Euler's and Lagrange's method of variable elements became so laborious
that Encke devised a special process for these, which can be applied
to many other disturbed orbits. [8]

When a photograph is taken of a region of the heavens including an
asteroid, the stars are photographed as points because the telescope
is made to follow their motion; but the asteroids, by their proper
motion, appear as short lines.

The discovery of Eros and the photographic attack upon its path have
been described in their relation to finding the sun's distance.

A group of four asteroids has lately been found, with a mean distance
and period equal to that of Jupiter. To three of these masculine names
have been given--Hector, Patroclus, Achilles; the other has not yet
been named.


FOOTNOTES:

[1] Langrenus (van Langren), F. Selenographia sive lumina austriae
philippica; Bruxelles, 1645.

[2] _Astr. Nach._, 2,944.

[3] _Acad. des Sc._, Paris; _C.R._, lxxxiii., 1876.

[4] _Mem. Spettr. Ital._, xi., p. 28.

[5] _R. S. Phil. Trans_., No. 1.