The astronomer about to seek for a new planet directs his telescope towards that part of the sun's path which is on the meridian at midnight; there, if anywhere, lies the chance of success, because that is the region in which such a body is nearer to the earth than at any other part of its course. He steadfastly compares his chart with the heavens, and usually finds the stars in the heavens and the stars in the chart to correspond; but sometimes it will happen that a point in the heavens is missing from the chart. His attention is at once arrested; he follows the object with care, and if it moves it is a planet. Still he cannot be sure that he has really made a discovery; he has found a planet, no doubt, but it may be one of the large number already known. To clear up this point he must undertake a further, and sometimes a very laborious, enquiry; he must search the Berlin Year-Book and other ephemerides of such planets and see whether it is possible for one of them to have been in the position on the night in question. If he can ascertain that no previously discovered body could have been there, he is then entitled to announce to his brother astronomers the discovery of a new member of the solar system. It seems certain that all the more important of the minor planets have been long since discovered. The recent additions to the list are generally extremely minute objects, beyond the powers of small telescopes. Since 1891 the method of searching for minor planets which we have just described has been almost abandoned in favour of a process greatly superior. It has been found feasible to employ photography for making charts of the heavens. A photographic plate is exposed in the telescope to a certain region of the sky sufficiently long to enable very faint telescopic stars to imprint their images. Care has to be taken that the clock which moves the camera shall keep pace most accurately with the rotation of the earth, so that fixed stars appear on the plate as sharp points. If, on developing the plate, a star is found to have left a trail, it is evident that this star must during the time of exposure (generally some hours) have had an independent motion of its own; in other words, it must be a planet. For greater security a second picture is generally taken of the same region after a short interval. If the place occupied by the trail on the first plate is now vacant, while on the second plate a new trail appears in a line with the first one, there remains no possible doubt that we have genuine indications of a planet, and that we have not been led astray by some impurity on the plate or by a few minute stars which happened to lie very closely together. Wolf, of Heidelberg, and following in his footsteps Charlois, of Nice, have in this manner discovered a great number of new minor planets, while they have also recovered a good many of those which had been lost sight of owing to an insufficiency of observations.