DIAMETER (from the Cr. &a, through, µErpov, measure)  , in geometry, a line passing through the centre of a circle or conic section and terminated by the curve; the "principal diameters" of the ellipse and hyperbola coincide with the "axes" and are at right angles;" conjugate diameters " are such that each bisects chords parallel to the other . The diameter of a quadric surface is a line at the extremities of which the tangent planes are parallel . Newton defined the diameter of a curve of any order as the locus of the centres of the mean distances of the points of intersection of a system of parallel chords with the curve; this locus may be shown to be a straight line . The word is also used as a unit of linear measurement of the magnifying power of a lens or microscope . In architecture, the term is used to express the measure of the lower part of the shaft of a column . It is employed by Vitruvius (iii . 2) to determine the height of a column, which should vary from eight to ten diameters according to the intercolumniation: and it is generally the custom to fix the lower diameter of the shaft by the height required and the Order employed . Thus the diameter of the Roman Doric should be about one-eighth of the height, that of the Ionic one-ninth, and of the 'Corinthian one-tenth (see ORDER) .