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Newton?€?s Opticks Book 1 opens with a set of definitions and axioms, so one might expect to find the theorems contained therein to be proved from said definitions and axioms via deductively valid rules of inference. But they?€?re not. Instead, Newton employs ?€?proof by experiment?€?: each theorem is proved via a series of experiments, which are represented by geometrical diagrams and accompanying text. Newton?€?s axioms and definitions do not feature explicitly in these proofs?€¡±they are not even mentioned in the discussions. I address two questions in relation to this case. First, how does ?€?proof by experiment?€? function as a proof? Second, what roles do axioms and definitions play in the trajectory from experiment to proven theorem? I argue that this case is revelatory of Newton?€?s understanding of experimental philosophy and the probative force of his (in)famous experimentum crucis.