My reference to the older form of instrument is chiefly with the view of directing attention to the sensitive focussing system, applied in the first instance to the nose-piece; now placed below the coarse adjustment. It certainly is a delicate form of fine adjustment. This model possesses other points of interest well worth preserving, which fully entitle it to occupy the prominent place given in the list of the house of Ross. In the Ross-Jackson “Histological and Bacteriological Microscope” much attention seems to have been given to eliminate certain weak points in the earlier Ross-Jackson model--defects still extant in stands of certain English and foreign makers--while retaining the more practical improvements of both constructions. Steadiness is secured by an extension of the tripod or claw-foot and the shorter and more solid uprights that sustain the whole weight of the instrument. [Illustration: Fig. 55.--The Ross-Jackson Histological Microscope.] [Illustration: Fig. 56.--Powell and Lealand’s Students’ Microscope, with Amici prism arranged for oblique illumination, the Sub-stage and Condenser being detached.] The Ross-Jackson, then, survives, together with the original tripod stand of Hugh Powell’s, upon which he expended all the resources of the practical optician, and applied the early principles involved in the Lister-Jackson instrument, but from different points of view. However, there is hardly a choice between one and the other in workmanship, both opticians having furnished microscopes of a typical class and very high order. The firm of Powell and Lealand have but one form of stand, from which they have never been tempted to deviate. It is supported on a true tripod base, forming a solid and substantial support to the body, which is of such a length as to give as nearly as possible the standard optical interval of 10 inches between the posterior principal focus of the objective and the anterior focus of the eye-piece; the variation in the optical tube length does not exceed a quarter of an inch with objectives of 1/2 inch and upwards. The arm on which the body is fixed is 5-3/4 inches long, which not only gives a clearance of 3-1/2 inches from the optic axis, but also permits of the introduction of a long fine-adjustment lever. [Illustration: Fig. 57.--Powell’s larger No. 2 Instrument.] [Illustration: Fig. 58.--Powell and Lealand’s Students’ Microscope arranged for direct illumination. _A._ Secondary or Sub-stage racked up to bring the Achromatic Condenser close to the object.]