Letter to Professor Stokes, Sec. R.S., containing Observations made at Malta on a Planetary Nebula

By William Lassell, Esq., F.R.S.

Malta, 26th Sept. 1862.

My dear Sir, – In directing my large equatoreal upon the well-known planetary nebula situated in AR 20^h 56^m N.P.D. 101° 56' (1862), it has revealed so marvellous a conformation of this object that I cannot forbear to send you a drawing of it, with some description of its appearance. With comparatively low powers, e.g. 231

and 285, it appears at first sight as a vividly light-blue elliptic nebula, with a slight prolongation of the nebula, or a very faint star, at or near the ends of the transverse axis. In this aspect the nebula resembles in form the planet Saturn when the ring is seen nearly edgewise. Attentively viewing it with higher powers, magnifying respectively 760, 1060, and 1480 times, and under the most favourable circumstances which have presented themselves, I have discovered within the nebula a brilliant elliptic ring, extremely well defined, and apparently having no connexion with the surrounding nebula, which indeed has the appearance of a gaseous or gauze-like envelope, scarcely interfering with the sharpness of the ring, and only diminishing somewhat its brightness. This nebulous envelope extends a little further from

the ends of the conjugate than from the ends of the transverse axis; indeed, it is but very faintly prolonged, and only just traceable towards the preceding and following stars. There is a star near its border, northwards, in the projection of the conjugate axis.

The breadth or thickness of the ring is, unlike that of Saturn, nearly uniform, or equal in every part, so that its form most probably is either really elliptic, and seen by us in a line nearly perpendicular to its plane; or, if really circular, and seen foreshortened, a section throughout any part of it, limited by the internal and external diameters, must be a circle. In other words, it will be like a circular cylinder bent round. It could scarcely fail to bring to my mind the annular nebula in Lyra, especially as there is a conspicuous central star (proportionally, however, much brighter than that which is in the centre of that nebula), and yet the resemblance is only rudely in form, for this ring is much more symmetrical and more sharply defined, suggesting the idea of a solid galaxy of brilliant stars.

The ring is not perfectly uniform in brightness, the south-preceding part being slightly the most vivid. The transverse axis is inclined to the parallel of declination about 13°. A series of micrometrical measures of the length and breadth of the ellipse gives a mean of 26.2" for the transverse, and 16.6" for the conjugate axis.

The accompanying drawing has not been at all corrected by these measures, but is the result of several sketches made during different observations, and is a faithful transcript of the appearance of the nebula to my eye, when most favourably seen.

The object is, as may be supposed, one of extreme difficulty, requiring in the highest degree the combination of light and definition in the telescope, and a favourable state of atmosphere, – which will further appear when I state that it was not until I was favoured with an unusually fine night, and had applied a power of 1480, that the whole of the details were brought out.

I confess I have been greatly impressed by the revelation of this most wonderful object, situated on what perhaps we may consider as the very confines of the accessible or recognizable part of the universe, affording ground for the inference that more gorgeous systems exist beyond our view than any we have become acquainted with.

I am, &c.,

WILLIAM LASSELL.