: Armed with a photometer originally designed for evaluating telescopes, Richard Potter in the early 1830s measured the re(integral)ective power of metallic and glass mirrors. Because he found significant discrepancies between his measurements and Fresnel's predictions, Potter developed doubts concerning the wave theory. However, Potter's measurements were colored by a peculiar procedure. In order to protect the sensitivity of the eye, Potter made certain approximations in the measuring process, which exaggerated the discrepancies between the theory and the data. Potter's measurements (...) received strong criticisms from wave theorists, not because they felt they needed to defend their theory, but because they believed that Potter was wrong in using the eye as an essential apparatus in the experiments. Potter's photometric measurements and the subsequent debate reveal the existence of two incompatible sets of measuring procedures, each of which consisted of a body of practices concerning how photometric instruments should be used properly. In the debate, the differences regarding measuring procedures shaped the participant's judgments of experimental evidence and eventually their evaluations of the optical theories. (shrink)