Wiener has given an explicit solution for the servo mechanism which minimizes the r.m.s. error when the system is following a signal contaminated with noise. The paper describes an elementary graphical method for evaluating Wiener's results, and makes use of a pair of transparent cursors which have already been described. An example is given, from which it appears that the accuracy of the method may be sufficient for engineering purposes.An intermediate result of the graphical method gives the minimum phase corresponding to a given attenuation characteristic.