. . . - 1876 1877 Do. All north of M = 47" filled with faint nebulosity, except, of course, the dark streak on its preceding edge. South of this pretty black, but less so than r'. No change in r except with regard to ScHROETEB'ssecond bridge, q. v 2l6 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. The second bridge o/ SCHROETER (in r). Observer. Date. Remarks. SCHROETER . . . 1797 Jan. 25 (see Fig. 13). The second bridge is plainly laid down. HERSCHEL . . . 1826 The north half is plainly indicated. HERSCHEL . . . 1837 No bridge laid down. LAMONT .... 1837 Do. -DE Vico. . . . 183?" Do. LASSELL. . . . 1847 Do. W. C. BOND 1848 Do. LIAPONOFF . ftM|fl? 1851 Do. LASSELL . 1854 Do. SCHMIDT . . . 1861 Do. LASSELL .... 1862 The north one-third is plainly indicated. WEBB .... 1863 Not laid clown. G. P. BOND . . . 1865 No bridge laid down in engraving, but plainly drawn on several different charts. . WEBB ., . 1866 No bridge laid down. ROSSE .... 1867 A second bridge without nucleus is laid down. SECCHI .... 1868 No bridge laid down. D'ARREST . 1872 Do. WINLOCK . . } TROUVELOT . . 5 1874 Do. TEMPEL .... 1876 Do. HOLDEN. . . . 1877 Second bridge, as in LASSELL, 1862, and SCHROETER, 1794. SCHROETER, 1797, lays down the second bridge in r, HERSCHEL in England (1826) shows it. Having this first drawing before him in 1837 at the Cape of Good Hope, he does not show it. LASSELL, 1862 (4-foot reflector), shows it faint. BOND (1865) does not show it in the engraving, but it is faintly but plainly shown in his drawings. ROSSE (1867) shows it. In 1874-1875 I did not see it, although r was examined in 1875 carefully. It was first seen here in 1876. It is now brighter than at that time. These facts, taken in connection with the general details given under the heading r, seem to me to prove a change in its brightness ; for how otherwise could SCHROETER see in 1797 what escaped HERSCHEL (1837) and LASSELL (1854) with superior advantages I MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. Pons tSchroeteri and 217 Pom S. 8+ ( >bserver. Date. Remarks. P A a. 61 Noi th half alone given. o 1826 175 1837 163.5 72 HEUSCHEI. . . . LAMONT . . . 1837 80 52 [The south end of fans .SV/mvAv/is 28" noith of 6'] [g v not given]. LAMONT 1837 [160??] '- LASSELI. . . 1847 180?? - - - - W. C. BOND . . 1848 154 - - LlAI'ONOKF*. 1851 71.9 42.3 In the direction of 669 and 685 (text, p. 79). I. \SSEI.I. . . . 1854 , 180?? SCHMIDT . . . 1861 139 - - No nucleus; the north two-thirds only are laid down. STRUVK .... 1 86 1 The fons crosses the whole of Sinus magnus, and & _ the brightest part ; but ^, is sometimes wanting. is usually LASS E r. i, . . . 1862 175 V No nucleus ; the north two-thirds only are laid down. WEIili .... 1863 . . ( )n the line joining 669 and 685. g u not drawn. G. P. BOND . . . 1865 157-5 7 44 Bridge continuous ; break nearly as in ROSSF. in various drawings. WEIIB .... 1866 . . Parallel to line 669-685, but following this line a little. R< ISSE .... 1867 175 1868 . . 7o 39 Length of bridge 78". There is a decided break in the bridge about its middle point. tfi formerly visible, but not seen in 1868 (text, p. 19). In 1857 the /WAT Schrot-tt'ri rvwrr.r towards the west. In 1867 conca-'f. SECCHI .... D'ARREST . . . 1872 175 Length of bridge 40''. WlNLOCK . - - ) TROUVKI.OT . . > 1874 168.2 Perhaps a break indicated. ,<> not given. TKMI-EI jg-5 . . Nucleus very much brighter than the rest. HOI.DEN . . . 1877 170.2 77.4 40.8 There is a break on each side of .-b, one in A5= 22", and the other in A 5 = 60". .v,, stellar at first ; not so later in the work. Variations in brightness seem to me to be proved, as remarked by Om> and SECCHI. See their memoirs as previously referred to. A Tvr, \T >i STRIA r 218 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. G and lacm Lassellii. Observer. South s. f. bright edge. point. SCHROKTER IlERSCHEL. HKRSCHEI.. LAMONT . DE Vico . LASSELI. W. C. BOND LlAl'ONOFF LASSKLL . SCHMIDT . STRUVE LASSEU, G. P. B>M> WEBH . . ROSSE . . SKCCHI . D'ARREST . WIN LOCK . TROUVEI.OT TEMPEI. HOI.DEN ] )ate. 1794 1826 1837 1837 1839 1847 1848 1851 1854 1861 1861 1862 90.4 1865 95.0 1866 ..; 1867 91.0 1868 Lacus Lassellii. p Art A<5 68 1 on following edge 152 117.5 68 1 in middle of 6. South edge of 6 i suddenly much brighter. . : 681 on following edge [681 on follmping t>\ .... - ; No bright sharp following point. Discovered lactis Lassellii. Plainly laid down. Plainly laid down though different from its present appearance. No Incus Lassellii. No lams Lassellii. 