AN ELEMENTARY APPROACH TO THE QUANTUM THEORY OF SCATTERING BY A POTENTIAL Introduction Stationary Scattering States. Calculation of the Cross Section Scattering by a Central Potential. Method of Partial Waves Complements ELECTRON SPIN Introduction Special Properties of Angular Moment 1/2 Non-Relativistic Description of a Spin 1/2 Particle Complements ADDITION OF ANGULAR MOMENTA Introduction Addition of Two Spin 1/2's. Elementary Method Addition of Two Arbitrary Angular Momenta. General Methods Complements STATIONARY PERTURBATION THEORY Description of the Method Perturbation of a Non-Degenerate Level Perturbation of a Degenerate Level Complements AN APPLICATION OF PERTURBATION THEORY: THE FINE AND HYPERFINE STRUCTURE OF THE HYDROGEN ATOM Introduction Additional Terms in the Hamiltonian The Fine Structure of the N=2 Level The Hyperfine Structure of the N=1 Level The Zeeman Effect of the Hyperfine Structure of the 1s Ground State Complements APPROXIMATION METHODS FOR TIME-DEPENDENT PROBLEMS Statement of the Problem Approximate Solution of the Schrödinger Equation An Important Special Case: Sinusoidal or Constant Perturbation Complements SYSTEMS OF IDENTICAL PARTICLES Statement of the Problem Permutation Operators The Symmetrization Postulate Discussion Complements APPENDICES Fourier Series and the Fourier Transform The Dirac "Function" The Lagrangian and Hamiltonian in Classical Mechanics