The History of Ophthalmology
Part 4: The Nineteenth Century: Seeing the Eye
This century gave us the ophthalmoscope, first called an eye mirror. After a series of ever-closer attempts made by many people, Hermann von Helmholtz, a German doctor and physicist, saw the potential of such a tool, devised his own version of it, and began teaching about it. Finally eye doctors could see the inside of a human eye and get a good view of diseases such as glaucoma and diabetic retinopathy.
At first the doctor held a lens in front of his own eye and another one in front of the patient’s eye. A light source was nearby, as it was soon discovered that the ambient light would always be inadequate. The long-sought-after trick for successfully seeing inside a person’s eye is to place your own eye in the path of the light rays entering and leaving the patient’s eye. This allows you to see what is being illuminated inside the eye.
Modernized versions of this instrument are still central to ophthalmological diagnoses:
- An ophthalmoscope is a hand-held device with several magnifying lenses up to x15, a light source, and an aperture in the center. The eye doctor shines it into a patient’s eye, looking through the aperture and one of the lenses to get a clear and magnified view of the interior of that eye. The eye’s structures can thus be clinically examined (e.g., cornea, lens, conjunctiva).
- The Phoropter is another instrument with many lenses used to measure an eye’s optical power. The patient sits with her chin and forehead against supports, looking through an aperture with one eye. The doctor flips from lens to lens, asking you to read the smallest line of letters you can see.
Since the phoropter gives an objective assessment of each eye’s refractive error, the doctor can write a prescription for glasses or contact lenses. It uses the diopter as the unit of measurement of an eye’s optical power.
- A slit lamp is a floor-standing instrument with a strong light which can be narrowed down to a slit. It is used with a microscope to see highly magnified views of eye structures. The patient rests his chin and forehead on the curved supports to keep the head still. Eyedrops are used to dilate the pupils when deeper structures need examining, such as the retina or optic nerve.
The Snellen Eye Chart
Herman Snellen was a Dutch ophthalmologist who devised an eye chart in 1862 to test a person’s visual acuity. This Snellen chart is still used today, in several versions. It consists of rows of letters which decrease in size but increase in number with each descending row.
To describe the traditional chart briefly:
A person standing 20 feet from the chart who can clearly read the sixth row down is said to have 20/20 vision.
If a person standing 20 feet from the chart cannot read this line, but can read the line 3 rows higher, he is said to have 20/40 vision. These letters are twice as wide and twice as high as those on the 20/20 row. A person with 20/20 vision could read these letters standing 40 feet away.
If a person standing 20 feet away cannot read even the top row with a single very large letter, even wearing the best possible glasses, she is defined as being legally blind. This person’s vision is 20/200.
History of Ophthalmology Part 1: The Ancient World
History of Ophthalmology Part 2: The Middle Ages: Spectacles
History of Ophthalmology Part 3: Anesthesia and Infection
History of Ophthalmology Part 5: The Twentieth Century: Swift Progress
History of Ophthalmology Part 7: Leading up to LASIK
History of Ophthalmology Part 8:LASIK Into the Twenty-First Century