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 gleam of these small stars; indeed a few feet of mist would have more power of obstructing the stellar light than cometary material scores of thousands of miles thick.

It is true that the central portions of many of these comets often exhibit much greater density than is found in the exterior regions. Still, in the great majority of such objects, there is no opacity even in the densest part sufficient to put out a bright star. In the case of the more splendid bodies of this description, it may be supposed that the matter is somewhat more densely aggregated as well as more voluminous. Still, however, it will be remembered that the great comet of 1858 passed over Arcturus, and that the star was seen shining brilliantly notwithstanding the interposition of a cometary curtain millions of miles in thickness. So far as I know, no case is known in which the nucleus of a really bright and great comet has been witnessed in the act of passage over a considerable star. It would indeed be extremely interesting to ascertain whether in such a case the star experienced any considerable diminution in its lustre.

There is a delicate method of testing the quantity of matter which a comet might possess in the vicinity of its nucleus. If there were any substantial quantity of gaseous matter, it is plain that though the rays from the star might not be altogether extinguished, or might not even be largely reduced in lustre, yet the apparent direction of the star must be deflected from its course by refraction through the comet. It is well known that when a telescope is used to observe a star, it is not generally pointed exactly in the direction of the star, because the light which it is sought to observe has been bent in, its passage through