For a good many years the discoveries of asteroids were made by means of the telescope. When, in the course of searching the heavens, an object was noticed which did not appear upon any of the recognised star charts, it was kept under observation for several nights to see whether it changed its place in the sky. Since asteroids move around the sun in orbits, just as planets do, they, of course, quickly reveal themselves by their change of position against the starry background. The year 1891 started a new era in the discovery of asteroids. It occurred to the Heidelberg observer, Dr. Max Wolf, one of the most famous of the hunters of these tiny planets, that photography might be employed in the quest with success. This photographic method, to which allusion has already been made in dealing with Eros, is an extremely simple one. If a photograph of a portion of the heavens be taken through an "equatorial"--that is, a telescope, moving by machinery, so as to keep the stars, at which it is pointed, always exactly in the field of view during their apparent movement across the sky--the images of these stars will naturally come out in the photograph as sharply defined points. If, however, there happens to be an asteroid, or other planetary body, in the same field of view, its image will come out as a short white streak; because the body has a comparatively rapid motion of its own, and will, during the period of exposure, have moved sufficiently against the background of the stars to leave a short trail, instead of a dot, upon the photographic plate. By this method Wolf himself has succeeded in discovering more than a hundred asteroids (see Plate XIII., p. 226). It was, indeed, a little streak of this kind, appearing upon a photograph taken by the astronomer Witt, at Berlin, in 1898, which first informed the world of the existence of Eros. [Illustration: PLATE XIII. MINOR PLANET TRAILS Two trails of minor planets (asteroids) imprinted _at the same time_ upon one photographic plate. In the white streak on the left-hand side of the picture we witness the _discovery_ of a new minor planet. The streak on the right was made by a body already known--the minor planet "Fiducia." This photograph was taken by Dr. Max Wolf, at Heidelberg, on the 4th of November, 1901, with the aid of a 16-inch telescope. The time of exposure was two hours. (Page 227)] It has been calculated that the total mass of the asteroids must be much less than one-quarter that of the earth. They circulate as a rule within a space of some 30,000,000 miles in breadth, lying about midway between the paths of Mars and Jupiter. Two or three, however, of the most recently discovered of these small bodies have been found to pass quite close to Jupiter. The orbits of the asteroids are by no means in the one plane, that of Pallas being the most inclined to the plane of the earth's orbit. It is actually three times as much inclined as that of Eros.