ioo POPULAR SCIENCE MONTHLY. 



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 ac- 

 counting for this progression has been the supposition that an undis- 

 covered 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 un- 

 successful. 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 re- 

 quires 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 propor- 

 tions 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, ob- 

 tained 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 ex- 

 posures 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 recog- 

 nized as known stars, by means of the February Mount Hamilton 

 plates. 



It is probable that any such planets would be well within the re- 

 gion 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 



