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might in a very big hall—front rows full, middle rows rather scattered, back rows quite empty. It does not need a great stretch of our imagination to think of a cluster of stars of this kind, for we have many instances in the sky; Fig. 85 is a picture of one for you to look at. Looking at them from outside

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Fig. 85.—The Star Cluster in Hercules.

we can very easily see the way in which they thin out. When we ourselves are inside such a cluster, it is not so easy; but by careful counting of the different classes of stars, and reasoning upon the counts, we may be able to do it still.

But we must be careful, and there is especially one mistake of which we must be careful; a possibility of mistake at which I have already hinted. There