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Besides these, brilliant comets appeared in 1859, 1860, and 1881. How long we may have to wait for another no one can say. It is probable that Halley's comet, when it appears eight or ten years hence, will at least be visible to the naked eye, but no one can predict even its apparent brightness. At its return in 1835 it was so small an affair that it was difficult to explain the excitement it caused in 1456 and later, except by supposing a great diminution in the dimensions, at least of its tail.

The question of the exact nature of a comet is still in doubt. In the case of large and bright comets, it is possible that the nucleus may be a solid body, though probably much smaller than it looks. Some light on the question is thrown by an observation, which is unique, made at the Cape of Good Hope when the great comet of 1882 made a transit across the sun's disk, as Mercury and Venus are sometimes known to do. Unfortunately, astronomers generally were not prepared for such a phenomenon, as the comet had been visible only in the southern hemisphere, and the transit occurred only a week or two after its first discovery. Hence it happened that the Cape Observatory was the only one at which an observation of the greatest interest in astronomy could be made; and here the circumstances were extremely unfavourable. The sun was about to set behind Table Mountain as the comet approached it. By careful watching, two of the astronomers, Messrs. Finlay and Elkin, were enabled to keep sight of the comet until it actually disap-