In a review article on tonometry several years ago,1 a number of modifications in the construction of the conventional Schiøtz tonometer2 were described. In particular, the shape of the hammer on the lever arm had been altered from the usual downward curve to an upward one in the form of an epicycloid. Statements were made concerning the superiority of this arrangement, but no supporting proofs or references were given.
An attempt to verify the mechanical principles led to an independent study of the most desirable shape for the contacting surface of the hammer. It will be shown that the theoretically ideal shape is that of the involute of a circle, rather than any epicycloid. Furthermore, the concave downward form with this curve provides practically the same advantages as the concave upward one. Nothing beyond the standard concepts of polar coordinates and the calculus will be required.
ASKOVITZ SI. A Note on the Construction of the Schiötz TonometerMathematical Analysis of the Hammer Contact Surface. AMA Arch Ophthalmol. 1957;57(1):7-10. doi:10.1001/archopht.1957.00930050009002