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Rh the stars generally; and it is not only a matter of interest but of great importance. Let me take an instance from this room. You know how the front row is a little bit shorter than the next row and won't seat so many people; the row behind will seat a few more, and so on till we get to the back row, which will seat a good many. If we knew the distance between the rows we could count how many people there ought to be in each succeeding row. And it is very much the same in the case of the stars. If we count the bright stars, we know how many there ought to be of the next order of magnitude; and how many there should be next after that. Suppose we count them, and there are not so many—suppose we counted the outside row and found there were not so many people as it would hold, we would say, "Why this audience is beginning to thin off at the back!" So with the stars, we are inclined to think from the counts we make that they begin to thin off in the distance; and perhaps if we go far enough we should not find any more stars. Look, for instance, at this table of the numbers of stars of successive magnitudes. Up to and including the second magnitude (let us say, the front two rows), there are 38 stars, and we can calculate (though I will not bother you with the details) that when we add the third row, or magnitude, we ought to have 151, very nearly four times as many; but we only find in. Adding the next row, or magnitude, if 38 is right, we ought to get 603, but only find 300, and so on.