This term, investment opportunity, seems to be the nearest expression in popular language to suggest or denote the technical magnitude
r employed in this book. The full expression for
r is the rate of return over cost, and both cost and return are differences between two optional income streams. So far as I know, no other writer on interest has made use of income streams and their differences, or rates of return over cost per annum. The nearest approximation to this usage seems to be in the writings of Professor Herbert J. Davenport, particularly his
Economics of Enterprise, (Macmillan, New York, 1913) pages 368, 379, 381, 394, 395, 396, 410, 411. [Original text reads
Economics of Entreprise—Econlib Ed.]
Part I, Chapter 1