The year 1830 was, however, a propitious period in the history of the modern microscope, as in January of that year Mr. Lister published his epoch-making paper, “On the Improvement of the Achromatic Microscope.” This appeared together with certain personal practical directions (for no man was ever more anxious to communicate his knowledge than Mr. Lister) to the before-mentioned opticians, which led up to changes lasting until 1840, when, by the efforts of this gentleman and his personal friends, “The Microscopical Society of London” came into existence. Among the more prominent members of the Society was Mr. George Jackson, a name still well known to microscopists, and who, jointly with Mr. Lister, gave us the Jackson-Lister form of microscope. This was forthwith accepted as a perfect model. Soon after Andrew Ross effected a further change in the instrument, shown in [Fig. 54] in its complete form as left by this optician. It is here represented as having a bar movement, with a claw foot bolted to two uprights to carry the trunnions with the body and stage. This base, is insufficiently wide and extended to carry so large an instrument with its centre of gravity so high. The coarse adjustment bar also was rectangular, and the fine adjustment a lever, with the milled head in the middle of the bar, which involved a certain amount of tremor; withal it was an instrument of excellent workmanship, and its defects were not regarded as irremediable. Messrs. Ross, however, preferred to construct an entirely new model designed by Zentmayer, the “Ross-Jackson-Zentmayer,” to which I shall refer presently. A later model, however, has to some extent taken its place, “the Histological and Bacteriological Microscope,” [Fig. 55].
Fig. 54.—An early Ross-Jackson Microscope.
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.
Fig. 55.—The Ross-Jackson Histological Microscope.
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 ½ inch and upwards. The arm on which the body is fixed is 5¾ inches long, which not only gives a clearance of 3½ inches from the optic axis, but also permits of the introduction of a long fine-adjustment lever.
