object-glass of a telescope along a diameter. Each half is mounted so
as to slide a distance of several inches each way on an arc whose
centre is the focus. The amount of the movement can be accurately
read. Thus two fields of view overlap, and the adjustment is made to
bring an image of one star over that of another star, and then to do
the same by a displacement in the opposite direction. The total
movement of the half-object glass is double the distance between the
star images in the focal plane. Such an instrument has long been
established at Oxford, and German astronomers have made great use of
it. But in the hands of Sir David Gill (late His Majesty's Astronomer
at the Cape of Good Hope), and especially in his great researches on
Solar and on Stellar parallax, it has been recognised as an instrument
of the very highest accuracy, measuring the distance between stars
correctly to less than a tenth of a second of arc.

The superiority of the heliometer over all other devices (except
photography) for measuring small angles has been specially brought
into prominence by Sir David Gill's researches on the distance of the
sun--_i.e.,_ the scale of the solar system. A measurement of the
distance of any planet fixes the scale, and, as Venus approaches the
earth most nearly of all the planets, it used to be supposed that a
Transit of Venus offered the best opportunity for such measurement,
especially as it was thought that, as Venus entered on the solar disc,
the sweep of light round the dark disc of Venus would enable a very
precise observation to be made. The Transit of Venus in 1874, in
which the present writer assisted, overthrew this delusion.

In 1877 Sir David Gill used Lord Crawford's heliometer at the Island
of Ascension to measure the parallax of Mars in opposition, and found
the sun's distance 93,080,000 miles. He considered that, while the