A Simple Optical Apparatus for Measuring the Corneal Thickness, and the Average Thickness of the Human Cornea *

PREVIOUS methods of measuring the corneal thickness in the living eye have recently been extensively reviewed and criticized by von Bahr (1948a), who devised an ingenious method (1948a, b) of overcoming the practical difficulties and theoretical objections of its forerunners. He employed an apparatus consisting of two rotating glass plates, coupled by gearing, which were mounted in front of the condensing lens and microscope of a slit lamp, so that the aperture of each was half covered by one of these plates. On rotation, the reflection of the slit in the corneal endothelium may be displaced in the field of view of the microscope and brought into coincidence with that in the epithelium. By noting the angle of rotation, the thickness of the cornea may be obtained from a curve derived by calculation. Von Bahr claimed for this instrument that it was simple and easy to handle and gave a satisfactory degree of accuracy and consistency. We were projecting a series of experiments in which changes in the thickness of the cornea in rabbits were to be followed after its metabolism was interfered with in various ways. Accordingly, a similar apparatus was constructed and attached to a Haag-Streit slit lamp. Experience showed, however, that there were disadvantages in this arrangement which made it difficult to use with a normal eye, and quite impossible with disturbed corneae for which it had been intended. There are eight images of the slit formed by this optical system when operating correctly, and in our model an extra " ghost " image due to a double reflection within the glass plates. Two of these images, one an epithelial, and one an endothelial reflex, have to be aligned, but this is not easy as the former is some thirty times the brighter, and its dazzle conceals the other when they are adjoining. Again, the closeness of the glass plates to the slit-lamp and microscope lenses resulted in the light which traversed the first