This monograph was written for the Conference on the New Instructional Materials in Physics, held at the University of Washington in summer, 1965. The measurement of very small distances and sizes, and the concept of models are discussed in the introduction. The optics of slits is dealt with in chapter 2. Chapter 3 presents the optics of holes. The phenomena of interference and diffraction are included in chapter 4. The author discusses the experimental determination of interatomic distances in a gas molecule for observation of interference effects in electron scattering in chapter 5. The monograph concludes with a number of references to physical optics. (LC)