The instrument used by Tycho Brahe for the purpose, called the _Quadrans Maximus_, is represented in Fig. 18. In this there is the quadrant B, D, one pointer being placed, as shown at the bottom, near H, and the other at the top, C. These pointers or sights were directed at the star by moving the arm C, H, on the pivot A, and turning the whole arm and divided arc round on the axis N, R. The altitude of the star is then read off on the quadrant B, D, and the azimuth, or number of degrees east or west of the north and south line, is then read off on the circle Q, R, S. The screws Y, Y, served to elevate the horizontal circle, and level it exactly with the horizon, and the plummets W and V, hanging from G, were to show when the circle was level or not; for the part A, G, being at right angles to the circle should be upright when the circle is level, so that if A, G, is upright in all positions when moved round the circle in azimuth, the circle is horizontal. Here, then, is an instrument very different in principle from what we had before. In this case the heavens are viewed from the most general standpoint we can obtain—the horizon; but observations such as these refer to the position of the place of observation absolutely, without any reference to the position of the body with respect to the equator or the ecliptic; but knowing the latitude of the place of observation _and the time_, it was possible for a mathematical astronomer to reduce the co-ordinates to right ascension and declination, and so actually to look at the position of these bodies with reference to the celestial sphere. Tycho also had various other instruments of the same kind, differing only in the position of the quadrant D, B, and of the circle on which the azimuth was measured. These instruments are the same in principle as our modern alt-azimuth, which will be described hereafter, one form having a telescope and the other being without it. [Illustration: FIG. 19.—Tycho’s Sextant. ] Fig. 19 is yet another very important instrument invented by Tycho Brahe; it is the prototype of our modern much used sextant. It was used by Tycho Brahe for determining the distance from one body to another in a direct line; a star or the moon, say, was observed by the pointers C, A, while another was observed by the pointers N, A, by another observer. The number of degrees then between N and C gave the angular distance of the two bodies observed. This instrument was mounted at E, so that it could be turned into any position. Not only then had this instrument its representative in our present sextant, but it was used in the same way, not requiring to be fixed in any one position. We also find represented in Tycho Brahe’s book another form of the same instrument, the sight A being next the observer, instead of away from him, so that he could observe the two stars through the sights N and C without moving the eye. In this form only one observer was required instead of two as in the last.