Direct measurements of membrane unstirred layers.

1. The unstirred layer of fluid bounding the posterior surface of the rabbit cornea and contact lens has been directly measured using an optical apparatus.2. Small polystyrene latex spheres (< 0.25 mu diameter) were used as an indicator because of the excellent light-reflective properties; other indicators were used (e.g. carmine particles) and had a similar behaviour.3. With the bulk solution unstirred the layer was 350 mu thick on the cornea and 150 mu thick on the contact lens. Following vigorous stirring the layer was reduced to 65 mu on the cornea and < 20 mu on the contact lens.4. The present direct measurements of the unstirred layers thickness confirm previous indirect measurements.