An addition to the specular microscope was described which allowed it to record the thickness of the excised [rabbit] cornea automatically as a function of time. The focus of the instrument was scanned mechanically through the tissue, and the position of the reflecting surfaces was detected by a photoelectric system and marked on a chart recorder. The system was able to follow thickness changes over periods of many hours and with an accuracy greater than obtainable by manual operation. This system was helpful in the evaluation of a new medium which considerably extends the useful lifetime of the corneal endothelial fluid pump.