mark could be placed. Kepler was the first to suggest this. Gascoigne
was the first to use it. Huyghens used a metal strip of variable width
in the focus, as a micrometer to cover a planetary disc, and so to
measure the width covered by the planet. The Marquis Malvasia, in
1662, described the network of fine silver threads at right angles,
which he used in the focus, much as we do now.

In the hands of such a skilful man as Tycho Brahe, the old open
sights, even without clocks, served their purpose sufficiently well to
enable Kepler to discover the true theory of the solar system. But
telescopic sights and clocks were required for proving some of
Newton's theories of planetary perturbations. Picard's observations at
Paris from 1667 onwards seem to embody the first use of the telescope
as a pointer. He was also the first to introduce the use of Huyghens's
clocks for observing the right ascension of stars. Olaus Romer was
born at Copenhagen in 1644. In 1675, by careful study of the times of
eclipses of Jupiter's satellites, he discovered that light took time
to traverse space. Its velocity is 186,000 miles per second. In 1681
he took up his duties as astronomer at Copenhagen, and built the first
transit circle on a window-sill of his house. The iron axis was five
feet long and one and a-half inches thick, and the telescope was fixed
near one end with a counterpoise. The telescope-tube was a double
cone, to prevent flexure. Three horizontal and three vertical wires
were used in the focus. These were illuminated by a speculum, near the
object-glass, reflecting the light from a lantern placed over the
axis, the upper part of the telescope-tube being partly cut away to
admit the light. A divided circle, with pointer and reading
microscope, was provided for reading the declination. He realised the
superiority of a circle with graduations over a much larger
quadrant. The collimation error was found by reversing the instrument