The Fitzwilliam Museum in Cambridge will soon allow visitors access to one of its hidden treasures. The Founder’s Library, which has never before been regularly accessible to the public, will open for tours in December. The ornately decorated room, designed by Charles Cockerell in 1848, houses 7,000 books donated by the seventh Viscount Fitzwilliam. The museum is hoping to present changing displays of rare books and archival documents in the library after its planned extension is completed in five to seven years.