The Ophthalmoscope

1895

An Instrument Which Reveals the Innermost Recesses of the Living Eye.

To the oculist Professor von Helmholtz gave the ophthalmoscope, and thus made it possible to investigate the conditions of the inmost recesses of the living eye. If the eye be illuminated, a portion of the light returns from the hinder surface, is brought to a focus by the lenses of the eye itself and forms an image of the retina in the external space. To see this was no easy matter. If the patient's eye were focused on a luminous object, the image would coincide with the source of light, and even if otherwise visible would be lost in the glare. If he looked elsewhere, the image would move, but inasmuch as the lenses cannot be adjusted to the clear vision of any object nearer than about ten inches that is the minimum distance from the eye at which it can form the image of its own retina. To see this clearly an observer without appliances must place himself at least ten inches from the image — that is, at 20 inches from the patient. At that distance the view would be so limited that no result could be obtained.

Von Helmholtz, however, convinced himself that if these difficulties could be overcome the image of a brightly illuminated retina could be seen. He made the observations through a small hole in the center of a mirror, which reflected light into the eye under examination. Then by means of a lens he shifted the position of the image backward until the relative positions of the observer and the patient were such that, according to calculation, the retina should be visible.

Again and again he tried and failed, but was convinced of the validity of the theory, and at last the experiment succeeded. From that time the oculist has been able to look into the darkness of the pupil and to see through the gloom the point of entry of the optic nerve and the delicate network of blood vessels by which it is surrounded. — Fortnightly Review.