^{page 349 note *} It may be doubted whether these rules–familiarly known as the “zigzag” rules–are even yet sufficiently well known and appreciated. They should unquestionably be studied, and fully grasped in all their elegant simplicity, by all students and others who are concerned with interpolation. A high authority, the late Ralph Todhunter, M.A., F.I.A., wrote as follows on the 7 April 1922 to the writer of the present note : “I do not find it necessary to remember anything about interpolation except the zigzag rules”; to which perhaps should be added “and Everett's formulæ.”