“…The interest in this problem is shown by the large number of optical studies (e.g. Bruhat & Grivet, 1935;Konstantinova, Ivanov & Grechushnikov, 1969;Rogers, 1977;Kobayashi, Takahashi, Hosokawa & Uesu, 1978;Horinaka, Tomii, Sonomura & Miyauchi, 1985;Kobayashi, Asahi & Takahashi, 1987) of this crystal. However, it seems that crucial assessments of the accuracies of the published results have not been made, presumably because of a lack of knowledge of the possible systematic errors.…”