In section 3.3 of Philosophy of Natural Science, Hempel argues that crucial tests are not sufficient enough to prove a given hypothesis or to disprove them. Hempel states what some may believe why a crucial test can prove or disprove a hypothesis. If there are two competing hypothesis which involve the same subject and no available evidence favors one or the other, then there exists a test, which will produce conflicting outcomes for the different hypotheses. This test is the so-called crucial test would then presumably refute one hypothesis while supporting the other. Hempel then presents his side of this argument using an example of past experiments involving the nature of light. He describes how Foucault performed an experiment involving the velocity of light through air and water. This experiment was meant to show whether light consists of waves or extremely small particles as presented by Newton. Foucault’s experiment was performed and the resulting outcome was used to refute Newton’s view of light as small particles traveling at a high velocity. Hempel believed that this test was not strong enough to completely support or refute either view of light. The experiment relied on the assumption that light as waves would travel faster in air than in water. However Hempel argues that the conception of light as streams of particles was too indefinite to assume that it would travel slower in air without additional assumptions about the motion of particles and their surrounding medium. So even though the results may seem to support and prove the wave hypothesis, it doesn’t necessarily disprove the particle theory. In fact Einstein later proposed a theory, which eliminated the classical wave theory, using support from an experiment by Lenard in 1903. But again as in the previous example one of the hypotheses was not definitely refuted, in this case being the wave theory. M.-M. V.