143-6 71-5 1872 90 | 1874 1876 ! - - 1877 I 89.7 Concave towards the south ; south edge brighter. No sharp following point, at least not as in BOND. 136.8 68.4 ; I 5 83 South edge convex towards soiith if at all; nearly straight. - - - i See Notes. 144 81-87 192.8 79.6 Roughly speaking the preceding half ^> folloiving half; a ////// brighter on the south edge, but not much ; its south shore is "directed towards the north." 192 55 South edge concave towards the south. 170 76.3 General shape as in Naval Observa- tory drawing, but brightness dif- ferent. . i Following half ]> preceding half . 79 : The preceding half ^> folltmnng half. Does not extend quite to Siiinsi/iagnns. Connects with Sinus m n near TT as connected. Black. Do. Completely black. See Man. Not., R. \. S., vol. 24, p. 178. Its south border is 10" or 12" south of ff. Black. The north half the brighter. Not laid down. Filled with faint nebulosity. 1874 Following half brighter than preceding half. \ 1876 South part nebulous; north part black. 1876 North part brighter, but all very dark. Probably constant. Q, P, R. Observer. Date. North edges. Remarks. / o HERSCHEL . . . 1837 93-95 LAMONT. . . . 1837 1045? [R, P, and S not laid down.] DEVico. . . . 1839 117? \V. C. BOND . . 1848 97-5 Cuts off a little of R. SCHMIDT 1861 105 ? LASSELI 1862 98 Cuts off a little of R. G. P. BOND . . . 1865 115 Do. ROSSE .... 1867 1868 101.0? Cuts off 1\. 1 Angle not easily measurable. It does nut agree with Kuv-h and r.<>\n. SKCCIII .... D'ARREST . . . 1872 i5 WIN LOCK . - ) TROUVELOT . . > 1874 97.0 Cuts off a little of K. HOLDEN. - - - 1876 105-7 j Probably constant. 220 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. Q. Observer. Date. Following point. Remarks. Aa A8 n n LE GENTIL . . . 1758 . . Same R. A. as 708. LAMONT .... 1837 162.4 40. 2? [671 on north edge of brightest nebulosity, or a little north of the edge in a slightly fainter part.] LASSELL .... 1847.' - - - - W. C. BOND. . . 1848 "150 [Same R. A. as 708.] *A diagram of 1848. Jan. 17, gives 150". Annals If air. Coll., v, p. 176. LlAPONOFF . . - 1851 146.5 22. O Precedes 708 by 4".6. Cj well defined. SCHMIDT . - . 1861 - - - - Follows 708 by from 7" to 8". LASSELL. . . . 1862 124.3 6.8 Same R. A. as 78. WEBB .... 1863 - - - - The line joining 669 and the extreme point of Q cuts the line 708-741 at G. P. BOND . . . 1865 145 20 ??ths of its length measured from 708. Q, N, O, P quite bright. 46 Q and.N quite bright. WEBB .... 1866 - - - - Appears to have its following point in same R. A. as 708. Q, X, O, P are all fainter than E, F, G, I, H, D, and [A ?] ROSSE .... 1867 US 21 SECCHI .... 1868 153-5 . - D'ARREST . . . 1872 192 32 WlNLOCK . - ) TROUVELOT - > 1874 155 - - TEMPEI 1876 . . . Same R. A. as 708. HOLDEN. . . . 1877 150 28 Do. Probably constant. T" and Observer. Date. r". HERSCHEL . . . 1826 HERSCHEL . . . 1837 Perfectly black Filled with faint nebulosity Perfectly black. Almost empty of nebulosity. LAMONT. . . . 1837 Same. . DE Vico . . . ! 1839 Black. LASSELL. . . . 1847 W. C. BOND. . . 1848 Filled with faint nebulosity Full of curdled nebulosity. LASSELI 1854 SCHMIDT . . . 1861 LASSELL. . . . 1862 G. P. BOND . . . 1865 ROSSE .... 1867 Contains nebulosity Not laid down Almost totally black Completely black. See Mon. Not., R. A. S., vol. 24, p. 1 78. . do Contains nebulosity. Pretty nearly totally black. Nebulous particularly u\\Jollcwiti one half. Faintly nebulous. Very faintly nebulous. D'ARREST . . . 1872 WlNLOCK > 1874 Uniformly filled with faint nebulosity. Same Very faint nebulosity. TROUVELOT. . > TEMPEL .... 1876 Black Nebulous. HOLDEN. . . . 1877 Quite dark empty Filled with nebulosity, except a black streak close to o TT. The various evidence points to changes in brightness. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 07T. 221 Observer. Date. Whole mass. Remarks. P HERSCHEL . . . 1826 o \ No nucleus o, but a detached brighter spot about where n now i.s. HERSCHEI, . . . i839 no? No nuclei o and it. W. C. BOND. . . 1848 99-5 No nuclei o and it. LASSKLL .... 1862 90 No nuclei o and n ; at the follmvin* end it joins <|iiite marked nebulosity. G. P. BOND. . . 1865 93 ? In original drawing from which engraving was made/ 90 . ROSSE . . . . WINI.OCK . . . j TROUVEI.OT . . > 1867 1874 93-o 90 No nucleus o; the nucleus it in A= 144". o and it not laid down. TEMPEI 1876 The mass on is laid down. IIol.UKN . . . 