The thieves who stole six paintings from President Bok's house this summer continue to elude the Harvard police, the Cambridge police and the Federal Bureau of Investigation.

But Jeffrey S. Kahn, Harvard Police captain, said yesterday the search is still in its early stages. In such a "long slugging type of case" the present lack of progress is "not at all surprising," Kahn said.

President Bok said yesterday he is optimistic that authorities will find the painting, because "objects like that are pretty hard to sell."

Bok said he has taken some extra security measures because of the thefts. These new precautions are "nothing dramatic" because "I'm not sure the things we have would be worth taking away," Bok added.

The six stolen paintings, including works by Eugene Boudin, a French Impressionist, and Gerrit Berckheyde, a Dutch painter, are reportedly worth $380,000. Bok borrowed five of the paintings from the Fogg Museum, and owns the sixth.

Seymour Slive, director of the Fogg Museum, said yesterday the Fogg is reviewing its policy of lending out paintings.

The paintings taken from Bok's residence are the first works of art to be stolen while on loan from the Fogg.