178 



A. I. DARLING 



Fig. 6. Ground section of early caries of human dental enamel mounted in 

 cjuinoline (R.I. 1.62) and seen by polarized light in the 45° position. (X 130.) 

 (Reproduced b\ kind permission of the editor of the British Dental Journal.) 



Their explanation has, however, caused considerable difference of 

 opinion, which has concerned chiefly the translucent zone, the dark 

 zone, and Nishimura's (1926) and Gustafson's (1957) third zone 

 just superficial to the dark zone. Gustafson ( 1961 ) believed and has 

 now shown l^y microradiography that the increased negative bire- 

 fringence of the translucent zone can at times be due to increased 

 mineralization. Usuallv, however, it is caused by a loss of the form 

 birefringence present in normal enamel (Darling, 1958). Recent ad- 

 vances in the understanding of the birefringence of the carious lesion 

 have arisen largely from the appreciation by Keil ( 1936 ) that spaces 

 resulting from the carious process produce form birefringence to 

 give the picture seen by polarized light in most mountants. This 