1875 1 it ^> o; both stellar in appearance, but not so much so as ;. . Probably constant. SPECTROSCOPIC OBSERVATIONS- OF THE NEBULA. The spectroscopic apparatus of the Naval Observatory and the relation of aper- ture and focal length of the 26-inch equatorial were such that no advantage could have been gained by devoting time to the re-examination of the spectrum of the nebula. A few extracts from work done by other astronomers, notably HUGGINS, SECCHI, and VOGEL are given herewith, and for similar observations reference is made to the original papers, whose titles will be found in the List of Books and Memoirs at the end of the Introduction. The most important of these is the paper of Profess,. r D'ARREST published in 1872. The main facts to be noted seem to be that this nebula is certainly gaseous and similar in constitution to other gaseous nebulae. The changes made out in the brightness of its parts are thus more credible than if it were a true stellar nebula, for, as is remarked by GEORGE BOND, "the variability of nebuliv, it' dependent on the variability of the stars of which they may be supposed to be formed, would require the greater portion of these stars to grow faint at one and the same- time. The improbability of this explanation is greatest where the number of stars of which the nebula is composed is the greatest, i. e, where it is brightest" The gaseous nature of some, at least of the small stars near the trapezium, is, I think, indicated by their peculiar behavior under different magnifying powers. As has been before remarked most of them are best seen with low powers. The space within about the trapezium is really nebulous and not void, was first suspected by BOND and HERSCHKI, The spectrum of the nebula seem* to consist of four lines. The positions of the three brightest are about A 5004 B 495-8 C 222 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. These seem to indicate the presence of hydrogen and nitrogen in the nebula, and that these are its principal constituents. We have yet to learn the true inter- pretation of the third line in the spectrum. A fourth line Hy has been occasion- ally seen. Dr. HUGGINS' paper in the Proceedings of the Royal Society, 1865, January 26, contains the important remark that "the positions in the spectra of <*, /?, y j , trapezii, which correspond to the positions in the spectrum of the three lines of the nebula were carefully examined, but in no one of them were dark lines of absorption detected" as might have been expected if the nebula was nearer to us than these four stars. In Dr. HUGGINS' paper u on the spectra of some of the stars arid nebula,"* he describes at some length his later observations of the nebula of Orion (p. 541), and corroborates his former measures, and after some general considerations as to the extinction of light in space, says (p. 544), " the result of the re-examination of the spectrum of this nebula appears to give increased probability to the suggestion * * * that the substances hydrogen and nitrogen are the principal constituents." * * * " I am still unable to find any terrestrial line which corresponds to the middle line." Dr. HuGGiNst shows that the nebula of Orion is not receding from the earth with a velocity greater than 10 miles per second, nor approaching the earth faster than 20 or 25 miles per second. In general, nebulae have not shown motions to or from the earth as the fixed stars have. In the Proceedings of the Royal Society for 1874, March 26, Dr. HUGGINS returns to this question. His results are the same except that it is possible that the Orion nebula may be approaching the earth as fast as 30 miles per second approximately. v Mr. LE SUBURB in reporting his measures of the spectrum of the Orion nebula, states that the nebulosity within the trapezium is comparable in brightness with that immediately surrounding it, as is shown by the relative brightness of the spectra. In the Proceedings of the Royal Society (1872, p. 383), Dr. HUGGINS gives the results of a new examination of the spectrum. The principal point to be noted is that it is possible that the brightness of the 3d (and 4th) lines may vary in brightness relative to lines i and 2 from time to time. SECCHI, after describing his previous observations of the spectrum of this nebula in which the line H/? of hydrogen was seen while none