It is not given to all to penetrate the innermost sanctuary of the secrets of Nature, very few know the road that leads to it. Some, impatient, err by taking paths which seem to shorten the route; others find, at almost every step, cross-roads which perplex them, lead to the left and to Tartarus, instead of holding the right which lead to the Elysian Fields, because they have not, as Aeneas, a sibyl for a guide. Others think not to be mistaken in following the most beaten and most frequented ways. Yet all perceive, after long labors, that far from having reached their aim, they have either passed on one side or turned their back upon it.