The following sheets contain the results of observations begun about the year 1825, and prosecuted with more or less assiduity from that time up to the commencement of the present year, in pursuance of a design to review the nebulæ and clusters of stars discovered by my father; and perhaps, in so doing, to add to their number, and to extend in some degree our knowledge of the nature and physical construction of that mysterious and interesting class of bodies. It was my original intention to have deferred the publication of these observations until I should have been able to have presented their results to the Royal Society in the more complete form of a general catalogue of nebulæ and clusters visible in this latitude; in which all my father’s nebulæ should have been included, and their places determined by at least two observations. To have done this, however, would have required several years’ additional work; and the want of an extensive list of nebulæ arranged in order of right ascension, having, since the recent improvements in the achromatic telescope, and the increased assiduity of astronomers in the detection and observation of comets, become continually more pressing, and the deficiency more and more complained of, I have thought it on the whole a preferable course to supply that deficiency so far as I am able, not by the production of a catalogue pretending to a precision and a completeness I am unable yet to give it, but by simply stating the individual results of such observations as I have hitherto made; with no other preparation than that of reducing them all to a common epoch, arranging them in order of right ascension, and bringing together, in every case where the same object has been more than once observed, all the observations of it which occur. By so doing, two distinct ends are accomplished. 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.
On the geometry of baselines suitable for UT1 estimation with VLBI Intensive sessions