The Ophthalmoscope

The First Steps in Seeing,
by Robert Rodieck.

In The First Steps in Seeing, Robert Rodieck describes the ophthalmoscope.

Light passes into the eye through the pupil, and continues through its mainly transparent interior to reach the retina. The portion of the light that is not caught by the photoreceptors is either absorbed or scattered in all directions by the underlying tissues. Some of the scattered light passes back through the pupil and out of the eye. But when we look into another person’s pupil, the back of the eye, or fundus, appears black. This is because the optical pathway of the light that enters the eye and falls on a given region of the fundus is the same as that of the light scattered from that region, which leaves the eye through the pupil. In effect, in order to see the interior of the eye under ordinary conditions, one has to place one’s head into this common pathway of the light.

A brilliant young clinician, Hermann von Helmholtz (1821-1894), grasped this issue, and realized that all he needed to do to see the interior of another person’s eye was to devise an optical device by which he could get both his head and the light into the pathway. He did so by placing a piece of glass between his eye and the patient’s and angling the glass so that it partially reflected the light from a lamp into the patient’s eye… The piece of glass and the lamp formed a device termed an ophthalmoscope (Greek opthalmos = eye + skopion, from skopein = to see). Modern ophthalmoscopes have a built-in light source, colored filters to emphasize some aspect of the view, and lenses to correct for any error in the optics of the clinician or patient (i.e., lenses of the same power that they might use in spectacles.)

The picture below shows a simple ophthalmoscope, which consists of just a light source, a semi-reflecting mirror, and two eyes.

An ophthalmoscope.

An image of the retina, as might be seen using an ophthalmoscope, is shown below. The dark patch in the center is the fovea, where the cone density is greatest. The light patch to its right is the optic disc where the optic nerve enters the blood vessels converge.

An image of the retina.
From Häggström, Mikael (2014). “Medical Gallery of Mikael Häggström 2014.”
WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.008

Learn more about the ophthalmoscope and its history from a website maintained by the College of Optometrists. Learn more about Helmholtz in one of my previous posts. Learn more about the physics of the eye in Chapter 14 of Intermediate Physics for Medicine and Biology.

The ophthalmoscope is yet one more example of how physics contributes of medicine and biology.