- 2, in o which the curve
**QOP**is the evolute and QPR FIG. - (3) The angle
**QOP**, which the radius vector makes with the fundamental plane. - The angle
**QOP**is the latitude of the place and the angle NOQ its longitude. - After the image of the body is brought into coincidence with the cross threads, the instrument is turned through 180° on the axis, which results in the line of sight of the telescope pointing in a certain direction OQ, determined by the condition QOZ = ZOP. The telescope is then a second time pointed at the object by being moved through the angle
**QOP**. Either of the angles QOZ and ZOP is then one half that through which the telescope has been turned, which may be measured by a graduated circle, and which is the zenith distance of the object measured from the direction of the axis OZ.