mirror or prism, G; at F, therefore, we have an image of the star, which can be examined together with the cross-wires at the eyepiece, M. There is in this instrument no necessity for the accurate adjustments that there is in the case of the transit, the surface of the mercury being always horizontal, and so giving an unaltering datum plane. [Illustration: FIG. 131.—Reflex Zenith Tube. ] When the star is perfectly vertical, its image will fall on a certain known part in the eyepiece; but, as it leaves the vertical, the angle of incidence of its light on the mercury alters, and likewise that of reflection, so that the position of the image changes, and this change of position in the eyepiece is measured by movable cross-wires and a micrometer screw, similar to that employed for reading the circle in the transit circle. At the present time γ Draconis is the star which passes very nearly through the zenith of Greenwich, and observations of this star are accordingly made at every available opportunity. We now pass to an enlargement of the sphere of observation of the transit circle in order that any object can be viewed at all times when above the horizon; in this case the transit circle passes into the alt-azimuth, or altitude and azimuth instrument, astronomical theodolite, or universal instrument. A reference to Fig. 132—a woodcut of an ordinary theodolite—will show the new point introduced by this construction. Imagine the upper part of the theodolite fixed with its telescope and circle in the plane of the meridian—we have the transit circle; swing the theodolite round through 90°—we have the prime vertical instrument. Now instead of having the upper part fixed let it be free to rotate on the centre of the horizontal circle—we have the alt-azimuth. In the description of the instruments used in Tycho’s observatory (Chapter IV.), we described another instrument by which Tycho and the Landgrave of Hesse-Cassel endeavoured to make observations out of the meridian; and we may remember that they almost had to give the matter up in despair, because they could not find any clocks sufficiently good to enable them to fix the position of the star. [Illustration: FIG. 132.—Theodolite. ] [Illustration: FIG. 133.—Portable Alt-azimuth. ]