Richard P. Feynman to Roberta Berry, December 18, 1974 / Richard P. Feynman to B. E. Bushman, January 7, 1975 / David A. Marcus to Richard P. Feynman, January 13, 1975 / Richard P. Feynman to David Marcus, February 18, 1975 by Michelle Feynman & David A. Marcus narrated by Johanna Parker / Richard P. Feynman narrated by Richard Poe