642 



POPULAR SCIENCE MONTHLY. 



opened: Ceres, the first asteroid, was found on the first day of the 

 nineteenth century. Bode's law, therefore, appeared to have found 

 confirmation here, for, though there was no single great planet, as else- 

 where, nevertheless the small army of fragments seemed to point to 

 some abortive attempt of Nature to form a world in the usual order, 

 or else' to an explosion of one already formed. In either case the dis- 

 tance of the 'mean asteroid' might be expected to follow the law, which 

 it was found approximately to do. It seems a pity that the law, having 

 survived so many tests, should go to pieces at last on what was per- 

 haps the final test which remained to be applied. When Neptune was 

 discovered, however, in 1846, it did not conform to the law at all. 

 The following table gives a comparison between the true distances and 

 those which result from Bode's law, the distance of the earth being 

 taken as unity: 



Planet. 



Mercury 



Venus 



Earth 



Mars 



Mean Asteroid. 



Jupiter 



Saturn 



Uranus 



Neptune 



The discovery of asteroids has been much simplified by the in- 

 crease of star maps, and especially by the advances in celestial pho- 

 tography. One feature, which is incidental to the duration of the 

 photographic exposure, renders the detection of such objects compara- 

 tively easy. When a photographic plate is exposed to the sky in a 

 camera or telescope, if there is no clockwork, so that the instrument 

 remains at rest, the images of the stars are drawn out into lines or 

 trails. Ordinarily, however, the instrument is kept in motion by a 

 driving clock, so that it exactly follows the stars in their apparent 

 daily motion, and the images of the stars result as circular dots on 

 the plate. An asteroid, however, from its nearness has so rapid an 

 apparent motion among the stars that, if an exposure is made of an 

 hour or more, its image is spread out in a line, while the images of 

 stars remain circular. On some of the plates, for example, made with 

 the great Bruce photographic telescope at Arequipa, several hundred 

 thousand stars appear. On one of these plates, which had an ex- 

 posure of four hours, seven asteroid trails were found. If these as- 

 teroids had formed circular images, similar to those of the stars, their 

 detection among the several hundred thousand images on the plate 

 would have been an enormous labor and would have required other 



