Empirical starting points:  constitution and construction

     Early proposals
          J.J. Thomson's
          Rutherford's

     A comment:  the model-making tradition

     Bohr's model
          How Bohr came to Rutherford
          Imposing a nonclassical stability condition on allowed orbits
          What are the experimental consequences?

Discrete spectra

     Background:  observations and apparatus
     Signatures of the chemical elements
     How are spectral lines produced?  Models of oscillating electrons

Returning to Bohr's model

     Hypotheses about light emission:  transitions between orbits
     Two quantization conditions, two fundamental principles
     What it explains:  spectra, the periodic table
     The correspondence principle