Instrumentation for the measurement of corneal thickness by optical means was apparently first accomplished by Blix.1 The method used involved the observation of the specular reflection from the epithelial and endothelial surfaces of the cornea. The apparatus consisted of two identical microscope tubes arranged horizontally and converging at an angle of 40° to a point in front of the tubes (Fig 1). One tube contained an illuminated diaphragm, the image of which was situated at the point of intersection of the microscope axes. By moving the tubes simultaneously and symmetrically along the line bisecting the angle between them, the image of the diaphragm could first be observed as reflected from the epithelial surface, and then the apparatus adjusted to cause the endothelial reflex to coincide with the point at which the epithelial reflex had been observed. The difference in adjustment gave the apparent distance between the two surfaces. The
DONALDSON DD. A New Instrument for the Measurement of Corneal Thickness. Arch Ophthalmol. 1966;76(1):25–31. doi:10.1001/archopht.1966.03850010027008
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