THE night of August 13-14, 1898, was an important date in the history of astronomy. In that night, nearly a hundred years after the discovery of the first asteroid (in the first night of the nineteenth century) Dr. Gustav Witt, thendirector of the Urania Observatory in Berlin, with the assistance of the writer, discovered the little planet Eros, which possesses a unique importance for astronomers. The paths^of the asteroids lie between the orbits of Mars and Jupiter, which are so distant from each other that the existence of an intermediate planet had long been suspected, and such a planet was also required to complete the series of the planets, in accordance with the law of Bode and Titius. All attempts to fill this gap failed until January 1, 1801, when Piazzi discovered the small planet Ceres, the orbit of which was computed by Gauss, and found to lie between the orbits of Mars and Jupiter. Within a few years three other asteroids, Pallas, Juno, and Vesta, were discovered and 323 of these minor planets were known in 1891. Then Dr. Max Wolf commenced a systematic search for asteroids by a photographic method, based on their motion relative to the stars. In a photograph made with an equatorial telescope and a long exposure the fixed stars appear as points, amid which the image of an asteroid or other planet is conspicuous as a short line. This method, though not free from practical difficulties, proved very fruitful. Wolf discovered more than a hundred asteroids by its aid and Charlois also employed it very successfully. A few years later Dr. Witt adopted the photographic method. In 1896 he discovered an asteroid which he named Berolina in honor of the city of Berlin, and in 1898 he made a second discovery which is more important than those of all the asteroids. He and I made photographs nearly every night in August, doing all the work ourselves. On the night of August 13-14 we maae a special photographic search for the asteroid Eunike, or No. 185, which had not been observed since 1889 and which should be found, according to its computed orbit, in the vicinity of the star Beta, in Aquarius. The plate was exposed nearlytwo hours and was immediately developed, in accordance with our custom, so that in the morning it was dry and ready for examination. We found the traces of Eunike and another known asteroid, Althrea, but we also found a third faint and indistinct trace, apparently made by a body in very rapid motion, for the length of the mark was about 0.016 inch, twice the length of the other traces. At first we thought that we had discovered a comet. On the following night the great refractor of the observatory was directed to the spot and a star-like object of the tenth or eleventh magnitude was revealed. The reduction of the observations showed an apparent motion much more rapid than that of any known asteroid, but, as the object was evidently not a comet, it was classed as an asteroid. Its remarkable speed caused it to be studied by many astronomers and within a short time numerous observations were available for the determination of the orbit. The calculation resulted in the surprising discovery that the new planet could not be a member of the known group of asteroids, as its orbit lay almost entirely inside the orbit of Mars. Eros approaches the earth more closely than any other body except the moon. It is a remarkable circumstance that it was discovered when it was near its aphelion and consequently in conditions very unfavorable for observation. A few years earlier it must have been very near the earth and visible to the naked eye but, apparently,nobody saw it. After the orbit had been determined its traces were found on photographs of that period, but it had never been recognized as a planet. Chance played a part in our discovery. Prof. Char- lois, in Nice, photographed the same region on the same night but did not develop his plate at once. When the news of our discovery reached him he developed the plate and found the trace of Eros. Dr. Witt's good fortune is the more remarkable as we were working with very crude apparatus. Our photographic telescope was made up of parts of several instruments, roughly patched together. The cross-wires were Illuminated by a small electric bulb, inclosed in a box which was made of pieces of cigar boxes, and was tied to the tube with string. At first the box was covered with a cloth cap, belonging to another instrument, in order to protect our eyes from the heat and light of the lamp, but one night this cap caught fire and thereafter we got along without it. The driving clock, which should have compensated the earth's rotation and kept the telescope directed to the same point of the sky, was a masterpiece, for it defied mastery by the most skillful mechanics. Only the habit of years enabled it to turn the telescope at all, at least in the positions in which we commonly used it, and it did not run correctly for five seconds at a time, so that it was scarcely safe to remove the eye from the telescope or the hand from the regulating wheel during the exposure, which was never shorter than two hours! Dr. Witt gave an amusing description of our difficulties at a meeting of astronomers at the observatory in the following winter, when the new planet Eros was christened with a “Salamander,” according to good old academic custom. The importance of Eros to astronomy is due to its smallness and its close approximation to the earth at certain epochs. It is too small to be measured directly, but its diameter is inferred from photometric measurements to be about 10 miles. Yet great hopes have been founded on this tiny planet. At times Eros comes so near to the earth that its distance can be measured with very great accuracy, and from the distance thus obtained the distance of the sun and all the dimensions of the solar system can be determined with greater exactness than is attainable by any other method. In 1892, before we knew of the little planet's existence, it must have come within 15 million miles of us. The next near approach will occur in 1931, when the distance between Eros and the earth will be less than 13% million miles—about half the distanceof Venus, when in transit over the sun's disk. Observations of transits of Venus have been mainly relied on for the determination of the earth's distance from the sun, which is the unit of astronomical measurement, but there will be no transit of Venus until 2004—and then it may rain! Eros exhibited remarkable fluctuations in luminosity during its fairly close approximation to the earth in the winter of 1900-1901. These variations gave rise to the most diverse conjectures concerning the physical condition of the little planet. Some astronomers pictured it as a double or binary planet, analogous to the binary stars, others gave it an irregular form, the different parts of which would reflect the sun's light unequally, etc. The peculiar phenomenon gradually disappeared and its cause remains a mystery.— Translated for the Scientific American Supplement from Gartenlaube