Instead of estimating the intensity minimum of the light being blocked by the adjacent teeth, a relatively inaccurate procedure, Cornu made pairs of observations on either side of the extinction maxima, averaging the values obtained with the wheel spun clockwise and counterclockwise.
Instead of estimating the intensity minimum of the light being blocked by the adjacent teeth, a relatively inaccurate procedure, Cornu set up an optical arrangement whereby he could adjust the intensity of the light on either side of a tooth for equal luminance. In addition, he averaged the values obtained with the wheel spun clockwise and counterclockwise.