(Plate XXVI.). [Illustration: PLATE XXVI. 1 2 Photographs of Swift's Comet. By Professor E. E. Barnard. 1. Taken April 4, 1892; exposure 1 hour. 2. Taken April 6, 1892; exposure 1 hour 5 minutes.] Anyone who desires to become a comet-hunter must, in addition to the possession of the supreme requisites, patience and perseverance, provide himself with an instrument of at least 4 inches aperture, together with a good and comprehensive set of star-charts and the New General Catalogue of nebulæ with the additions which have been made to it. The reason for this latter item of equipment is the fact that many telescopic comets are scarcely to be distinguished from nebulæ, and that an accurate knowledge of the nebulous objects in the regions to be searched for comets, or at least a means of quickly identifying such objects, is therefore indispensable. The portions of the heavens which afford the most likely fields for discovery will naturally be those in the vicinity of where the sun has set at evening, or where he is about to rise in the early morning, all comets having of necessity to approach the sun more or less closely at their perihelion passage. Other parts of the heavens should not be neglected; but these are the most likely neighbourhoods. Most of us, however, will be content to discover our comets in the columns of the daily newspaper, or by means of a post-card from some obliging friend. The intimation, in whatever way received, will generally contain the position of the comet at a certain date, given in right ascension and declination, and either a statement of its apparent daily motion, or else a provisional set of places for several days ahead. Having either of these, the comet's position must be marked down on the star-map, and the course which it is likely to pursue must be traced out in pencil by means of the data--a perfectly simple matter of marking down the position for each day by its celestial longitude and latitude as given. The observer will next note carefully the alignment of the comet with the most conspicuous stars in the neighbourhood of the particular position for the day of his observation; and, guiding his telescope by means of these, will point it as nearly as possible to that position. He may be lucky enough to hit upon his object at once, especially if it be a comparatively bright one. More probably, he will have to 'sweep' for it. In this case the telescope must be pointed some little distance below and to one side of the probable position of the comet, and moved slowly and gently along, careful watch being kept upon the objects which pass through the field, until a similar distance on the opposite side of the position has been reached. Then raise the instrument by not more than half a field's breadth, estimating this by the stars in the field, and repeat the process in the opposite direction, going on until the comet appears in the field, or until it is obvious that it has been missed.