Badal Optometer
Optometer (op-tom-e-ter) is an instrument for determining the refraction of the eye.
The Badal optometer (a subjective optometer) relies upon the patient's judgement of sharpness or blurredness of a test object. It consists of a single positive lens and a movable target. The vergence of light from the target, after refraction through the lens, depends upon the position of the target. The patient is instructed to move the target towards the lens from a position where it appears blurred until it becomes clear. That point (converted in dioptric value) represents the refraction of the patient's eye. This is a crude and inaccurate instrument, in which the measurement is marred by accommodation, variation in retinal image size with target distance, large depth of focus and non-linearity of the scale.
A case including a Badal optometer, instructions, order form for spectacles and a frame sample. I bought this set at an antique store years ago. It's in almost "mint" condition. The order form (go to link below) was printed in 1928.
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