A FAMOUS ASTRONOMICAL PROBLEM 499 



station by Dr. Curtis, made no contribution because of the severe storm 

 conditions prevailing at the time of totality. The Egyptian cameras, 

 mounted by Professor Hussey, recorded a considerable number of 

 stars, but the sky, though clear in the usual sense, was full of dust, 

 and the sun and the surrounding region covered by the search were 

 at a low altitude. The Spanish cameras, photographing through 

 clouds which permitted only 20 or 30 per cent, of the light to pass, 

 recorded 55 stars down to abotit the seventh and eighth magnitudes. 

 All suspected images not occupying the positions of known stars were 

 proved to be defects in the films. 



The eclipse of 1908 in the South Seas was utilized by the Crocker 

 Expedition to cover a region extending east and west along the sun's 

 equator with duplicate exposures. Notwithstanding interference from 

 rain and clouds at the beginning of totality, clear sky prevailed during 

 the last two thirds of the four critical minutes. Dr. Perrine finds more 

 than 500 images of well-lcnown stars on the plates, and no images of 

 unknown bodies. Stars are recorded down to nearly the ninth visual 

 magnitude. 



It is not absolutely certain that intramercurial planets, revolving 

 around the sun in elliptical orbits would be seen in projection entirely 

 within the area 9°x29° lying along the solar equator and equally 

 east and west of the sun's center, yet there are exceedingly strong 

 reasons to believe such would be the case. The eight large planets and 

 the 650 ± minor planets in our system revolve around the sun in the 

 same direction and, excepting a small proportion of the asteroids, so 

 nearly in the sun's equatorial plane that the parts of their orbit planes 

 lying within the limits for intramercurial planets would be projected 

 upon the photographed area. The central plane of the zodiacal light 

 differs little from the sun's equatorial plane. It is certain, also, that 

 any intramercurial planets originally moving in planes inclined at 

 large angles to Mercury's orbit plane would gradually be compelled 

 by the attractions of Mercury and the other major planets to move in 

 planes inclined at small angles to the ecliptic. The coincidence of the 

 satellite planes in the systems of Jupiter and Saturn, and no doubt of 

 Uranus and Neptune also, with the equatorial planes of these planets 

 is another analogy of some weight. Admitting, for completeness, the 

 hypothesis of an extensive system of small planets moving in planes 

 making a variety of angles with the ecliptic and sun's equator, some 

 would certainly have been caught in the region photographed. A 

 single planet, or a half dozen planets, massive enough to meet the re- 

 quirements, moving in any orbit planes would no doubt have been dis- 

 covered a generation ago. In view of these facts, there is little reason 

 to fear that any planets effective in disturbing Mercury's motions were 

 north or south of the regions covered by photography. 



