A FAMOUS ASTRONOMICAL PROBLEM 497 



correctly, concluded that the objects seen were well-known neighboring 

 stars. 



The perfecting of dry-plate photography gave renewed interest 

 to the search for Vulcan, both when passing over tlie solar surface 

 and at times of eclipse. Although the sun has been photographed 

 almost daily during the past twenty years, at one observatory or an- 

 other, no experienced observer has seriously claimed that his plates 

 recorded an unknown planet crossing the sun. Neither were eclipse 

 searches more successful : the well-known bright stars lying nearly 

 in the direction of the sun were photographed, but no strange bodies. 

 Curiously enough, the optical principles governing the efficiency of 

 cameras in this search were overlooked for many years, and faint 

 objects near the sun — say stars fainter than the fourth magnitude — 

 were not observable, because their images, though formed on the photo- 

 graphic plates, were overwhelmed and buried from sight in the gen- 

 eral darkening of the photograph by the bright-sky background. It 

 was not until 1900 that the elements of the problem of photographing 

 faint bodies near the sun were comprehended. While preparing for 

 the eclipse of that year, three astronomers, Professor W. H. Pickering, 

 of Harvard College Observatory, and Messrs. Perrine and Campbell, of 

 tlie Lick Observatory, independently arrived at the same simple con- 

 clusion that the focal lengths of the intramercurial-search cameras 

 should be relatively long, in order to reduce the intensity of the sky 

 exposure on the plates without reducing the intensity of the star 

 images, and thus let the latter be seen on the negative. The principles 

 involved are so simple as hardly to call for elucidation. 



Let the two cameras have lenses of equal aperture, say 3 inches, 

 of equal transparency and capable of covering equal angular fields of 

 view, say a circle 10 degrees in diameter. Let one be of short focus, 

 21 inches, and the other of long focus, 135 inches. The powers of 

 tlie two lenses to record stellar points on the sensitive plates in focus, 

 under good atmospheric conditions, are not very imequal, for the two 

 lenses collect equal quantities of light and condense the light into 

 images of very nearly the same size. Both collect the same quantity 

 of sky light, but the longer-focus camera spreads it (more thinly) over 

 an area (135)"/(21)^ = 41 times the greater. It is evident that 

 faint-star images hopelessly lost to view on the sky-blackened small 

 plate may be seen with ease on the nearly clear glass of the large plate. 

 "We may safely say that the large plate will show images of stars 3 or 3^ 

 magnitudes fainter than the small plate. The same advantage exists 

 for small intramercurial planets as for stars, provided the exposures do 

 not exceed two or three minutes in length, as they seldom do at eclipses. 

 In longer exposures on intramercurial planetoids the advantage would 

 usually be lost, as their rapid (and unknown) motions would cause their 



VOL. LXXIV. — 32. 



