In the historical review presented to my readers on the evolution of the modern microscope, I have for the most part relied upon my long and close association, extending over a period of upwards of half a century, with microscopy. I need hardly say I could have very much extended my remarks with pleasure and profit had space permitted, and thereby much increased the number of names of manufacturers, who have well-established reputations for the quality of their work, and whose instruments, more or less complete in design, realise the wants of students and of that large class of present-day workers engaged in microscopical pursuits to whom economy of outlay is almost a first consideration. No valid reason, however, can be assigned for splitting up, as some writers do, the several forms of microscopes into some six different classes, which implies inferiority in mechanical details or finish, whereas the difference wholly consists in luxurious appliances to save time, and in accessories for special work or original research. Before bringing these remarks to a close, it is my wish to direct the student’s attention to one or two points of importance in connection with the use of the instrument, viz.: variations in body-lengths of microscopes, especially between those of English and of Continental manufacture. The _optical-standard_ measurement adopted in this country for the body-tube-length is 10 inches; and for its _mechanical_, 8-3/4 inches. That of Continental opticians is, optical-tube-length 7·08 inches, or 180 mm.; the mechanical, 6·3 inches = to 168 mm. Professor Abbe constructed an apochromatic immersion objective especially for the English optical tube-length of 10·6 inches (= to 270 m.m.), and mechanical tube-length somewhat less in measurement. This may be taken to mean a slight increase in the standard value of the tube, and therefore the addition of the rack-and-pinion to the draw-tube, now generally made a part of the microscope, is certainly of some practical value. This difference, however, when working with the English body-tube of 10 inches, may be discarded; it is, in fact, only where the shorter Continental body is in use, that so small a difference of tube-length exercises a disturbing effect over adjustment. Moreover, an object placed on the stage of the shorter body microscope will not be seen with the same distinctness by the draughtsman should he wish to make use of the _camera lucida_. The _optical_ tube-length of the body is measured from the back lens of the objective to the front lens or principal focus of the eye-piece; the _mechanical_ tube-length from the end of nose-piece of objective to the top lens of the eye-piece. [Illustration: The Hartnach Students’ Model Microscope.] CHAPTER III. Applied Optics:--Eye-pieces; Achromatic Objectives; Condensers.