You will see the great variety characterising these discoveries; some of them are the result of deliberate search, others have come accidentally, and some even contrary to expectation. Of the great majority of the earlier ones it may be said that enormous diligence was required for each discovery; to identify a planet it is necessary to have either a good map of the stars or to know them thoroughly, so that the map practically exists in the brain. We need only remember Hencke's fifteen years of search before success to recognise what vast stores of patience and diligence were required in carrying out the search. But of late years photography has effected a great revolution in this respect. It is no longer necessary to do more than set what Sir Robert Ball has called a "star-trap," or rather planet-trap. If a photograph be taken of a region of the heavens, by the methods familiar to astronomers, so that each star makes a round dot on the photographic plate, any sufficiently bright object moving relatively to the stars will make a small line or trail, and thus betray its planetary character. In this way most of the recent discoveries have been made, and although diligence is still required in taking the photographs, and again in identifying the objects thus found (which are now very often the images of already known members of the system), the tedious scrutiny with the eye has become a thing of the past. TABLE SHOWING THE NUMBER OF MINOR PLANETS DISCOVERED IN EACH DECADE SINCE 1850. 1801 to 1850--altogether 13 discoveries. 1851 to 1860-- " 49 " 1861 to 1870-- " 49 " 1871 to 1880-- " 108 " 1881 to 1890-- " 83 " 1891 to 1900-- " 180 announcements In 1901 " 36 " " 1902 " 50 " " 1903 " 41 " --- Total 609 [_N.B._--Many of the more recent announcements turned out to refer to old discoveries.] [Sidenote: Scarcity of names.]