of the other characteristic lines of the substance could be observed, examines the question as to whether the presence of this line indicates that hydrogen is indeed present in the nebula. A Geissler tube containing hydrogen gave, in his spectroscope, the three lines Ho:, H/?, H.y ; when the light from this tube was enfeebled by reflection before entering the spectroscope only one line, H/?, was seen. He concludes, first, that the presence of a single line is sufficient to prove the existence of an elementary substance in a celestial body ; second, that the monochromatic nature of the light of nebulae is probably only apparent, and that there are probably other spectral lines not seen Philosophical Trun suctions, 1868, p. 529. t Proceedings of the Royal Society, 1868, May 14, p. 384. t Proceedings of the Royal Society, 1870, March 3, p. 242. In the Comptes Rendus, vol. 66, p. 643. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 223 on account of their faintness ; third, that, as we do not see certain lines whose abso- lute brilliancy is greater than H/?, we may conclude that certain substances are not present in the nebula; and fourth, that substances there present act by radia- tion and not by absorption, as in stars. Dr. VOGEL* gives the results of four nights' observations in 1871 on the spectrum, as below : 1. W. L = 500.4 M. M. 2. =495-8 " 3- =486.1 " The uncertainty is about db o.i 5 M. M. The first line is the brightest, the second the faintest; no fourth line was seen. The different parts of the nebula gave ahvuv- the same spectrum, and the relative brightness of the three lines was always the same. The first line coincides with a double line in the atmospheric spectrum. The second lino does not agree in position with the lines of any known terrestrial substance. The third line coincides with the hydrogen line H/?. Dr. BREDicuiNf gives as the normal spectrum of gaseous nebula 1 , the following positions of the three brightest lines : A = 5003.9 1.2 B = 4957.9 db 114 C =4859.2 dz 3- 1 The probable errors are computed by supposing the nebula 3 G. C. 4964, 4628, 4234, 4447, 4390, 4510, and 4373 to have the same spectrum, in fact, and the small differences to be due to accidental errors of observation. * Ast. Nacli., p. 78, col. 245. t In the Moscow Observations, vol. ii, p. 60. 224 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. * PART IV. CONCLUSIONS TO BE DERIVED FROM THE FOREGOING MEMOIR. It may be well to summarize the foregoing work, in order to review, briefly, the ground over which we have gone and the conclusions which are to be gained. The object of the work was twofold : First, to make such a detailed study and description of the central and brighter portions of this nebula, that a repetition of the work would be easy and short, and so that the question of any future change in the parts con- sidered can be settled definitely and beyond a doubt and without any great labor. The form in which the observations are classified in Part II seems to me to satisfy this condition. The accuracy .of the micrometer measures is sufficient for the purpose, and greater than I anticipated ; for example, the JS of the brightest part of F is io2 // .oo // .3 (5 nights); of the brightest part of G is 65". 3 o".8 (6 nights); the position-angle of the frons is 50. 3 o.5 (4 nights), and so on. These are sur- faces and not points it will be remembered. It will be noted that the first object of my work is precisely that proposed to himself by LE GENTIL in 1758. The second object was to completely and thoroughly discuss the large mass of material already on hand derived from the observations of 224 years (1656-1880). All available drawings were examined, and thirty-eight are here engraved (nearly all on the same scale), and abstracts have been made of all available observations arid are here given. Several unpublished series and drawings have been printed for the first time, notably those of LASSELL, SCHMIDT, and LANGLEY. By this examination the epoch of the first trustworthy observations has been carried back from 1824 to 1758. LE GENTIL'S figure of the central part yields evidence comparable in value with the first figure of Sir JOHN HERSCHEL. I conclude that there have been (1) possible changes of brightness in the masses of J and 15; (2) changes in the brightness of A ; (3) changes in the brightness of E ; (4) changes in the brightness of D ; (5) undoubted