“…Histological Studies.-A variety of procedures under this heading has been adopted to evaluate drug release from vehicles. Strakosch (96) has studied the penetration of salicylic acid, sulfur, resorcinol, and other substances by examining histological sections for evidence of keratolysis. Dyes.-One of the early methods used for studying penetration of substances into skin employed an oil-soluble dye to color the ma-nique which is used will be determined by the type of radiation, the half-life, and the solubility of the radioactive isotope being studied.…”