Under the action of the solar forces, the coma also very frequently forms itself into concentric luminous arcs, separated by comparatively dark intervals. These luminous semi-circles vary in number, but sometimes there are as many as four or five at a time. All great comets show these concentric curves more or less, but sometimes only a portion is visible, the rest of the coma having a different structure. When great comets approach near the Sun, their coma is generally composed of two distinct parts, an inner and an outer coma, the inner one being due to the luminous jets issuing from the nucleus, which, never extending very far, form a distinct, bright zone within the fainter exterior coma. The tails of comets, which are in fact a prolongation of the coma, are likewise extremely variable in form. They are sometimes straight like a rod; again, are curved like a sabre, or even crooked like an S, as was that of the comet of 1769. They are also fan-shaped, pointed, or of the same width throughout. Many of these appendages appear longitudinally divided through their middle by a narrow, darkish rift, extending from the nucleus to the extremity. This peculiarity appears in the comet shown on Plate XI. Sometimes the dark rift does not commence near the nucleus, but at some distance from it, as I observed in the case of comet 1881, III., on June 26th. This dark rift is not a permanent feature of a comet's tail, but may be visible one day and not at all the next. Comet 1881, III., which had shown a dark rift towards the end of June, did not exhibit any such rift during July and August, when, on the contrary, its tail appeared brighter in the middle. Coggia's comet, which showed so prominent a dark rift in July, 1874, had none on June 10th. On the contrary, the tail was on that date very bright along its middle, as also along each of its edges. The tail of a comet does not invariably point directly away from the Sun, as above mentioned, and sometimes the deviation is considerable; for instance, the tail of the comet of 1577 deviated 21° from the point opposite to the Sun.