Dear Tracy: Have just read with great pleasure your Sigma Xi lecture in the American Scientist.  A beautiful lecture.  Much amazed by the repeated references to the "entirely different mechanism mentioned on page 46".  Feel now obliged to pursue the theoretical study of this little model and its variants a little further and hope to do so some time soon with the help of a suitable graduate student.
Kindest regards
Max Delbruck
February 3, 1949