crucial experiment a means of deciding between rival theories that, providing parallel explanations of large classes of phenomena, come to be placed at issue by a single fact. For example, the Newtonian emission theory predicts that light travels faster in water than in air; according to the wave theory, light travels slower in water than in air. Dominique François Arago proposed a crucial experiment comparing the respective velocities. Léon Foucault then devised an apparatus to measure the speed of light in various media and found a lower velocity in water than in air. Arago and Foucault concluded for the wave theory, believing that the experiment refuted the emission theory. Other examples include Galileo’s discovery of the phases of Venus (Ptolemaic versus Copernican astronomy), Pascal’s Puy-de-Dôme experiment with the barometer (vacuists versus plenists), Fresnel’s prediction of a spot of light in circular shadows (particle versus wave optics), and Eddington’s measurement of the gravitational bending of light rays during a solar eclipse (Newtonian versus Einsteinian gravitation). At issue in crucial experiments is usually a novel prediction. The notion seems to derive from Francis Bacon, whose New Organon (1620) discusses the ‘Instance of the Fingerpost (Instantia – later experimentum – crucis),’ a term borrowed from the post set up at crossroads to indicate several directions. Crucial experiments were emphasized in early nineteenth-century scientific methodology – e.g., in John F. Herschel’s A Preliminary Discourse on the Study of Natural Philosophy (1830). Duhem argued that crucial experiments resemble false dilemmas: hypotheses in physics do not come in pairs, so that crucial experiments cannot transform one of the two into a demonstrated truth. Discussing Foucault’s experiment, Duhem asks whether we dare assert that no other hypothesis is imaginable and suggests that instead of light being either a simple particle or wave, light might be something else, perhaps a disturbance propagated within a dielectric medium, as theorized by Maxwell. In the twentieth century, crucial experiments and novel predictions figured prominently in the work of Imre Lakatos (1922–74). Agreeing that crucial experiments are unable to overthrow theories, Lakatos accepted them as retroactive indications of the fertility or progress of research programs. See also BACON, FRANCIS ; CONFIRMATION; DUHEM ; PHILOSOPHY OF SCIENC. R.Ar.