_Watson’s Edinburgh Students’ Microscope_ (Fig. 71) is a thoroughly efficient one for all practical purposes, great care having been bestowed upon its smallest details, and it is not difficult to perceive the reason of its popularity among students. The tripod form of foot ensures great steadiness and firmness; the body carries the smaller 0·92 eye-piece, and with draw-tube closed is of the Continental length. The draw-tube is graduated to millimetres, and when fully extended the body measures 10 inches. The stage is provided with mechanical and rotary movements; the compound sub-stage with centring screws, rack and pinion to focus, and a means of lifting the condenser out of the optical axis when not required for use. Notwithstanding, none of the movements are at all cramped; a clear distance is maintained beneath the stage, affording plenty of room for manipulating the mirror. Both coarse and fine adjustments work with smoothness, the latter being on Watson’s latest improved principle--one revolution of the milled head moves the body 1/300 of an inch. The stage is of extra large size, to allow of the use of large culture-plates. No Continental stand of higher price compares with the Edinburgh microscope. Its height when placed in the vertical position is 11-1/2 inches. [Illustration: Fig. 71.--Watson’s Edinburgh Students’ Microscope.] [Illustration: Fig. 72.--Sub-stage of Edinburgh Students’ Microscope. This view of underside of stage of students’ instrument shows the mirror set at an angle for oblique illumination, and sub-stage turned aside.] The various sizes of oculars adopted by opticians and at present in vogue cause considerable confusion. A standard size is specially needed for students’ and small microscopes. The standard long used by Continental manufacturers is 0·92 of an inch. The adoption of this size would place the eye-piece in the same position as that of the universal screw for the objective, formulated by the Royal Microscopical Society many years ago. The desirability of using standard sizes has been fully recognised by Messrs. Watson and they are now adapted to most of their microscopes. The English diameter, 1·35 of an inch, known as the “Ross” size, is retained in all their microscopes of large size. Watson’s Mechanical Draw-tube. [Illustration: Fig. 73.--Watson’s Mechanical Draw-tube (full-size).]