d of parallaxes." Its principle
consists in substituting successive morning and evening observations
from the same spot, for simultaneous observations from remote spots, the
rotation of the earth supplying the necessary difference in the points
of view. Its great advantage is that of unity in performance. A single
mind, looking through the same pair of eyes, reinforced with the same
optical appliances, is employed throughout, and the errors inseparable
from the combination of data collected under different conditions are
avoided. There are many cases in which one man can do the work of two
better than two men can do the work of one. The result of Gill's skilful
determinations (made with Lord Lindsay's heliometer) was a solar
parallax of 8.78", corresponding to a distance of 93,080,000 miles.[776]
The bestowal of the Royal Astronomical Society's gold medal stamped the
merit of this distinguished service.

But there are other subjects for this kind of inquiry besides Mars and
Venus. Professor Galle of Breslau suggested in 1872[777] that some of
the minor planets might be got to repay astronomers for much
disinterested toil spent in unravelling their motions, by lending aid to
their efforts towards a correct celestial survey. Ten or twelve come
near enough, and are bright enough for the purpose; in fact, the absence
of sensible magnitude is one of their chief recommendations, since a
point of light offers far greater facilities for exact measurement than
a disc. The first attempt to work this new vein was made at the
opposition of Phocaea in 1872; and from observations of Flora in the
following year at twelve observatories in the northern and southern
hemispheres, Galle deduced a solar parallax of 8.87".[778] At Mauritius
in 1874, Lord Lindsay and Sir David Gill applied the "diurnal method" to
Juno, then conveniently situated for the purpose; and the continued use
of similar occasions affords an unexceptionable means for improving
knowledge of the sun's distance. They frequently recur; they need no
elaborate preparation; a single astronomer armed with a heliometer can
do all the requisite work. Dr. Gill, however, organized a more complex
plan of operations upon Iris in 1888, and upon Victoria and Sappho in
1889. A novel method was adopted. Its object was to secure simultaneous
observations made from opposite sides of the globe just when the planet
lay in the plane passing through the centre of the earth and the two
observe