"The Constructio was evidently written at several different times. The order of its contents is peculiar, and there are to be seen in it evidences of different stages of the discovery. Its object was to explain fully the mode in which he had calculated the Tables and incidentally the reasoning on which they were based, but there are no historical references to the way in which he originally arrived either at the idea of a Table of Logarithms or at the method of constructing it."