Page:Popular Science Monthly Volume 64.djvu/519

Rh stars have been discovered by Professor Pickering, Mr. H. C. Bailey and the other members of the observatory.

At the present time the equipment of the Arequipa Station of the observatory consists of the following instruments: the thirteen-inch Boyden telescope, an instrument so constructed that, by a change in the position of the lenses, it may be used either for visual or for photographic work; the twenty-four-inch Bruce photographic telescope, the most powerful instrument of its class in the world, a gift of the late Miss Catherine Bruce, of New York; the eight-inch Bache photographic telescope; a five-inch refractor, and several smaller instruments of different kinds.

In general the work carried on in Arequipa is the extension to the southern sky of that previously begun in Cambridge. This is well illustrated by the Harvard photometry. With the large meridian photometer alone more than a million light comparisons have been made. The greater part of this work was done in Cambridge by the director and his assistants, but about two hundred thousand observations have been made by the writer in Arequipa and elsewhere in South America. This work, planned and begun by Professor Pickering a quarter of a century ago, now furnishes not only precise determinations of the magnitudes of all the brighter stars in the sky, but also the magnitudes of certain zones of fainter stars, by which the estimated magnitudes of the stars included in the various great catalogues can be reduced to the photometric scale.

With another Pickering photometer, during the last year, several thousand light comparisons of Eros were made by the writer. Eros is that remarkably interesting little planet which at times comes so near the earth as to be our nearest celestial neighbor. Eros is a variable planet, undergoing striking changes in light. The above observations showed that during the year 1903 the complete light-cycle was only 2h 38m 6s.1. If these changes are due to the rotation of the planet, the true period may be that given above, or, more probably, twice that amount, 5h 16m 12s.2.

Visual observations of variable stars have been regularly carried on since the establishment of the station, although the results have not yet been published. These observations are now made by Messrs. Manson and Wyeth. A determination of the longitude and latitude of the station was made in 1897 by Professor Winslow Upton, of Brown University. The result was, longitude 4h 46m 12s west of Greenwich. The latitude is south 16° 22′ 28″. The longitude of the observatory in Cambridge is 4h 44m 31s. It follows, therefore, that Arequipa is about thirty miles west and four thousand miles south of Cambridge.

Photographic work has always occupied a large share of the time at Arequipa. Several photographic instruments are kept employed