M. Hoek of Utrecht has found cases in which the orbits of two or more comets exhibit a common point of intersection in distant space, and infers their derivation from the same origin.
_Further Observations required._—One of the chief and essential features in cometary work is the accurate determination of positions. But this entails the possession of expensive instruments, and a knowledge which amateurs have not always acquired. This department of labour can well be left to the trained hands at large observatories, where, fortunately, it meets with every attention. Ordinary observers will merely require to know the approximate place, and this is to be found by estimating the difference in R.A. and Dec. between a comet and a known star. The position of the latter may be found in a good catalogue and corrected for precession; then, allowing for the observed differences, the comet’s place may be assigned to within very small limits of error. A low power, embracing a field of 1° or more, is best adapted for these observations, as it is more likely to include a catalogued star, and will exhibit the comet, especially if a large one, to the best effect.
The announcement of a new comet is always read with avidity by amateurs, and their first desire is to see it for themselves. This they may readily do by marking its place on a star-map or globe, and noting its relative place amongst the stars near. The telescope should then be directed towards the point indicated, and if the comet is not presented in the field, the instrument should be moved a little so that the surrounding region may be examined. If failure still attends the effort, the observer should point the telescope a few degrees E. or W. of the suspected point, and then carefully sweep over the place of the comet. It will then be picked up, unless it is too faint for his aperture. The first announcement of a comet generally gives the position at discovery, and the daily rate and direction of motion. The latter must of course be allowed for when the search is instituted.
The physical aspects of comets are as diversified as they are variable. No two comets are exactly alike, nor does the same comet exhibit a permanency of detail. Of course, when these objects are enormously distant, and barely visible, many of them appear to present similar characteristics; but under the closer and more expanded views obtainable near perihelion the resemblance vanishes, and every comet is seen to possess features peculiar to itself. To trace these features, and to record them by delineation and description, forms one of the most interesting branches in which amateurs may engage. Much has been learnt of previous comets by successively noting their transitions of form and brightness, and the same scrupulous attention should be given to future comets.