Johnston Optometer 1882

The Johnston Optometer is one of the rarest instruments in the history of optometry. This example was rescued from an abandoned house. Only the sharp eye of its current owner saved it from the junk pile forever. It is signed and dated, making it a precisely documented piece of American optical history.

The Inventor and Patent

George Johnston of Detroit, Michigan invented this optometer. US Letters Patent No. 254,139 was granted on February 28, 1882. The application was filed September 18, 1881. The instrument is signed:

JOHNSTON OPTICAL CO.
PAT. Feb 28, 1882
DETROIT MICH.

What the Optometer Does

The Johnston optometer measured refractive error — the strength of eyeglasses needed. The patient viewed an object through the eyepiece. The examiner rotated the dials until the image appeared most clear. The dial position indicated the prescription.

Johnston’s key innovation was testing for astigmatism alongside short and farsightedness simultaneously. Before his instrument, testing for astigmatism required a separate set of cylindrical lenses and a slow, tedious process. Johnston’s design made the combined test both easy and accurate. The angle of astigmatism could also be determined mathematically.

Mechanical Design

The instrument uses four sets of revolving lens discs. Each disc carries a full set of test lenses of different focal powers. The discs mount on shafts at angles to each other. A hollow half-globe allows light to pass centrally through the instrument. The operator rotates each shaft independently using hand wheels and index fingers. A spring-pawl system prevents accidental lens displacement. Each disc also contains one blank opening, allowing clear vision when that set is not needed.

A Known Limitation

Accommodation — the eye’s natural ability to self-focus — could interfere with measurements. This was a recognized challenge with all optometers of the era. Despite this, the Johnston optometer represented a serious and elegant attempt to objectify the refraction process in 1882.

Rarity and Collectibility

Very few Johnston optometers ever appear for sale. This example is a highly prized collectible for anyone interested in early American optometry and ophthalmology history.