The amount of material available for such a work is immense, and a. Volume much more bulky than the present one could easily have been compiled. The writer, has, however, striven to keep before him the handbook ideal, which shall appeal more to the gen eral reader, and perhaps encourage him to prosecute a personal enquiry into those aspects of the subject which may interest him most. The knowledge that space was limited has perforce led to the ruthless rejection of a mass of interesting facts and theories, but it is hoped that, while striving to render the subject as brief as pos sible, nothing of vital importance or interest has been omitted.