We begin then by referring to the hole in the wall at which Tycho is pointing (see Fig. 112), and the circle, of which the hole was the centre, opposite to it, on which the position of the body was observed, and its declination and right ascension determined. This then was Tycho’s arrangement for determining the two co-ordinates, right ascension and declination, measured from the meridian and equator. It is to be hoped that the meaning of right ascension and declination is already clear to our readers, because these terms refer to the fundamental planes, and distances as measured from them are the very A B C of anything that one has to say about astronomical instruments. We know that Tycho had two things to do. In the first place he had to note when a star was seen through the slit in the wall, which was Tycho’s arrangement for determining the southing of a star, the sun, or the moon; and then to give the instant when the object crossed the sight to the other observer, who noted the time by the clocks. Secondly, he had to note at which particular portion of the arc the sight had to be placed, and so the altitude or the zenith distance of the star was determined; and then, knowing the latitude of the place, he got the two co-ordinates, the right ascension and declination. How does the modern astronomer do this? Here is an instrument which, without the circle to tell the altitude at the same time, will give some idea of the way in which the modern astronomer has to go to work. In this we have what is called the Transit Instrument, Fig. 113; it is simply used for determining the transit of stars over the meridian. It consists essentially of a telescope mounted on trunnions, like a cannon, having in the eyepiece, not simple cross wires, but a system of wires, to which reference has already been made, so that the mean of as many observations as there are wires can be taken; and in this way Tycho’s hole in the wall is completely superseded. The quadrant is represented by a circle on the instrument called the transit circle, of which for the present we defer consideration. [Illustration: FIG. 112.—Tycho Brahe’s Mural Quadrant. ] [Illustration: FIG. 113.—Transit Instrument (Transit of Venus Expedition). ] [Illustration: FIG. 114.—Transit Instalment in a fixed Observatory. ]