How did Fresnel's theory of light lead to piecemeal realism?

In the philosophy of science, the case of Augustin-Jean Fresnel serves as a primary example for the development of "piecemeal realism." This perspective emerged as a refinement of scientific realism in response to critiques—such as Larry Laudan’s "pessimistic induction"—which argued that because many successful past theories were later proven false, we should not assume current successful theories are true.12

Fresnel’s theory of wave propagation was spectacularly successful in predicting phenomena like diffraction and interference, including the famous Poisson spot. However, Fresnel based his work on the existence of an all-pervading ether, a medium that modern science has rejected.13

Piecemeal realism resolves this by rejecting the "all-or-nothing" assessment of theories. Instead of labeling Fresnel’s theory as entirely true or false, it distinguishes between the parts of the theory that generated its predictive success—such as the mathematical treatment of wave propagation—and the auxiliary hypotheses, like the ether, that did not. By focusing on the specific components that are "put to work" in successful predictions, realists argue that we can maintain a commitment to the approximate truth of those specific elements while acknowledging that other parts of the theory were mistaken.1