This is done as follows:—On the same side of the wires as the lamp is a convex lens and reflector so arranged that the rays from the wires are reflected through a hole in the tube, and again down the tube to the eyepiece, where the images of the wires are brought to a focus at the same place as the stars to be measured, so that any eyepiece can be used. The wires show as bright lines in the field, and they are worked about in the field just as real wires might be by moving the wires outside the tube. A sheet of metal can be moved in front of the distance-wires so as to obstruct the light from them at any part of their length, and their bright images appear then abruptly to terminate in the field of view, so that faint stars can be brought up to the terminations of the wires and be measured without being overcome by bright lines. ----- Footnote 16: It is not too much to say that the duty on glass entirely stifled, if indeed it did not kill, the optical art in England. We were so dependent for many years upon France and Germany for our telescopes, that the largest object-glasses at Greenwich, Oxford, and Cambridge are all of foreign make. Footnote 17: These details are given from the _Forces of Nature_ (Macmillan). CHAPTER XXI. THE ADJUSTMENTS OF THE EQUATORIAL. As the equatorial is _par excellence_ the amateur’s instrument, and as in setting up an equatorial it is important that the several adjustments should be correctly made, they are here dwelt upon as briefly as possible. They are six in number. 1. The inclination of the polar axis must be the same as that of the pole of the heavens. 2. The declination circle must read 0° when the telescope is at right angles to the polar axis. 3. The polar axis must be placed in the meridian. 4. The optic axis of the telescope, or line of collimation must be at right angles to the declination axis, so that it describes a great circle on moving about that axis. 5. The declination axis must be at right angles to the polar axis, in order that the telescope shall describe true meridians about that axis. 6. The hour circle must read 0h. 0m. 0sec. when the telescope is in the meridian. When these are correctly made the line of collimation will, on being turned about the declination axis, describe great circles through the pole, or meridians, and when moved about the polar axis, true parallels of declination; and the circles will give the true readings of the apparent declination, and hour angles from the meridian. To make these adjustments, the telescope is set up by means of a compass and protractor, or otherwise in an approximately correct position, the declination circle put so as to read nearly 90° when the telescope points to the pole, and the hour circle reading 0h. 0m. 0sec. when the telescope is pointing south. First, then, to find the error in _altitude_ of the polar axis.