change in the brightness of SCHROETER'S second bridge ; (6) undoubted changes of brightness in SCHROETER'S bridge and in the appear- ance of r/ , its nucleus ; (7) a possible change in the position of the south edge of G since 1837 ; (8) a probable change in the brightness of T" (9) a probable change in the brightness of ;. (10) a certain change in the development of the mass // near D. There is no evidence whatever for any change of form other than that which may be due to such changes of brightness ; as in the cases of A, D, //, etc I do not find any change of the Messierian branch near 793. The connection of the stars of the trapezium with the nebula appears to me to bo settled by the conclusions of my paper reducing Professor HALL'S observations of these stars, and by various former observations, such as the important one by Dr HUGGINS- of those portions of the spectra of a, /?, ;/, $, trapezii, near the places of the nebula spectral lines i, 2, 3, and others. MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 225 The change in the brightness of D, h, and other masses, is shown by the Wash- ington Observations taken alone. These also show the feasibility of making tolerably accurate photometric observations of the relative brightness of two nebulous masses. Certain of the masses have varied in brightness during the period of observations. A new nebulous patch (h) has been seen from the time of its origin, when it was stellar in appearance and faint, until now, when it is bright and of measurable dimensions. It appears to me, then, to have been shown that the figure of the nebula of Orion has remained the same from 1758 to now (if we except a change in the shape of its apex (E) about 1770, which appears quite possible) ; but that in the brightness of. its parts undoubted variations have taken place, and that such changes are even now going on. I have not hesitated to give the conclusions to which I have been led in the course of this work, although I am aware that all of these may not be accepted on a first reading. With regard to any subject of this kind, every competent judge has a body of opinion derived ' partly from his own experience and partly from judgments formed from time to time by examinations of the work of others. In general, this body of opinion leans to the view that the phenomena presented by the celestial bodies, are, for long periods of time, quite constant. For example, accounts of supposed changes in the conformation of the lunar craters are received and rightly received with a measure of grave doubt, and yet no one is disposed to deny that real changes are now taking place from moment to moment, just as they have in the past; but each par- ticular recorded evidence of change is regarded with doubt, and a full measure of proof, depending on sufficient observation, is justly demanded. A competent observer is, however, still bound to put his observations on record. It appears to me that I have less reason to hesitate in recording my own judg- ments upon the phenomena here described, as the observations themselves are given in full detail, and the materials for an adequate judgment are spread out for inspection. At least, I can be sure that all the existing evidence is impartially presented in such a way as to be readily added to in the future, and I cannot myself doubt but that the principal conclusions here set down will be confirmed by others. APP. V -29 226 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. ADDENDUM. PHOTOGRAPHIC RESULTS OF DR. HENRY DRAPER. The first photograph of the nebula of Orion was made by Dr. HENRY DRAPER in September, I88o, and the unavoidable delay which has occurred in printing- the present memoir enables me to include an account of the astonishing results which he has attained. A wood-cut which I had prepared from his first photograph was found to be so unsatisfactory thkt Dr. DRAPER most generously offered to supply the necessary photolithographic reproductions of his last negative (taken March 14, 1882) to accom- pany the brief account I had prepared. The full page photolithograph is here given as figure 40. I requested Dr. DRAPER to prepare some account of his work to be presented with it, and I print below a memorandum which he has kindly furnished. "MEMORANDUM TO ACCOMPANY THE PHOTOGRAPH OF THE NEBULA IN ORION SENT TO PROFESSOR HOLDEN FOR HIS MEMOIR. BY HENRY DRAPER, M. D. "As far as I know, no