The historical antecedents of Frege's treatment of binocular vision in "The thought" were the physiological writings of Johannes Mueller, Hermann von Helmholtz, and Emil du Bois-Reymond. In their research on human vision, logic was assigned an unexpected role: it was to be the means by which knowledge of a world extended in three dimensions arises from stimuli that are at best two-dimensional. An examination of this literature yields a richer understanding of Frege's insistence that a proper epistemology requires us to recognize the existence and importance of nonsensible sources of knowledge.
"Optics of Thought: Logic and Vision in Müller, Helmholtz, and Frege." Notre Dame J. Formal Logic 41 (4) 365 - 378, 2000. https://doi.org/10.1305/ndjfl/1038336881