The National Gallery of Art -- significantly strengthening one of its weakest areas -- has acquired 166 rare American drawings worth at at least half a million dollars from the remarkably broad-based collection of John Davis Hatch.

When Hatch began 50 years ago to buy Thomas Coles and Winslow Homers, Copleys and Kensetts, American master drawings could still be purchased for a song. Choice works by such artists today might fetch as much as $100,000 each.

Unlike most collectors, Hatch refused to focus on a single artist, period or technique. He bought drawings large and small, finished works and sketches, figure studies, landscapes, self-portraits and cartoons. What makes his collection so impressive -- and valuable to the Gallery -- is that it manages to survey more than 200 years of American art history.

Twenty-four of the Hatch drawings will be purchased by the Gallery through its Ailsa Mellon Bruce Fund; the remaining 142 have come to Washington as gifts.

Because old drawings can be shown only intermittently (they tend to fade in light), and because they are not very decorative (they tend, instead, to be intimate and subtle), the museum and its donors have long preferred to go for paintings. Though the Gallery does own some first-rate Homers and John Marins, it has almost never purchased drawings by Americans -- its collection had so many gaps that its curators could not decide which ones should be filled.

"We have acquired, overnight, a collection we can build on," said John Wilmerding, the Gallery's curator of American art. "Though there are no works by Thomas Eakins in the Hatch collection, we now know what we need."

The Hatch collection includes five Coles, three Copleys, four Jasper Cropseys, two Asher B. Durands, five Horatio Greenoughs, half a dozen Homers, five Eastman Johnsons, two Emanuel Leutzes, four Jon Van der Lyns and works by John Singer Sargent, John Sloan, George Luks and one of Charles Dana Gibson's "Gibson Girls."

Other artists represented, such as Ignatz M. Gaugengigl, are not at all well known.

John Davis ("Jerry") Hatch, a Massachusetts Yankee now in his mid-70s, was for many years a museum man himself. He became director of the Seattle Art Museum in 1929, assistant director of Boston's Isabella Stewart Gardner Museum in 1932, and director of the Norfolk Museum in 1950. In 1976, to celebrate the Bicentennial, 100 of his drawings were displayed in Dublin, London, Paris, Liverpool and Cambridge. That exhibition's catalog was written by John Wilmerding, who since has helped attract many of those drawings to the National Gallery of Art.

Wilmerding is organizing a show called "American Light" which will open here in February. It will include a number of Hatch drawings.

Until then, the drawings will be available for viewing -- by appointment only -- in the West Building's Print Room on weekdays from 10 a.m. to noon and from 2 to 4 p.m.