photograph of any nebula has been taken except in my observatory. The first photograph of the nebula in Orion was made on September 30, 1880, with rny CLARK telescope of 1 1 inches aperture and an exposure of 51 minutes. It comprised the brightest parts of the region in the neighborhood of the trapezium and showed the condensed masses well. In March, 1881, a number of photographs of this object were taken, the best being on March 1 1 with an exposure of 104 minutes. By comparison with the former picture this made a marked advance, and minute stars down to the 14.7 magnitude of POGSON'S scale were shown. An account of it was read before the French Academy of Sciences and printed in the Comptes Hendus, April 1 8, 1881. "On March 14, 1882, the negative was made from which the photolithographic enlargement in this memoir was produced. The instrument used was the CLARK tele- scope of 1 1 inches aperture mounted on the equatorial stand and driven by the clock which I had constructed. The exposure was from 7 h o8 m to 9 h 25; that is, 137 minutes: gelatino-bromide plates were employed. The night was clear but cold and windy. The mean temperature was 27 Fahr.; the wind NNW. and in gusts, the strongest pressure being 5 pounds per square foot about nine o'clock; the whole travel of the wind during the exposure was 35 miles. The variation in the force of the wind is one reason why the stars show some ellipticity under this magnifying power; the gusts of course displaced the telescope somewhat, though the mounting is firm and the clock-work strong. "In the photograph the larger stars are much overexposed, the proper time to make a good picture of the trapezium being about 2 minutes. The twinkling of these stars is therefore recorded on the sensitive plate, and gives to them an excess of size. If a photograph should be taken on a steady night the stars of the trapezium would be easily separated, and in the original negative of this picture, in a strong light, the separation can be seen. The variation in size of the stellar images gives an idea of he lias found i \vith wir - reason vvh f course display "ire of ti 1 f L jh should be tal M a be md in t well. In March, i ber of photogi . ale w< and pr the the of these trapezium would r ' : - FHUTUGRAFH DF THE NEBULA IN DRICN. Taken by Professor Henry Draper M.D, .March 14th 1BB2, Exposure 3n artotypB enlargement by E, Bierstadt fram ** Dri ^ f ^ * la brighter then the nebula are overexposed, This picture when compared with that taken increased extent and shows stars of the 14,7 magnitude of Fo ? ?nn's, scale. Fie, MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OP ORION. 227 the relative magnitude of the stars, though that estimate requires correction for the color of the stars. It must be remembered that no one enlargement can do justice to the original negative; various exposures, various intensities of light, and various points of view are necessary for a complete examination. "During the month of March, 1882, 1 also made four photographs of the spectrum of the nebula in Orion, which are described in the number of the American Journal of Science for May, 1882. Two of these were made with the slit spectroscope that I usually employ for photographing spectra of the stars and they show two lines in the ultra-violet plainly, beside the traces of two others. The first-mentioned two are hydrogen 7, A 4340, and hydrogen 6, A 4101 ; the others are too faint to give a good estimate of the wave length. "The other spectrum photographs, taken without a slit, show that two of the con- densed masses preceding the trapezium give a continuous spectrum, and, therefore, contain either gas under pressure, or liquid, or solid matter.* "271 MADISON AVENUE, New York, April 29, 1882." Although it is still too soon to give a final discussion to the photographic results attained by Dr. DRAPER, I cannot refrain from pointing out some of the conclusions which may be drawn from this marvelously perfect representation of the nebula. If we compare the frontispiece with Fig. 40 we shall be able best to appreciate the important advance which has been made. BOND'S engraving is the most accurate drawing that has been made, even as a map, and as a picture it is decidedly the best representation of a single celestial object which we have by the old methods. The work of observing alone extended over years and consumed many precious hours. I have before said