The following variation seems an improvement on the usual method of tracing a refracted ray. In the usual method when an incident ray OP is given, any radius OP is taken and a circle described. PA is drawn perpendicular to the surface AB. OA is then sub-divided into the number of units expressed in the numerator of the fraction μ, and OB is measured backwards, the number of units expressed by the denominator. BQ is then drawn perpendicular to AB to meet the circle in Q. OQ is the refracted ray.