A complete transit observation of a star consists therefore of two operations. The observer, as we have already described, sees a star entering the field of the telescope, and as it swims forward, he presses the galvanic button, which sends a signal to the chronograph as the star comes up to each of the ten vertical wires in succession. But, beside the ten wires, there are others. Two vertical wires lie outside the ten of which we have already spoken, and there is also a horizontal wire. The latter can be moved by a graduated screw-head just above the eye-piece, and as the star comes in succession to these two vertical wires, this horizontal wire is moved by the screw-head, so as to meet the star at the moment it is crossing the vertical wire, and the observer presses a second little button, which records the position of the horizontal wire on a small paper-covered drum. Then, the transit over, the observer leaves the telescope and comes round to the outside of the west pier. Here he finds seven large microscopes, which pierce the whole thickness of the pier, and are directed towards the circumference of a large wheel which is rigidly attached to the telescope and revolves with it. This wheel is six feet in diameter, and has a silver circle upon both faces. Each circle is divided extremely carefully into 4320 divisions--these divisions, therefore, being about the one-twentieth of an inch apart. There are, therefore, twelve divisions to every degree (12 × 360 = 4320), and each division equals five minutes of arc. The lowest microscope is the least powerful, and shows a large part of the circle, enabling the observer to see at once to what degree and division of a degree the microscope is pointing. The other six microscopes are very carefully placed 60° apart--as equally placed as they possibly can be. These microscopes are all fitted with movable wires--wires moved by a very fine and delicate screw; the screw-head having divisions upon it so that the exact amount of its movement can be told. Each of the six screw-heads will read to the one five-thousandth part of a division of the circle; in other words, to the one hundred thousandth part of an inch. Using all six microscopes, and taking their mean, we are able to _read_ to the one-hundredth of a second of arc. If, therefore, the observations could be made with perfect certainty down to the extremest nicety of reading which the instrument supplies, we should be able to read the declination of a star to this degree of refinement. It may be added that a halfpenny, at a distance of three miles, appears to be one second of arc in diameter; at three hundred miles it would be one-hundredth of a second. It need scarcely be said that we cannot observe with quite such refinement of exactness as this would indicate. Nevertheless, this exactness is one after which the observer is constantly striving, and tenths, even hundredths, of seconds of arc are quantities which the astronomer cannot now neglect.