Metal Optometer

Nickel-plated over brass, it originally cost $0.75. Patent applied for around 1875.This was used to help determine the power of glasses a person would require. The instrument is made of brass and steel. The sliding disc has embossed dots and says, “Count the dots”.
The main bar has etched markings for concave and convex.
The problem with a device like this is that it does not control well for accommodation, the muscular contraction that occurs inside the eye in an effort to look at near objects.