It is a commonplace remark that every question solved is a step towards new problems waiting solution. It is equally obvious that many difficulties must be encountered by every author who uses his best endeavours to supply a standard volume or even a fairly comprehensive text-book on the Microscope, one that will remain a sure guide for any lengthened period. Such a success I regard as scarcely possible. I may, however, notice that my earlier work has met with a great amount of appreciation, and its utility acknowledged in the past by a demand almost unprecedented, edition after edition being called for. It is hardly necessary to add that my task has been accomplished with an earnest desire to assist in diffusing a love for an instrument which has been my constant companion for upwards of sixty years.[1] Moreover, I have a firm conviction of the real utility of the Microscope in the work of education, its practical value in many branches of science, art, and manufacturing industries. These are my chief reasons for applying myself once more to the task of revision, rewriting, and rearranging and bringing this book as far as possible into line with the knowledge gained in chemical pathology and bacteriology. It will be noticed that in the first part, my subjects have as far as possible been treated from a historical point of view. This method has enabled me to affix dates of introduction of special inventions and improvements made in the instrument and its appliances. The enlargement of my pages has enabled me to devote more space to bacteriological processes, and by the further addition of plates and several hundred illustrations to more fully elucidate the subject matter of my text. In an Appendix I have introduced a selection of “Formulæ and Methods” of staining, mounting, etc., also tables of the “Metrical System,” now in general use in the laboratory; together with comparative thermometric values, all of which I trust may prove of service to the student.