Berkel, Klaas van. “H.A. Snellen, Two Pioneers of Electrocardiography. The Correspondence Between Einthoven and Lewis from 1908-1926”. BMGN - Low Countries Historical Review, vol. 101, no. 2, Jan. 1986, pp. 259-60, https://doi.org/10.18352/bmgn-lchr.2718.