25
Of the Great Nebula in the Sword-handle of Orion.
(54) It may easily be supposed that in a situation so favourable for viewing thismagnificent object as the Cape, where it passes the meridian at an altitude of 60°, with theadditional advantages of a sky of perfect purity, and of mirrors in a constant' course ofrepolishing, I should eagerly seize the opportunity to re-examine my earlier delineation of it,with a view to the detection of change, the correction of error, and the observation of furtherparticulars as to its form, extent, and structure which had escaped previous observation.Although considerable pains had been taken with my figure of 1825 (engraved in the 2nd vol.of the Memoirs of the Astronomical Society) to express the gradations of light and the generalform of the nebula and its principal branches, and although in both these respects that figure,taken altogether, may, I believe, fairly claim to be considered more correct than any other ofthe same object which has yet appeared,*' as well as more exact in many of its details; yetthe first glance obtained of it under these more favourable circumstances sufficed to convinceme of the necessity of executing a redelineation of it, based upon a micrometric survey andprojection of the stars contained within its area, and comprehending a multitude of nebulousbranches, convolutions and other details, of whose existence I had never before had the leastsuspicion. The figure of 1825 was executed without the aid of micrometric measurements, or
* I am aware but of four representations of this nebula which have appeared 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, thoughrather a coarsely executed figure, and confined solely to the denser part of the nebula, or those regions whichI have termed the Front, Occiput , and Fauces, yet contains some valuable particulars respecting the apparentbreaking-up of the nebula (especially about the front and occiput) into patches and knots; particulars veryunsatisfactorily expressed in my figure of 1824, but in which my observations of 1834 and 183 7 fully confirmDr. Lamont’s remarks. In his figure he has (perhaps intentionally) omitted to express the remarkable
effusion of the nebula from the “ Front” 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 disagreedecidedly. The two figures of Sig. Rondoni, which are given in the Report of Observations made at theCollegio 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 ofthe nebula (such, at least, as I have ever seen it), which they resemble in fact hardly more than they do
one another. Nor should I have thought it necessary to do more than simply mention them, were it not
that one of them has been referred to by Mr. Hunt, in his recent work entitled “ Researches on Light,” asan instance of direct photographic representation impressed on a lithographic stone by the light of the nebulaitself. Were this the case, the high authority which a self-impressed picture would claim must necessarilylead to the absolute rejection both of Dr. Lamont’s and my own figures, or else to the conclusion of changesboth in the nebula itself, and in the situations, relative brightnesses, and nebulous appendages of the starsadjacent to it, of a very extraordinary and capricious kind. In fact, however, the inscription on the marginof Sig. Rondoni’s figure simply expresses (as I understand it) that his original drawing (probably an eye-draft)was subsequently transferred to a surface of stone by a photographic process. I purposely avoid all com-ment on the remarks which accompany these two representations, leaving astronomers to form their ownjudgment on them. The other representation above alluded to is that of Sig. Devico himself, in the year1839, printed in the Annals of the Collegio Romano for 1838, which, though much less inaccurate in manyrespects than Sig. Rondoni’s, is by no means free from objection on that score.
H