never lost a chance of adding to his stock of careful observations.[1]

The account of all these instruments and observations, printed at his
own press on the island, was published by Tycho Brahe himself, and the
admirable and numerous engravings bear witness to the excellence of
design and the stability of his instruments.

His mechanical skill was very great, and in his workmanship he was
satisfied with nothing but the best. He recognised the importance of
rigidity in the instruments, and, whereas these had generally been
made of wood, he designed them in metal. His instruments included
armillae like those which had been used in Alexandria, and other
armillae designed by himself--sextants, mural quadrants, large
celestial globes and various instruments for special purposes. He
lived before the days of telescopes and accurate clocks. He invented
the method of sub-dividing the degrees on the arc of an instrument by
transversals somewhat in the way that Pedro Nunez had proposed.

He originated the true system of observation and reduction of
observations, recognising the fact that the best instrument in the
world is not perfect; and with each of his instruments he set to work
to find out the errors of graduation and the errors of mounting, the
necessary correction being applied to each observation.

When he wanted to point his instrument exactly to a star he was
confronted with precisely the same difficulty as is met in gunnery and
rifle-shooting. The sights and the object aimed at cannot be in focus
together, and a great deal depends on the form of sight. Tycho Brahe
invented, and applied to the pointers of his instruments, an
aperture-sight of variable area, like the iris diaphragm used now in