how much labor was spent upon the mechanical execution of the steel plate; scores of revises were criticised and read. Dr. DRAPER'S negative was made in 137 minutes, and for nearly every purpose is incomparably better than the other. The color and tint of the nebula, which is wonderfully preserved in BOND'S engraving, is lost in the photograph; and yet, if the latter is held up between the eye and a window, the pictorial effect is most striking. The amount of preparation for the two works is not to be estimated by years or hours, but it may be left out of account in a comparison. It required the best efforts of each observer to attain the results. The telescope used by Dr. DRAPER is an ii-inch photographic refractor, made by ALVAN CLARK & SONS. The minimum visibile for such an aperture is 14.4 on ARGELANDER'S scale. In the accompanying Table A, I have given a list of the stars laid down by BOND which are found upon two photographic prints which Dr. DRAPER has sent me. Table B contains a list of such of BOND'S stars as are exterior to the brightest central nebula, and absent from these two prints. Naturally the original negative would show far fainter stars; but in my comparisons I have been confined to the use of these two prints, and I have included no star in Table A which is not shown in its true position on both the photographic prints examined. * A private letter of Dr. HIHHHXS informs n.e that in a photograph ,,f tin- speetrnm ,,f tin- nelmla taken in April, 1882, five Hues are shown ; the four previously known in the v.s.l.le speetrum. and a n. 228 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. Taking Table A we see that stars as faint as 11.5 are plainly and well shown. No. 793, which is immersed in nebulosity and is only of the 11.7 magnitude, is yet clearly seen. There are shown in the photolithographs five stars fainter than the 13.0 magnitude, viz, Nos. 435, 650, 653, 657, 778, whose magnitudes on ARGELANDER'S scale are 13.1, 13.1, 139, 13:1, 13.1, respectively. In Table B the brightest star is of the 11.9 magnitude, and this star (808) is marked variable by BOND.* TABLE A. BOND'S number. ARGELANDER'S magnitude. Remarks. BOND'S number. ARGELANDER'S magnitude. Remarks. 427 10.7 Very faint in photograph. 670 10.8 - 430 11.7 Very faint in photograph. 685 8-3 435 I3-I Very faint in photograph. 690 10.3 449 10.5 700 ii. 5 453 II. 2 Very faint in photograph. 705 ii. 5 467 8.7 707 II. 2 479 10. 708 9.6 505 ' 9-6 709* 12.3 506 II- 3 724 10.5 523 10. I 732 ii. 5 551* 10. I 734 9.0 554 9.0 741 IO.O 558 10.7 746 10.8 570 9.4 750 10.8 580 12.3 757 IO.O 598? 12.3 Doubtful in photograph. 778* 13.1 635 10.5 781 10.8 647* 12. I Just visible. 784 10.8 650 I3-I 785 10.8 653 13-9 793 ii -7 In the nebula ; faint in pho- 657* 13-1 822 10.7 tograph. 663* ii. 7 848 9-9 669 9.8 863*? 12.5 Doubtful in photograph. TABLE B. BOND'S number. ARGELANDER'S magnitude. Remarks. BOND'S number. ARGELANDER'S magnitude. Remarks. 508 12.3 641* 14.8 5>o I3-I 652* 13-9 5i6 13-5 767 13.9 524* 12.5 789 14.8 532 14.2 797* 15-0 545 I3-I 805 13-9 552 14.9 808* II. 9 566 13-3 826 14.8 615 14.2 832 13.9 N. B. Those stars marked with the asterisk (*) were reported to bo^variable by BOND. * It should be borne in mind that if these minute stars have a slightly reddish tint that also might account for their absence in the photograph. [H. D.] MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. 229 The stars marked with asterisks were counted variable by BOND, and the posi- tions of some of them in the lists are suggestive. Undoubtedly No. 778 was near its maximum, and this instance suggests an important application of photography in the detection of variable stars. It is, however, when we examine the details of the nebulous structure, as shown by the photograph, that we can best appreciate the astonishing advances which have been made. The most important evidence to be obtained from Dr. DRAPER'S photograph is in relation to the brightness of the different portions of the Huyghenian region. The photograph gives, of course, the brightness as shown by the chemical decomposition of the salts of silver on the plates used. This is not directly comparable with their brightness estimated by the eye or measured with a photometer. Still it must be remembered that the plates used by Dr. DRAPER are sensitive to rays lower in the spectrum than 6, for example, and therefore represent the results of eye