This volume brings together six published and two new essays by the noted philosopher of science, Peter Achinstein. It represents the culmination of his examination of methodological issues that arise in nineteenth-century physics. He focuses on the philosophical problem of how, if at all, is it possible to confirm scientific hypotheses that postulate `unobservables' such as light waves, molecules, and electrons? This question is one that not only was of great interest to nineteenth-century physicists and methodologists, but continues to occupy philosophers of science up to the present day.