too difficult even for his unrivalled powers of observation. The minor planets were also found to be remarkable for the great inclination and eccentricity of some of the orbits; the path of Pallas, for example, makes an angle of 35° with the ecliptic, and its eccentricity is 14, so that its least distance from the sun is not much more than half its greatest distance. These characteristics suggested to Olbers that the minor planets were in reality fragments of a primeval planet of moderate dimensions which had been blown to pieces, and the theory, which fitted most of the facts then known, was received with great favour in an age when "catastrophes" were still in fashion as scientific explanations.
The four minor planets named were for nearly 40 years the only ones known; then a fifth was discovered in 1845 by Karl Ludwig Hencke (1793–1866) after 15 years of search. Two more were found in 1847, another in 1848, and the number has gone on steadily increasing ever since. The process of discovery has been very much facilitated by improvements in star maps, and latterly by the introduction of photography. In this last method, first used by Dr. Max Wolf of Heidelberg in 1891, a photographic plate is exposed for some hours; any planet present in the region of the sky photographed, having moved sensibly relatively to the stars in this period, is thus detected by the trail which its image leaves on the plate. The annexed figure shews (near the centre) the trail of the minor planet Svea, discovered by Dr. Wolf on March 21st, 1892.
At the end of 1897 no less than 432 minor planets were known, of which 92 had been discovered by a single observer, M. Charlois of Nice, and only nine less by Professor Palisa of Vienna.
The paths of the minor planets practically occupy the whole region between the paths of Mars and Jupiter, though few are near the boundaries; no orbit is more inclined to the ecliptic than that of Pallas, and the eccentricities range from almost zero up to about 13.
Fig. 89 shews the orbits of the first two minor planets discovered, as well as of No. 323 (Brucia), which comes nearest to the sun, and of No. 361 (not yet named),
