of Mercury. The perihelion of its orbit moves forward at least 40″ in a century more than theory calls for. The most plausible way of accounting for this progression has been the supposition that an undiscovered planet, or a group of small planets, exists within the orbit of Mercury. The search for such objects has been a well-defined eclipse problem; the sun-lit sky prevents effective search by every-day methods. Organized efforts to discover such bodies by visual means were made at the eclipses of the late seventies and early eighties, but they were unsuccessful. Photographic methods, though not planned for efficiency in that particular problem, were applied in the nineties. Early in the year 1900 it occurred independently to Professor W. H. Pickering, of Harvard College Observatory, and to Messrs. Perrine and Campbell, of the Lick Observatory, that efficiency in the photographic method requires the cameras to be of relatively long focus, in order to reduce the intensity of sky illumination on the photographic plate; and each of these astronomers, unknown to the other two, fixed upon the proportions which such instruments should have. Their results were in good general accordance. The first attempt to apply this method was made by Professor Pickering at the eclipse of May, 1900, with camera lenses three inches in aperture and 135 inches in focal length, but no evidence was secured. Mr. Abbot, of the Smithsonian Institution party, obtained one photograph with a similar lens, covering a limited area of the sun's surroundings, which recorded eighth magnitude stars. Four suspicious images on the plates were noticed; but whether they were ordinary photographic defects or images of real objects could not be determined, as the required second plate of the same region was not secured by this party or others.
The last word on the subject is by Perrine, who applied the method in Sumatra in May, 1901. His four telescopes, making three exposures each, secured negatives in duplicate of a region 6° wide and 38° long—19° on each side of the sun, in the direction of the sun's equator. Through thin clouds covering two thirds of this area, one hundred and sixty-two stars, including several as faint as the ninth magnitude, were photographed; and through thicker clouds covering the remaining third, eight stars, four of them between 6.0 and 6.5 visual magnitudes, were recorded. While these instruments were in use in the preceding February at the Lick Observatory, exposures were made on the region of the sky which would be occupied by the eclipsed sun in May. All objects on the Sumatra eclipse plates were recognized as known stars, by means of the February Mount Hamilton plates.
It is probable that any such planets would be well within the region covered, provided their orbit planes make a small angle with the sun's equator. The earth was very nearly in the plane of the sun's
