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 CHAPTER XV.

COMETARY STATISTICS.

are usually supposed to be distasteful to the general reader, and I shall not here submit any very elaborate ones; and those which are presented may be passed over altogether if the reader likes, for they are not essential to a study of the subject of comets from a descriptive or observational point of view. And, indeed, there is another reason why they may be pardonably neglected: for if the truth must be told, many of those which appear in this chapter are in a certain sense untrustworthy. A moment's reflection will make it plain why such should be the case. Comets have, at the best, very ill-defined boundaries; and as a large telescope will reveal a greater spread of cometary material than a small one, it follows, as a matter of course, that dimensions measured in a small telescope may differ very much indeed from those obtained by means of a large telescope. Discrepancies are bound, therefore, to occur in the measurements arrived at by different observers using, even on the same day, telescopes of different optical power.

The same remark applies to the differences which exist between the sensitiveness of the eyes of different observers, a fact which very often becomes very pronounced, when it is