This paper offers an ethnomethodological clarification of Husserl's concepts of "Galilean physics" and "regressive inquiry". It employs the reader's textual-practical operationalization of these concepts. With the use of a simple optical prism as a perspicuous case of a scientific instrument, the reader will be asked and instructed to make a self-reflexive inquiry into the practical contingencies of the prismatic field of reflection. The reader will discover that the geometric structures of the reflective field of the prism is an achievement and formalization of the reader's own operatlonalizations. I named these discoverable discovering operationspraxioms or practical axioms — the pre-scientific contingencies of the geometric structures of prismatic reflection.