Letter from William Lipscomb addressed to his son James and grandson Thomas, postmarked Jan. 3, 2006.  The Bohr radius (about 0.529 A) and the Rydberg constant (about -13.6 eV) of the hydrogen atom are derived from first principles. This is the entire letter.  It does not have or need an explanation or a signature.

Grandson Thomas used this letter to solve a problem in physical chemistry class on Nov. 16, 2013, two years after Bill's death.