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.
Cornu was estimating the intensity minimum of the light. This light was being blocked by the adjacent teeth. Cornu made pairs of observations on either side of the extinction maxima. Cornu was averaging the values obtained with the wheel. This wheel was spun clockwise and counterclockwise. The adjacent teeth were a relatively inaccurate procedure.