During the sixteenth and seventeenth centuries, ivory diptych sundials were widely used to determine the time by day or night. These elaborate portable sundials, which could be adjusted for different latitudes, incorporated various devices useful for merchants and others who traveled extensively in Europe. This catalog illustrates in detail Harvard's collection of eighty-three ivory diptych sundials, one of the largest holdings of these instruments in the world. The collection encompasses a comprehensive array of styles and designs from Nuremberg, Paris, and Dieppe, the major centers of their production, as well as from other parts of Europe.This catalog is the fourth publication of Harvard University's Collection of Historical Scientific Instruments, and the first to appear in twenty-two years. This collection, which was established in 1949 as a resource for the history of science and technology, has one of the three largest university holdings of its kind in the world. It comprises about 15,000 instruments covering a broad range of scientific disciplines dating from 1500 to the present. Illustrated catalogues of other parts of the collection are anticipated in the near future. These will include volumes on early telephones and phonographs, psychological instruments, and apparatus for teaching science in Colonial America.