McClean tonometer was first designed in 1919. It was an American hand-held contact tonometer somewhat similar to the Schiøtz tonometer. It did not have any weights and took a reading directly in millimeters of mercury pressure. Its readings were generally higher than other instruments (up to 40 mm Hg) so many considered it to be inaccurate. On the box the instructions describe normal as between 22 to 40 mm Hg.
There are two tiny vertical handles to hold between one’s fingers while the instrument is lowered onto the patient’s cornea. The post then indents the cornea away from the foot plate. Depending upon how firm the eye ball is the indicator needle deflects to a reading on the scale.
This example was made by E.B. Meyrowitz, Surgical Instruments Co., 520 Fifth Ave. New York.
There is a good discussion of tonometer history at the College of Optometrists website.