tance of an object of this kind from
a point of line such as a star is measured with the heliometer it is found
that a certain bias, somewhat difficult to allow for with certainty, is
introduced into the measures; and our confidence in the final results
suffers accordingly. After his observations of Mars in 1878, Dr. David
Gill was sufficiently impressed with this source of error to make three
new determinations of the sun's distance, using three of the minor planets
instead of Mars, in spite of the fact that they were sensibly farther
away; and his choice was justified by finding that the results from these
three different sets of observations agreed well among themselves, and
differed slightly from that given by the observations of Mars. Hence it
seems conclusively proved that one of these bodies is a better selection
than Mars in any case, and the discovery of Eros, which offered the
advantage of greater proximity in addition, was hailed as a new
opportunity of a most welcome kind. It was seen by a little calculation
that in the winter of 1900-1901 the planet would come very near the
earth; not the nearest possible (for it was also realised that a still
better opportunity had occurred in 1894, though it was lost because the
planet had not yet been discovered), but still the nearest approach which
would occur for some thirty years; and extensive, though somewhat hasty,
preparations were made to use it to the fullest advantage. Photography had
now become established as an accurate method of making measurements of the
kind required; and all the photographic telescopes which could be spared
were pressed into the service, and diligently photographed the planet and
surrounding stars every fine night during the favourable period. The work
of measuring and reducing these photographs involves an enormous amount of
labour, and is even yet far from completed, but we know enough to expect a
result of the greatest value. More than this we have not time to say here
about this great problem, but it will have been made clear that just when
astronomers were beginning to wonder whether it was worth while continuing
the monotonous discovery of new minor planets by the handful, the 433rd
discovery also turned out to be one of the greatest importance.

To canons for the advantageous prosecution of research, if we care to make
them, we may therefore add this--that there is no line of research,
however apparently unimportant or monotonous, w