'The basis of tonometry by applanation, even of tonometry as a whole, is Fick's law, or Imbert-Fick's law, which actually is a much more accurate name'. It is affirmatively cited solely in the ophthalmic and optometric literature, and in patent applications for new tonometers, but as a law of physics underlying clinical practice, it has been challenged both on theoretical grounds and on empirical, evidence-based, data. As the diagnosis and treatment success of glaucoma are primarily determined by tonometry, it seemed profitable to explore in historical context the genesis of this 'law' that was said to be the basis of the method.