An ophthalmoscope enables a physician to examine the interior of the eye to detect abnormalities or signs of disease on the retina and lens of the eye. It does this by directing a tiny beam of light through the pupil. The pupil is the black "window" of the eye.
Babbage and Helmholtz
The first ophthalmoscope was invented in 1847 by Charles Babbage (1792-1871), an English mathematician. Babbage gave the device to a physician for testing, but it was laid aside and forgotten. Four years later, German physician and physiologist Hermann von Helmholtz (1821-1894) developed his own version of the ophthalmoscope. Helmholtz was unaware of Babbage's instrument. Because he had better luck making his device known, Helmholtz is often credited as the sole inventor.
Helmholtz's instrument operated by using a mirror to shine a beam of light into the eye. The observer would look through a tiny aperture (opening) in the mirror. Helmholtz found that looking through the lens into the back of the eye only produced a red reflection. By attaching a condenser lens he obtained a clearer inverted image, which was then magnified five times. He called this combination of a mirror and condenser lens an indirect ophthalmoscope. It was used regularly for eye examinations until 1920.
Helmholtz also invented the ophthalmometer, which was used to measure the curvature of the eye. The eye's curvature determines whether the focal point of an object's image will be on the lens of the eye, or in front or behind the lens. If the focal point isn't on the lens, the person will be near-or far-sighted. In addition, Helmholtz studied color blindness and the speed of nervous impulses. He also wrote the classic Handbook of Physiological Optics.
Swedish ophthalmologist Allvar Gull-strand (1862-1930), who also studied physiological optics, developed another version of the ophthalmoscope. He also invented a slit lamp, used with a microscope, that enabled a physician to locate foreign bodies in the eye.
The Modern Opthalmoscope
The modern ophthalmoscope is a hand-held instrument. It contains a small battery-powered lamp that directs the beam of light by way of a mirrored prism. The observer looks through a tiny hole in the prism. The instrument magnifies the image and can be focused by a series of revolving lenses. The lens needed to focus the image gives the doctor an approximation of the glasses lens prescription needed to correct the patient's vision.
A new type of ophthalmoscope that can project a laser beam is used in eye surgery to correct a detached retina. Another, larger type of ophthalmoscope, called the binocular ophthalmoscope, is used in clinical research. It provides an image of the eye that is magnified fifteen times.