ABSTRACT
During the course of the foregoing work in the first half of 1954, a more definite and precise statement of the time-span instrument was made. The use of this instrument became a central feature in the analysis of the difficulties outlined: it must therefore be described at this stage. The form in which it is presented is the form it had reached by the spring of 1955. The broad features of the instrument, however, had remained unchanged from the beginning, so that the present description is a more detailed elaboration of an instrument that, in the early stages, was employed with the assistance of a fair amount of intuitive judgment.
