ClipArt ETC
"If B and C are points in the spiral and the ratio of AC to AB be given, the intermediate point D may be obtained by describing a semicircle on BC as a diameter and erecting a perpendicular at A." —Anthony, 1904

Drawing Equiangular or Logarithmic Spiral

"If B and C are points in the spiral and the ratio of AC to AB be given, the intermediate point D may…