Why the Pupil Looks Black
To an observer, the pupil of a healthy eye looks dark. It seems odd that light could enter the eye and not return, since the retina at the back is illuminated when light shines in. The reason is an optical one: the eye is built so that light returning from the retina follows the same path back toward the light source. An observer looking from the side, or from a position that blocks the lamp, sees almost nothing.
For centuries this meant that the inside of a living eye was hidden. Physicians knew a good deal about eye anatomy from dissection, and cataracts, the clouding of the lens, were treated surgically in ancient and medieval times, but problems at the back of the eye could not be seen. A patient might lose sight with no visible sign. Doctors could only judge from what the patient reported and from the look of the front of the eye.
Babbage and Helmholtz
The instrument that solved the problem used a simple optical idea. If the observer could look along the same line as the light shining into the eye, the red-orange glow of the retina would be visible.
The English mathematician and inventor Charles Babbage is reported to have built a device that did this in 1847. He showed it to an eye doctor, who is said to have shown little interest, and the idea was left aside. Because the story depends on later recollection, it is usually told with caution.
In 1851 the German physician and physicist Hermann von Helmholtz, then teaching in Königsberg, built an instrument he called the Augenspiegel, or eye mirror. He worked it out while preparing a lecture, to demonstrate how light returns from the eye. His first versions were made with a stack of small glass plates, similar to microscope cover slips, that reflected light from a lamp into the patient’s eye while the observer looked through the plates. They were crude and relatively cheap to make, and his publication in 1851 is usually cited as the start of ophthalmoscopy. Credit for the first idea is sometimes shared with Babbage, but it was Helmholtz who published a clear description and a method.
Improving the Design
Others quickly made the instrument easier to use:
- Within a year or two, the physician Christian Ruete in Germany developed an approach in which a concave mirror directed light and a lens produced a wider but upside-down view of the retina, a technique now called indirect ophthalmoscopy.
- Mirrors with a small hole in the center let the observer look directly through the mirror, giving a magnified upright image of a smaller area.
- Other lenses could be rotated into place to correct for the focus of the patient’s or examiner’s eye.
Albrecht von Graefe in Berlin and other eye specialists adopted the instrument and used it to describe diseases of the retina and optic nerve. In 1855 Carl Liebreich published an atlas of the ophthalmoscopic appearances, and similar illustrated books helped doctors learn what they were seeing. Practitioners in various countries described swelling of the optic nerve, retinal detachment and other conditions that could now be seen in life.
From Candle to Battery
Early ophthalmoscopes relied on a candle or oil lamp, and later a gas lamp, reflected into the eye, so examinations were usually done in a darkened room. Electric light sources were added in the later nineteenth and early twentieth centuries, and handles containing batteries made small, self-contained hand-held versions practical for clinics. The direct ophthalmoscope with an illuminated head and a dial of lenses became one of the common tools in a doctor’s bag, and it remains in use in much the same basic form.
More Than Eye Disease
Because the retina is the only place in the body where small blood vessels can be seen directly without surgery, the ophthalmoscope was used by physicians as well as eye specialists. Changes in the vessels could be linked to general conditions such as high blood pressure and diabetes, and this gave the instrument a place in general medicine. The same reasoning is why a retinal examination can still be part of a physical or diabetes check in some settings, although practices differ and this article does not provide medical guidance.
Later Developments
The twentieth century added many ways to look inside the eye:
- the slit lamp, a microscope with a narrow beam of light, for examining the front and middle parts of the eye
- fundus photography, which records the retina as a picture
- fluorescein angiography, which uses a dye to show blood flow
- optical coherence tomography, which makes cross-section images of the retina and is now a common form of diagnostic imaging
- research on retinal implants and other devices, which depend on detailed knowledge of the retina
Even with these tools, the ophthalmoscope retains a place as a simple, portable first look. It is a typical example of an invention that came from physics and was taken up by medicine, and it shows how an instrument can create a field, since modern eye medicine grew in part from the ability to see what was happening inside the living eye.