Page:Tycho brahe.djvu/350

320 by navigators. Nonius suggested attaching the pointer or alidade to some point on the circumference instead of to the centre, as the divisions on this plan might be made twice as large as usual. I only mention this proposal because Tycho (who does not allude to Nonius) constructed a large semicircle revolving round a vertical axis, with a long ruler turning on a pivot at one end of the horizontal diameter.

In addition to complete circles, quadrants were also used long before Tycho's time, though not extensively. Ptolemy describes a meridian quadrant attached to a cube of stone or wood, with a small cylinder in the centre of the arc, of which the shadow indicated the altitude of the sun on the graduation. Among the numerous instruments which Nasir al-din Tusi erected in the splendid observatory at Meragha, in the north-west of Persia (about A.D. 1260), was a Ptolemean mural quadrant, made of hard wood, and with a radius of about twelve feet. The limb was of copper, on which three arcs were drawn; the middle one was divided into degrees, while of the others, one showed every fifth degree, the other the minutes (all of them?). Nasir al-din mentions the instrument of Ptolemy, which evidently had served him as a model.

Tycho Brahe is therefore not (as supposed by some writers) the inventor of the mural quadrant, an instrument which up to the end of the eighteenth century was the most important one in astronomical observatories. Of course he cannot have known anything of the Arabian quadrants, but the