Ratio of fitted period of the fundamental to 3 times the fitted period for the third harmonic, for each slit. Unity is marked with a dashed grey line. The blue region denotes the approximate range in which the n = 3 node exists. The grey region shows an estimate of the credible intervals for this ratio. These are derived using the credible intervals on the periods measured separately for the two harmonics, and propagated through the formula P₁/3P₃ in the usual manner for errors.