The ophthalmometer

from GlobalCONTACT - 02/2022

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From the Roth collection

After the discovery of the refractive phenomena of a glass sphere, medieval physics and anatomy puzzled over how images were formed in the spherical human eye. Later, when the laws of refraction and especially the magnifying properties of a convex lens became known, it was assumed that the spherically curved cornea and eye lens were responsible for the formation of retinal images. However, it was not until the invention of the obscura camera or the photographic camera equipped with a magnifying lens that the formation of images in the eye could be finally explained. By Hans-Walter Roth

BEFORE THE SUCCESSFUL CONSTRUCTION OF THE OPHTHALMOMETER, THE CURVATURE OF THE ANTERIOR CORNEAL SURFACE COULD ONLY BE ESTIMATED WITH THE KERATOMETER OR PLACIDO DISC.

While the optical properties of the crystalline lens inside the eye, also known as the "lens cristallinea", were known early on, the light-refracting relationships of the cornea remained unsolved for a long time and it took until the 19th century to fully answer all questions. Today it is known that the lens, viewed in isolation in air, is a concave lens with light-scattering properties. For this discovery, it was necessary to accurately record the inner as well as outer curvature of the cornea in all its sections. This required the development of a measuring device with which not only the radii of curvature of the corneal anterior surface but also of its posterior surface could be determined. Since the refractive index of the cornea was known to be 1.33, the refractive power in all sections could be calculated directly. The instrument in use today, usually referred to as an ophthalmometer, astigmatometer, or more accurately as a keratometer, essentially dates back to the work of Helmholtz in 1851, who had already invented the ophthalmoscope in 1850, making it possible to examine the back of the eye in living subjects. Before the successful construction of the ophthalmometer, the curvature of the anterior corneal surface could only be estimated with the keratometer or placido disc. Now it was possible to measure the so-called corneal radii at any point. Javal in Paris, Gullstrand in Stockholm and Schiötz

in Oslo improved the technique, which remains the basis for optical measurement of the eye until today. Gullstrand received the Nobel Prize for Medicine in 1911 for this work.

OPHTHALMOMETER FROM AROUND 1900

The device shown here is an ophthalmometer from around 1900. It consists of a cast-iron three-legged frame on which a classic telescope is mounted and focused to a distance of about 10 cm. To the right and left of this are two light sources that can be moved around the central axis and with which a measurement figure can be projected onto the cornea. The basic principle is the mirroring of test images projected onto the cornea. In the beginning, this was done by lighting with a gas flame, although it is not known whether many eyelashes fell victim in the process. The device shown was subsequently electrified and an electric cable wrapped in green cotton suggests that this was already done at the beginning of the last century. The measurement is uncomplicated. At the beginning, the two test images are projected onto the center of the cornea, focused and brought to concurrence by varying the projection angles or rotating the axes. The astigmatism and the values for the corneal radii are calculated from the determined angles or the axial position. This device, made with high precision, is an example of the craftsmanship of the century before last. Despite its age of over one hundred years, it is still fully serviceable. Such historical ophthalmometers were irreplaceable especially in the manufacture and fitting of the contact lens. They also proved their worth in the diagnosis of keratoconus or in the fitting of an injured eye with scarring astigmatism. Even after such a long time, it still works with amazing precision and is in no way inferior to modern, laser-controlled devices. It is the gift of a colleague for the collection and comes, lovingly cared for over the years, from his grandfather's ophthalmological practice. n