Look up indentation (Schiøtz) tonometry. In Eastern Europe, applantation tonometry was developed and used around that time, where the patient was sedated with cocaine and a known-weight cylinder covered in hydroglycerinated Bismark brown solution (basically eye-safe ink) was placed on top of the cornea. Part of the cornea that pressed flat against the tool displaced the ink, which was then stamped on paper to measure the white circle.

Non-contact tonometry was first developed in 1972 by Dr B. Grolman and didn't change much since except now it's digital of course.