This is, on the face of it, a text-book. It has been written for learners, not for the learned. Its object is, not to extend the boundaries of the science by excursions into debatable ground, but to present its admitted truths in a form easily apprehended. By this statement, however, I do not wish to convey the idea that the treatise is unscientific in its character or its methods. I mean merely that I have studiously avoided cumbering my book with the many abstruse and still unsolved questions which environ the subject. Those questions are not without interest or value, and few persons have a keener relish for their discussion than the writer, whose life-long studies have been in that precise line of inquiry. But original investigation is felt to be out of place in a text-book for instruction in the elements. At the same time, it is believed, the attentive reader, who is familiar with the recent literature of the science, will find the subject brought up to the latest clearly ascertained results, while in some directions a decided advance has been made.