Al-Sufi's descriptions of the stars were, it is true, based on Ptolemy's catalogue, but his work is not a mere translation of that of his predecessor. It is, on the contrary, a careful and independent survey of the heavens, made from his own personal observations, each of Ptolemy's stars having been carefully examined as to its position and magnitude, and Ptolemy's mistakes corrected. In examining his descriptions, Schjellerup says, "We soon see the vast extent of his labours, his perseverance, and the minute accuracy and almost modern criticism with which he executed his work." In fact, Al-Sufi has given us a careful description of the starry sky as it appeared in his time, and one which deserves the greatest confidence. It far surpasses the work of Ptolemy, which had been without a rival for eight centuries previously, and it has only been equalled in modern times by the surveys of Argelander, Gould, Heis, and Houzeau. Plato remarked with reference to the catalogue of Hipparchus, _Cœlam posteris in hereditatem relictum_, and the same may be said of Al-Sufi's work. In addition to his own estimates of star magnitudes, Al-Sufi adds the magnitudes given by Ptolemy whenever Ptolemy's estimate differs from his own; and this makes his work still more valuable, as Ptolemy's magnitudes given in all the editions of the _Almagest_ now extant are quite untrustworthy. In the preface to his translation of Al-Sufi's work, Schjellerup mentions some remarkable discrepancies between the magnitudes assigned to certain stars by Ptolemy and Argelander. This comparison is worthy of confidence as it is known that both Al-Sufi and Argelander adopted Ptolemy's (or Hipparchus') scale of magnitudes. For example, all these observers agree that β Ursæ Minoris (Ptolemy's No. 6 of that constellation) is of the 2nd magnitude, while in the case of γ Ursæ Minoris (Ptolemy's No. 7), Ptolemy called it 2nd, and Argelander rated it 3rd; Argelander thus making γ one magnitude fainter than Ptolemy's estimate. Now, Al-Sufi, observing over 900 years ago, rated γ of the 3rd magnitude, thus correcting Ptolemy and agreeing with Argelander. Modern photometric measures confirm the estimates of Al-Sufi and Argelander. But it is, of course, possible that one or both stars may be variable in light, and β has actually been suspected of variation. Almost all the constellations afford examples of this sort. In the majority of cases, however, Al-Sufi agrees well with Argelander and Heis, but there are in some cases differences which suggest a change in relative brightness. Among other remarkable things contained in Al-Sufi's most interesting work may be mentioned the great nebula in Andromeda, which was first noticed in Europe as visible to the naked eye by Simon Marius in 1612. Al-Sufi, however, speaks of it as a familiar object in his time. Schjellerup says--