plished. In the first place, the series of observations thus arranged can be used, as a catalogue, for reference, and may serve the purposes of one, until a more perfect one can be produced-(valeat quantum). In the next place, the results so stated, carry with them their own weight and evidence. Where several observations of one and the same object occur, their agreement or disagreement will enable every one to assign to them their proper degree of credit,— to appretiate the amount of error, both accidental and inherent, to which the system of observation adopted is liable; and being thus impressed with a due notion of the degree of latitude with which each result is to be interpreted, he will readily perceive what reliance can be placed on single observations, unchecked by the context.

My mode of observing, the general character of the instrument employed, and the principal sources of error to which its determination of the places of objects is liable, are stated in considerable detail in my five catalogues of double stars discovered with it, published in the Memoirs of the Astronomical Society. To these, therefore, I will refer for the particulars in question: but it will be right here to mention, that a much greater latitude of error must unavoidably subsist in observations of nebulæ than in those of stars. Many of these objects present a large and ill-defined surface, in which it is not always easy to say where the centre of greatest brightness is situated. Vast numbers of them are so extremely faint as to be with difficulty discerned at all, or not until they have been some time in the field of view, or are even just about to quit it. In such cases the observations become hurried and uncertain; and this peculiar and fertile source of error and mistake is greatly increased by their excessively irregular distribution over the heavens,-crowded together in some places so as to allow hardly any interval between their transits,—while in others whole hours elapse without a single nebula occurring in the zone of the heavens under examination. In these crowded parts of the heavens, it is not only the number, but the variety and interest of the objects which distract attention and render it scarcely possible to proceed with that methodical calmness and regularity which is necessary to ensure numerical correctness, especially when the observer has continually present to his mind the rarity of his opportunities. It is only in the months of March, April, and May that the richer parts of the heavens can be advantageously observed, and

then only in the complete absence of the moon, and of twilight. When to these conditions we add those which arise from the variable and uncertain nature of our climate, it will be seen that a number of circumstances by no means frequently concurring, is necessary to produce a night in which it is possible to make any great progress in a review of nebulæ; and that in fact there is hardly any branch of astronomy which has a greater tendency to create a sense of hurry, of all things the most fatal to exact observation.

The observations which are here registered comprise, more or less, about 2500 nebulæ and clusters of stars, which is the number of those observed by my father; but of the two collections, only about 2000 are common to both, the remaining 500 of mine being new. It may serve to show the close and rigorous nature of my father's scrutiny, when I state, that among these 500 I can call to mind only one very conspicuous and large nebula, and only a very few entitled to rank in his first class, or among the "bright nebulæ." By far the greater proportion of them are objects of the last degree of faintness, only to be seen with much attention and in good states of the atmosphere and instrument. This is so far satisfactory, that it shows pretty evidently our knowledge of the nebulous contents of the northern hemisphere to be at length nearly complete, and that to make a further step, the powers of an instrument like the 40-feet reflector will be required.

It has generally been my practice to make some kind of sketch or drawing, sometimes more, sometimes less elaborate, of any nebula at all remarkable which presented itself. In many instances careful drawings have been made and repeatedly compared, at distant intervals, with the objects they have been intended to represent. The difficulty of making satisfactory representations of such objects is, however, extreme; and of those which accompany this paper, as specimens of the various classes of objects which the heavens present, I am rather disposed to apologize for the incorrectness than to vaunt the accuracy. General resemblance, however, I can vouch for; and where fidelity in details has been attempted, I should hope that glaring departures from nature will hardly be found to subsist. Among the figures will be found representations of some very extraordinary objects, which have not hitherto been sufficiently pointed out to the notice of astronomers, and of which indeed some of the

most remarkable peculiarities have escaped every former observer. These are briefly noticed in the descriptions appended to each observation, and more distinctly and at large in the explanations of the plates. Meanwhile it is only necessary for any reader to cast his eye over the figures, to be satisfied that many of these mysterious objects possess a symmetry of parts, and a unity of design, which, singular as their constitution must appear, strongly mark them as systems of a definite nature, each complete in itself, and subservient to some distinct purpose of which it is in vain for us to conjecture the nature.

The manner in which the observations are presented in the following pages requires little explanation. The first column contains a general number for convenience of future reference; the second, the number or name under which each nebula is to be found, either in my father's catalogues (in which case his class and number is set down,) or in the brief but important list furnished by MESSIER in the Mémoires de l'Académie des Sciences for 1771, and the Connoissances des Temps for 1783 and 1784, which are indicated by the letter M. When no previous mention of a nebula or cluster could be found in the writings of former observers, it is designated as Nova. A very few are due to STRUVE. These are noted by Z.

The third column contains the right ascension, and the fourth the north polar distance, for 18300. The manner in which these elements are obtained from the rough observations, or the process of reduction employed throughout the work, will be fully stated in an Appendix annexed to the observations, as well as other matters of detail which may be advantageously deferred. Under the head of each nebula, as above mentioned, all its observations are brought together, and for distinctness' sake, the hour and minute in R, and the degree in N.P.D. are only once set down for each, viz. in the upper line, or that which contains the earliest R of each. When the subsequent observations in Æ fall into the next minute, the seconds are counted on beyond 60; and when those in polar distance pass into the next degree, the same contrivance, i. e. counting the minutes beyond 60, is resorted to, to avoid doubling the initial entries and so misleading the eye. When no R or no P.D. has been observed, the fact is indicated by dotted lines. When only a rough approximation has been obtained (a case unfortunately too common), or when, none having been procured, the

necessary datum has been taken from the working list book, this is indicated by the sign. What is here meant by the working list will be explained in the Appendix.

The column headed "Description and Remarks" contains the abbreviated description taken down, at the time of observation, from the sweeping book. The following is the system of abbreviation used.

In all the descriptions the following order is, or ought to be, adhered to. First the degree of brightness is mentioned, characterized by e B, v B, pB, B, pF, F, v F, e F. Next, the size, generally indicated by L and S with their adverbial adjuncts e, v, p. Thirdly, the general form, as R, E, mE, &c. Fourthly, the degree and rate of condensation, as b M, sb M, vglb M, &c. If the nebula be resolvable, it is next expressed by r. After these come more distinct numerical particulars, as the estimated diameter, indicated by the number of seconds it was judged to subtend; the measured, or estimated length, breadth, or position with the meridian, &c.; and any other remarks which may have appeared desirable to note at the time. Clusters are characterized as Rich, or poor, or compressed, loose, or scattered, &c. For example:

"vB; vL; 1E; vgpmb M; 50"1; 45" br; pos 29°3 by microm. a * 9m dist 80"." Which expanded runs thus:

45° np

"Very bright; very large; little extended; very gradually pretty much brighter in the middle; 50′′ long; 45" broad; angle of position (reckoned always from the north, and from a meridian, in the direction north, following, south, preceding,) measured 29°3 by the micrometer. A star of the ninth magnitude is situated 45° north preceding the centre of the nebula, and at a distance from it of 80", (both by estimation)." Measured angles are always marked as above, in degrees and decimals.

Finally, the last column contains the sweep in which the observation occurs, numbered regularly on in order of time up to sweep 427, with only one interruption of the order, viz. that the sweeps marked 43,. . . . 49 precede sweep 1 in order, having been written down on separate sheets in the year 1823, before the commencement of my more regular plan of observation, and mislaid.