Extracted human teeth which were caries-free and with normal appearance were used .for the studies. 92 teeth were made up to the specimens. They were cut by means of a high speed carborundum wheel, so to obtain suitable surfaces for investigations. The cut surfaces were bisected through the long axis either in the bucco-lingual or labio-longual plane. The surface of the enamel and dentin of every specimen was artificially ground and polished to the