Phase retardation per reflection δ as a function of 1 − R. Filled grey points are the data reported in [6], points with arrow represent mirrors with an anisotropy lower than the sensitivity of the apparatus. Empty grey points are data taken from references [1,25–29,31]. Empty and filled diamonds are data from respectively [11] et [32]. The three empty square points are taken from reference [7] and were obtained with mirrors of increasing number of deposited layer, the birefringence increasing with the number of layers. To guide the eyes, the trend showed by [6] is represented by a dashed line.