The McLean tonometer was used to measure pressure in the eyeball. It could also be used to diagnose conditions such as keratitis, an inflammation of the cornea, and choroiditis, an inflammation of the layer between the sclera and the retina. Tonometry is also used to assess the risk for glaucoma. Today, doctors use a high-tech probe placed against the eye to accomplish the same goals.
From the Abraham Pollen Archives, Massachusetts Eye and Ear Infirmary.