THE tercentenary of the birth of Robert Hooke, Gresham professor of astronomy, surveyor to the City of London and curator of experiments to the Royal Society, occurs on July 18 of this year. Hooke appears to have kept a continuous diary for the greater part of his life; and parts of his daily journal have been found in three different libraries. The importance of the first part was realised by Dr. Jean Pelseneer of Brussels, who directed the attention of the Royal Society to its existence in 1928; with the permission of the authorities of the Guildhall Library, Mr. H. W. Robinson made extracts, some of which were published by Dr. Pelseneer in I sis (February 1931). Afterwards, Mr. Robinson discovered another and later part of the diary in the British Museum, where for more than a hundred and sixty years it had been catalogued as the diary of James Petiver. The Guildhall portion of Hooke's diary is the most important, and is full of interest from all points of view. It records meetings of the Royal Society of which no minutes occur; elections of which there are no official records extant; details of his work as architect, surveyor and contractor; his daily visits to the coffeehouses and taverns, where he joined in the discourses and gossip of the city men; details of his private life, his income, his purchase of books and necessaries of life.