This appears to have been the first really sensitive focussing method applied to the nose-piece; it was, and probably is, one of the most delicate systems ever applied to the microscope. It has enjoyed a long period of popularity, and I believe it still survives in Powell and Lealand’s instruments, which are very generally admitted to be of superior excellence for all purposes where extreme delicacy of focussing is an essential element.
The rival system of fine adjustment--the short lever and screw applied externally to the body-tube--known as the Lister-Jackson system, which appears to have been contrived to allow the body-tube to be supported more firmly on the limb or stem, has had its merits ably realised in the microscopes of Smith and Beck and their successors, but, except as modified by the successors of Andrew Ross (Schrœder’s form), it is, I believe, admitted that it has been superseded by other modifications lately introduced into the Ross-Jackson instrument.
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.