During the Revolutionary War, Gen. George Washington took a keen interest in the tiniest details of how his spies operated, right down to the formula for invisible ink. His focus on the minutia of the Culper Spy Ring, which had key Long Island links, is visible in a rare and historically important letter being auctioned in Manhattan tomorrow by Christie's.

The letter was written in 1779 by Washington to his chief of intelligence, Gen. Benjamin Tallmadge, and focuses - without naming him - on the man who historians have called the Revolution's most important spy, Robert Townsend of Oyster Bay. His code name was Culper Jr., and Washington, at his own request, never learned his true identity.

The 3 1/2-page letter is from the collection of the late publishing baron, Malcolm Forbes, and is expected to sell for $120,000 to $180,000.