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The Line of collimation is the imaginary line joining the optical centre of the object-glass to the intersection of the wires in the field of view of an astronomical telescope. When the axis on which the telescope turns is not exactly at right angles to the line of collimation, the defect from the right angle is called the Error of C. This error is corrected by viewing a distant object, first when the telescope is in a certain position, and again when the axis of rotation has been reversed on its bearings. It may also be corrected by means of an eye-piece so constructed that the observer can see at the same time the wires in the field of view, and their image formed by reflexion in a basin of mercury; this is called a Collimating eye-piece. The error can also be corrected by the use of a small telescope floated on mercury, the wires in whose field of view serve as a distant object; this instrument is called a Collimator. (Collimation should have been written from the first, Collineation; a false reading of collimare, in a passage of Cicero, for collineare -con, together, linea, a line-having caused the error. See Littre, s.v.)