der, at Bonn, sets down in his charts no less than 3,256 stars visible to the naked eye; and Heis, whose eye indeed was possessed of an abnormal power, seeing stars as points without rays, increased Argelander's list by 2,000 stars visible at Münster. Thus, not taking into the account the no less than twenty more degrees of the heavens visible from Alexandria than from Germany, the ancients noted hardly one-half of the stars which were visible to them! The defectiveness of their observations can be more easily understood from the fact that for instance they reckoned 474 stars of the fourth magnitude, only 271 of the fifth, and finally only 49 of the sixth magnitude; whereas the fact is, that the number of stars increases so rapidly in the order of magnitudes that each succeeding class embraces a much larger number than all the classes that precede it. In our latitudes Argelander makes out with the naked eye nineteen nebulæ and star-clusters, while Hipparchus mentions only two, and Ptolemy five, neither of them noting such prominent objects as the nebula in Orion and that in Andromeda. And such defective knowledge as this of the open-lying heavens persisted long after the invention of the telescope—for full 1,500 years. Among the old astronomers the Persian, Abdalrahman-Al-Sûfi, who lived in the tenth century, forms a notable exception; but he did not inspire his contemporaries or his successors with his own ardor, or prompt them to add to his labors.
The same is to be said of the southern heavens. The Arabians, surely, did not lack opportunity for acquiring a knowledge of many of its constellations. Ever since the days of Bartolomeo Diaz, it was a necessity for Europeans, on sea-voyages, to determine places by southern constellations. Ptolemy was acquainted with only a few of the principal stars of the antarctic hemisphere, and it was not till the beginning of the seventeenth century that Theodor von Emden regularly divided these regions of the heavens into constellations. It remained for Herschel, in recent times, to determine a number of open questions with regard to these southern constellations.
To account for this backwardness in the investigation of a subject which certainly possessed at least as much interest for the ancients as for ourselves, by declaring it to be the result of their superficiality, were an injustice to the olden time, seeing that in other respects it commands our unconditional admiration for its arduous achievements. That what they needed was to have their senses trained to this kind of work, and that, although they had keen appreciation of art, they never learned to look at things with the eye of the investigator of Nature, will be better understood from a statement of what they knew about individual celestial objects than of what they knew about the entire firmament.
That well-known group of stars, the Pleiades, which in the Fall adorns our eastern sky by night, serves well to show that in observing the stars something else is required besides a clear atmosphere and
