February 16, 2002 To the Editor: Re "Frayn Takes Stock of Bohr Revelations" (Arts & Ideas pages, Feb. 9) and "New Twist on Physicist's Role in Nazi Bomb" (front page, Feb. 7): Regarding the letters that Niels Bohr drafted but never sent to his friend Werner Heisenberg, it is important to make clear why controversy over Heisenberg's role has persisted, the fact that the small-scale German bomb program came to a halt in June 1942, and that it was Heisenberg who persuaded the authorities to kill it. In his unsent letters, Bohr said he had "wondered with what authorization such a dangerous matter, of such great political importance, could be taken up with someone in an occupied country." To this question Heisenberg could have answered truthfully that he had no authority to tell Bohr about the German bomb program; he did it on his own. Michael Frayn opens his play "Copenhagen" with the inevitable next question: "Why did he come?" asks Bohr's wife. "What was he trying to tell you?" But Bohr never sent the letter he had been trying to write, and Heisenberg had no chance to answer. THOMAS POWERS South Royalton, Vt., Feb. 13, 2002 The writer is the author of "Heisenberg's War: The Secret History of the German Bomb." Copyright 2002 The New York Times Company