 In The Mechanics of Meaning, David Hyder traces discussions of Helmholtz’s theory regarding “sensation-spaces and their relation to the philosophy of space and perception” by Boltzmann, Poincaré, Carnap, Russell, and Weyl (Hyder 2002, 19n). De Kock 2013 is an analysis of the problem of externality in Helmholtz’s theory of perception. Biagioli (forthcoming) analyzes Helmholtz’s debate with Albrecht Krause over the status of geometrical axioms; more generally, Biagioli’s work illuminates Helmholtz’s relationship to Poincaré and to Holder. Darrigol 2003 also places Helmholtz’s work in context, including with respect to related work by Grassmann, Fechner, and Wundt. Darrigol’s focus is Helmholtz’s theory of number and measure in physics and mathematics.