Description
Rare antique optometrist’s bichrome vision-testing spectacles, c. early twentieth century. These curious red and blue-lensed eyeglasses were used to not only test a patient’s vision, but also for a little diagnostic trickery when warranted.
Optometry manuals of the period stressed that the purpose of an examination was not to determine how poorly the patient sees, but rather what the patient has the ability to see. To prevent self-conscious exaggeration and to determine the strength of vision objectively, the optometrist might employ red and blue (or green) lenses, especially when one eye retains good vision. Colored charts were designed to deceive each color-filtered eye, making each eye respectively blind to certain letters (white letters on a pink ground would become invisible when seen through the red lens, for instance). Typically, the stronger eye would be covered with the red, which made — unbeknownst to the patient — certain stimuli only visible to the blue-filtered eye. This method provided for a more objective evaluation of a patient’s vision.
The red and blue lenses were also employed in the bichrome test (today more commonly called the red-green duochrome test) which is used in refraction to determine undercorrection or overcorrection. The test is based on a monocular endpoint in which each eye is tested separately. It is a subjective test that requires responses from the patient and is used to refine the spherical endpoint. Chromatic aberration, the basis of the test, occurs because different wavelengths of light are bent to a different extent. The longer wavelength (red) is refracted less than the shorter (green). If the letters on the red side stand out more, the optician should add minus power; if the letters on the green side stand out more, he or she should add plus power. Neutrality is reached when the letters on both backgrounds appear equally distinct.
Ophthalmology purposes aside, these red and blue spectacles are simply fun as a colorful sculptural object.
Recently acquired from an important private collection of antique eyewear.
Measurements
Frame front: 4 3/8 inches across
Temples: 6 inches long (at a curve)
Condition
Very good antique condition.
Shipping
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