and corrected his observations. His table of refractions enabled him
to abolish the error due to our atmosphere so far as it could affect
naked-eye observations. The azimuth circle of Tycho's largest quadrant
had a diameter of nine feet, and the quadrant a radius of six feet. He
introduced the mural quadrant for meridian observations.[2]

[Illustration: ANCIENT CHINESE INSTRUMENTS, Including quadrant, celestial
globe, and two armillae, in the Observatory at Peking. Photographed in
Peking by the author in 1875, and stolen by the Germans when the
Embassies were relieved by the allies in 1900.]

The French Jesuits at Peking, in the seventeenth century, helped the
Chinese in their astronomy. In 1875 the writer saw and photographed,
on that part of the wall of Peking used by the Mandarins as an
observatory, the six instruments handsomely designed by Father
Verbiest, copied from the instruments of Tycho Brahe, and embellished
with Chinese dragons and emblems cast on the supports. He also saw
there two old instruments (which he was told were Arabic) of date
1279, by Ko Show-King, astronomer to Koblai Khan, the grandson of
Chenghis Khan. One of these last is nearly identical with the armillae
of Tycho; and the other with his "armillae aequatoriae maximae," with
which he observed the comet of 1585, besides fixed stars and
planets.[3]

The discovery by Galileo of the isochronism of the pendulum, followed
by Huyghens's adaptation of that principle to clocks, has been one of
the greatest aids to accurate observation. About the same time an
equally beneficial step was the employment of the telescope as a
pointer; not the Galilean with concave eye-piece, but with a
magnifying glass to examine the focal image, at which also a fixed