PREFACE ACTS differ in importance, but it is a fundamental article of the faith of science that all facts are important in some degree. The import of some of them may not be clear at first, but continued collection brings about the possibility of valuable references. An orbit cannot be computed from one or two points given many are necessary. Similarly a number of facts the more the better-may be required before we see their meaning. But there is a meaning, and it is worth our while to amass details patiently. This is the modern spirit-to inquire of Nature instead of building philosophic word structures into the blue. Observation and record are the watchwords. A large acquaintance with particulars often makes us wiser than the possession of abstract formulas, hovever deep, says William James in the preface to his Varieties of Religious Experience. And particulars may be either subjective or objective. A dream is as much of a fact as a bomb is. It is a psychological fact the other is a...