Johnston Optometer

An optometer was used to measure a person’s refractive error (strength of eye glasses needed). This was determined by having the person view an object through the eyepiece then rotating the dials until the object was most clear. The main problem was that the natural ability of the eye to focus (accommodation) could throw off the result. These days, very few of these are ever offered They are considered a highly prized collectible for those interested in the field of optometry and/or ophthalmology.
This beauty was recently found in an abandoned house. Only the astute eye of its new owner prevented it from being tossed in the junk pile and lost forever.
It is signed:

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