Using artificial tear solutions with varying constituents in physiologic concentrations, surface chemical properties of the constituents have been studied. Both mucus (glycoprotein) and simple proteins are capable of lowering the surface tension of tears. Mucus has a great affinity for an oilwater interface. Conjunctival mucus spread by the action of the lids and adsorbed at the corneal surface is thought to be the principal factor in spreading the precorneal tear film. A hypothesis to explain the effect is presented.