L19A
       1. Motivation: The Curvature of a Surface Is
           Characterized by the Differential of Gauss Map
       2. Example: Cylinder
 
L19B
       1. Definition: Self-Adjoint Linear Map
       2. Proposition: The Differential of Gauss Map Is a Self-Adjoint Linear Map
       3. Definition: The Second Fundamental Form
 
L19C
       1. Definition: Normal Curvature
       2. Derivation: The Geometric Meaning of the Second Fundamental Form