A GENERALLY accepted feature of the histology of ground sections of carious lesions in human dental enamel is the so called ‘dark zone’. This zone, which lies at the border of the lesion, between the main body and the translucent zone or normal enamel, shows positive birefringence when the section is placed in quinoline and viewed by polarized light. It has been demonstrated that the positive birefringence in this zone is related to the presence of small inter-crystallite spaces which are inaccessible to media of large molecular size and contain air or vapour which is of low refractive index. This results in positive form birefringence which reverses the negative intrinsic birefringence of the enamel structure¹.