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III. in a sense, lost its individuality and become merged in the tail. Before the stage c is reached, it will often happen that a second envelope will have begun to rise as at a, so that two or even three envelopes, more or less concentric, may be visible at the same time, one inside another.

If, with the foregoing description clearly in his mind of the envelopes usually seen in the heads of large comets, the reader will turn to Chapter IX (post), and will examine the illustrations there given of the heads of the Comets of 1858 (vi.), 1861 (ii.), 1862 (iii.), and 1874 (iii.) in particular, he will have no difficulty in realising the features which generally present themselves in the heads of large comets; and which from time to time are described by different observers, under the varying terms of "jet", "fan", "luminous sector", "envelope", and so on. Bessel considered that the changes which he observed in the head of Halley's Comet in 1835 justified him in assuming that a systematic oscillation of the head and nucleus took place in the plane of the comet's orbit, almost amounting to a movement of rotation.

Three comets of recent date are noteworthy as having undergone tail transformations quite without precedent, though the credit of our knowledge respecting them is in part due to the assistance of photography, which has furnished records of changes more full and more accurate than eye observation could have done.

The first of these comets is Swift's Comet of 1892 (i.). On April 4, the tail was 20° long, bifid, straight, and slender. Between the 2 branches scarcely any cometary matter was visible. The next morning a new tail had appeared in the interspace, and each of the 3 main tails was found to be made up of several, side by side. At least a dozen distinct streaks of cometary matter could be counted. After the lapse of another day one of the original 3 tails had vanished and the other 2 had become blended. Then one of these brightened up and the other faded away. The bright one had a sharp