The Ritz theory of electromagnetism and optics is criticized in relation to the experimental evidence after the introduction of a simple and natural modification of the hypothesis concerning the velocity of radiation scattered by the electrons of a medium. It is argued that the theory is then in harmony with the electron theory of dispersion, accounts satisfactorily for aberration, the first-order Doppler effect from moving sources and interferometer experiments on binary stars. There is no evidence from binary stars which contradicts it. It is compatible with the second-order Doppler effect and possibly the Fizeau experiment since arguments are advanced which indicate that these phenomena depend essentially on the momentum and energy of radiation. Other phenomena are discussed. It is concluded that the best evidence against the theory comes from experiments on the lifetimes of fast mesons and the velocity of γ rays and light from moving sources. The justifications for the discussion are the desirability of having the experimental base of an important part of physics be as rigorous as possible, and the hope that it sharpens our understanding of existing evidence for special relativity and stimulates new and different experiments.