_Nachet’s Microscope_, a new form of which was first seen at the Antwerp Exhibition 1892, is very solidly built, and has all the qualities necessary for histological work. The stage rotates about the optic axis, and carries a movable slide holder. The coarse adjustment is by rack and pinion movement, the fine by the new system of micrometer screw (described in the journal of the Royal Microscopical Society of 1886), with divided head indicating the 1/400 part of a mm. The plane and convex mirror is mounted on a jointed arm. The draw-tube is divided into millimetres. The illuminating system, consisting of a wide-angled Abbe condenser (N.A. 1·40) with iris diaphragm, is raised or lowered by rack and pinion screws. The iris diaphragm, being mounted on a wheel, is worked by a tangent screw, which by a very slight movement causes the aperture of the diaphragm to pass from the centre to the periphery of the condenser. Altogether the arrangement of the sub-stage is novel, and the instrument is extremely well arranged and adapted to modern requirements. Nachet and Hartnack, of Paris, hold an almost equal rank as makers of first-class microscopes, and in point of excellence of workmanship fairy rival those of our English makers. [Illustration: Fig. 98.--Nachet’s Class Demonstrating Microscope.] There are very many other London and Continental makers of microscopes besides those especially mentioned, who have well-sustained reputations as opticians, and who, from want of space, I have been obliged to pass over. Messrs. Newton’s Students’ Microscope must be mentioned with respect. It is a good and useful instrument, has a firm stand with a reversible (rotatory) body movement, which seems to ensure steadiness when brought into the horizontal position for micro-photographic purposes. There are other opticians whose microscopes have stood the test of time--Messrs. Collins, Crouch, &c. It may, however, be taken as a well-established fact that those opticians known to manufacture the more highly-finished models also produce the more serviceable forms of students’ class-room, and other microscopes. The Bacteriological Microscope. The microscope required for bacteriological studies should be perfect in all its parts. With regard to the choice of an instrument, it is very much a matter of price, since the most perfect is usually the most costly; I shall therefore proceed to give a typical example of the instrument in use in a bacteriological laboratory. The microscope should possess the following qualifications, all of which are absolutely necessary for the study of such minute objects as bacteria and other micro-organisms.