Understanding Helmholtz's philosophy of science requires attention to his experimental practice. I sketch out such a project by showing how experiment shapes his theory of perception in three ways. One, the theory emerged out of empirical and experimental research. Two, the concept of experiment fills a critical conceptual gap in his theory of perception. Experiment functions not merely as a scientific technique, but also as a general epistemological strategy. Three, Helmholtz's experimental practice provides essential clues to the interpretation of his theory of perception. A case study from experimental investigation of hearing shows how he designed such studies in accordance with the epistemological commitments of the theory of perception. Yet, while the theory was important to his experiments, the soundness of the experimental strategy was epistemically independent of those commitments. Secondly, the case study illustrates how Helmholtz consistently held that causal inferences underwrite reference to a real, but indirectly experienced external world.