observations far more nearly than ordinary sensitive (wet) plates would do. The brightest part of the photograph is in the region about A, agreeing with all my own photometric work. D is faint; about as bright as E. My photometric results of 1880 give in the mean E m 0.95 D. The earlier ones make E relatively considerably fainter. My conclu- sion from my own observations was that E is now brighter with respect to D than in 1878, and from the whole series pf eye observations of all astronomers, that E has certainly increased in brightness. Formerly it was undoubtedly fainter than D. This was so from 1850 up to 1878 and even 1879. Now, by my photometer observations, and by the testimony of Dr. DRAPER'S photograph, it is about of the same brilliancy. In the case of the mass A, also, the photograph gives undeniable evidence of change. For quite a time A was not the brightest mass of the nebula ; .now, it cer- tainly is. In the photograph F is the brightest of the three masses F, G, H. My photometric observations agree with this. The mass N is the only one whose relative brilliancy is materially different in the photograph from its brilliancy as given in my own results. I attribute this in part, at least, to the effect of the proximity of the image of the star 685 to the mass N. This would alter the relative chemical effect of the masses near it and those far from any such influence, to some degree. It will be of interest to compare Dr. DRAPER'S photograph and Fig. 7, which gives a drawing made by myself through tourmaline plates, arranged so as to cut off the fainter portions of the Huyghenian region. The shape of the dark space bounded by E, F, and Gr, and I, is the same in both The Sinus Lamontii is alike in both ; the darker space, S, connecting T with the iS' Lamontii, is also similar, and so in other cases. The photograph, however, represents relatively far more light than Fig. 7, and if we were arranging our work in a seri, > in the order of light, we should put first BOND'S drawing (the frontispiece), then DRA- PER'S photograph (Fiff. 40), and lastly, Fig. 7. To comprehend the extreme faintnesi of some of the nebulous masses represented in the photograph a study should made of the Messierian branch, of the regio Picar<1iana, and especially of the mass of the Index Chart. The comparison of the photograph with the drawing of Lord Roi 230 MONOGRAPH OF THE CENTRAL PARTS OF THE NEBULA OF ORION. and of BOND (frontispiece), or with the Index-Map, should be made in order to appre- ciate their marked agreement. I have not given such comparisons in detail for the obvious reason that this pho- tograph comes as the beginning of a new epoch in such observations, and it will receive its proper discussion as the very first of a series of exact and automatic repre- sentations which we must thank the skill of Dr. DRAPER for inaugurating. I feel that the present memoir receives a new value in that it brings the work of the old period together for discussion, and leaves a clear field for the employment of the new, and far more satisfying methods. At the same time I must point out that the evidence to be derived from this photograph lends a great strength to the best previous drawings of the^ nebula, such as those of Lord ROSSE, BOND, and LASSELL. It has always been easy to object to drawings, estimates, and even photometric measures made on objects of so great faintness and difficulty as the nebulae; and the conclusions derived from such work have often been met with the criticism, easy to make and hard to answer that the personality of the observer was so much to be feared that such conclusions remained doubtful. I desire to emphasize the fact that all the important conclusions as to the present state of the nebula which I have derived from an examination of such drawings as those I have mentioned, are confirmed by this photographic repre- sentation, at least in so far as it is capable of giving any evidence at all, and that it comes to confirm, and not to destroy, our confidence in the faithful work of competent observers by the ordinary methods. ( U. S. NAVAL OBSERVATORY, Washington, D. C., December, 1880. ^ ( WASHBURN OBSERVATORY, Madison, Wisconsin, May, 1882. $ UNIVERSITY OF CALIFORNIA LIBRARY BERKELEY Return to desk from which borrowed. This book is DUE on th^last date stamped below. 12jun'50 P AUG 20 1953 LD 21-100m-ll,'49(B7146sl6)476 *s&- ' > S O LV Rffir >J H W : / : n &