CIRCLE, .MURAL, an Instrument used for determining the meridian altitude or zenith distance of a star. It consists of an astronomical telescope firmly fixed to a graduated circle, which moves about a horizontal axis, fixed in a strong vertical wall running north and south. In the common focus of the eye-piece and object glass of the telescope is a system of cross-wires (spider lines are generally used for the purpose), one being horizontal, and five vertical, with equal spaces between. The line joining the optical center of the object-glass with the intersection of the hori zontal and middle vertical wires, is called the line of collimation of the telescope, and when the instrument is in perfect adjustment, this line moves in the plane of the meridian.
Besides the above-mentioned fixed wire, there is a movable one, called a microm eter wire, which is moved by means of a screw, remaining always parallel to the fixed horizontal wire.
If the instrument be so adjusted that the image of a star, while passing across the middle vertical wire in the field of view, shall at the same time be bisected by the fixed horizontal wire, the star is at that moment in the line of collimation of the telescope. It is therefore at that moment in the meridian, and its meridian zen ith distance is the angle through which the circle would have been turned from the position it had when the line of collimation of the telescope pointed to the zenith. There is a fixed pointer, for the purpose of approximately reading the instrument. If the instrument were accurately adjusted, so that the pointer was opposite the zero point of the circle, when the line of collimation of the telescope pointed to the zenith, the arc intercepted between these two positions of the instrument would be the meridian zenith distance of the star.
Great nicety is required in " reading " the instrument; i.e., in determining exactly the arc through which the circle has moved in bringing the telescope from the ver tical to any other position. The rim is usually graduated at intervals of five min utes; and the eye could determine only the division nearest to the fixed index. But by means of a "reading microscope," or micrometer (q.v.), fixed opposite to the rim, the portion of the interval to the nearest division on the rim can be read to seconds. There are usually six such microscopes fixed opposite different points of the rim; and the " reading ' of the instrument is the mean of the "readings" of all the micro scopes. This tends to eliminate errors arising from imperfect graduation and adjust ment. If the instrument is properly adjusted, the zero point of the circle will be opposite the fixed pointer when the line of collimation of the telescope points to the zenith. In practice, however, this is not always accurately, or even approxi mately the case. As we shall immediately show, it is of no consequence, as the final result of every observation is the difference between two readings.
It is evident that the difference between any two readings of the instrument will represent the angle through which the line of collimation of the telescope moves in passing from one position to the other. It remains to show how a fixed point, viz.,
the nadir (q.v.), is observed, and then how an observation is taken of the star itself in its meridian passage.
We must explain here that the fixed horizontal wire in the eye-piece of the tele scope, in the instruments as now used, is only an imaginary line which determines the line of collimation of the telescope. It coincides with the position of the micrometer wire, when the screw-head of the micrometer marks zero.
To observe the nadir, a trough of mercury is placed underneath the instrument, and the telescope is turned so as to look vertically downwards into it, An image of the system of cross-wires which is in tl.e common focus of the object-glass and eye-piece, will be reflected hack again to nearly the same focus. Looking the telescope, the observer now adjusts it by means of a tangent screw till the reflected image of the hori zontal wire coincides with the real one. The final adjustment is perhaps most deli cately affected by turning the screw-head of the micrometer which moves the wire itself. When they coincide, the line joining the center of the phject-glass of the telescope with the intersection between the middle vertical and horizontal micrometric wire, will be vertical. Now, the angle between this and the line of collimation of the telescope, which, as we have said, joins the optical center of the object-glass with the intersection of the middle vertical and imaginary fixed horizontal wire, will, if the micrometer is in proper adjustment, be at once read off the micrometer screw-head. The instrument being clamped as above adjusted, the microscopes are read off, and the reading of the micrometer screw-head above mentioned being added to or subtracted from this read ing, as the case may be, the nadir reading of the instrument is determined. The zenith reading, therefore, which differs from it by 180°, is at once known.
• Again, to observe a star in the meridian, the instrument is previously adjusted so that the star, in passing the meridian, shall pass over the field of view of the telescope. As the image of the star approaches the center of the field, the observer adjusts the telescope by the tangent screw, so as very nearly to bring the image of the star to the horizontal wire. Finally, just as the star passes the middle vertical wire, lie bisects the image of the star with the horizontal wire by a touch of the micrometer screw-head. The circle being now clamped (or made fast), the "reading" is determined as before by reading the pointer and microscopes, and adding or subtracting, as the case may be, the reading of the micrometer. This reading now subtracted from the zenith-reading gives the meridian zenith distance of the star; and this, again, subtracted from DO', gives its meridian altitude above the horizon.
At the royal observatory of Greenwich, the principal observations are now made by an instrument which combines the mural C. with the transit instrument. See TRANSIT INSTRUMENT.