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sand. Possibly a very keen eye might see more than six thousand, but most eyes will see even less than five thousand. Of these only one half can be above the horizon at the same time, and of this half a great number will be so near the horizon as to be obscured by the great thickness of the atmosphere in that direction. The number which can readily be seen on a clear evening by an ordinarily good eye will probably range between fifteen hundred and two thousand. Stars visible to the naked eye are called lucid stars, to distinguish them from telescopic stars, which can be seen only by the aid of a telescope.

It is impossible to make even an estimate of the total number of telescopic stars. It is commonly supposed that between fifty and one hundred million can be seen with large telescopes, and it is now possible, with specially arranged telescopes, to photograph stars which are fainter than the smallest the eye can see in any telescope. There is no sign of any limit to the number. As we pass to fainter and fainter degrees of brightness the stars are found to be more and more numerous. All that we can say of the total number is that it must be counted by hundreds of millions.

We have, in fact, some reason for inferring that the great majority of the stars are invisible in the most powerful telescope we can make, owing to their distance. The distance of the great majority is such that only the brightest of them can become known to us.

Minute stars are here and there collected into clusters in various parts of the sky. Some of these clusters are