Fig. 2.—Lieberkuhn’s Microscope.
Benjamin Martin’s Microscope.—Benjamin Martin, about 1742, was busily engaged in making improvements in the microscope, and I may say he was certainly the first to provide accurate results for determining the exact magnifying power of any object-lens, so that the observer might state the exact amplification in a certain number of diameters. He devised numerous improvements in the mechanism and optical arrangements of the instrument; the rack and pinion focussing adjustments; the inclining movements to the pillar carrying the stage; and the rectangular mechanical motions to the stage itself. He was familiar with the principles of achromatism, since it appears he produced an achromatic objective about 1759, and he is said to have sent an achromatic objective to the Royal Society about that date. But an ingeniously constructed microscope by Martin found its way to George the Third, the grandfather of our Queen, and afterwards came into the possession of the late Professor John Quekett, of the Royal College of Surgeons, who presented it to the Royal Microscopical Society of London. This microscope will ever associate Martin’s name with the earliest and best form of the instrument, even should he not receive full recognition as the inventor of the achromatic microscope. On this account I introduce a carefully made drawing of so singularly perfect a form of the early English microscope to the notice of my readers. ([Fig. 3].) The description given of it by the late Professor Quekett is as follows:—“It stands about two feet in height, and is supported on a tripod base, A; the central part of the stem, B, is of triangular figure, having a rack at the back, upon which the stage, O, and frame, D, supporting the mirror, E, are capable of being moved up or down. The compound body, F, is three inches in diameter; it is composed of two tubes, the inner of which contains the eye-piece, and can be raised or depressed by rack and pinion, so as to increase or diminish the magnifying power. At the base of the triangular bar is a cradle joint, G, by which the instrument can be inclined by turning the screw-head, H (connected with an endless screw acting upon a worm-wheel). The arm, I, supporting the compound body, is supplied with a rack and pinion, K, by which it can be moved backwards and forwards, and a joint is placed below it, upon which the body can be turned into the horizontal position; another bar, carrying a stage and mirror, can be attached by a screw, L N, so as to convert it into a horizontal microscope. The stage, O, is provided with all the usual apparatus for clamping objects, and a condenser can be applied to its under surface; the stage itself may be removed, the arm, P, supporting it, turned round on the pivot, C, and another stage of exquisite workmanship placed in its stead, the under surface of which is shown at Q.”
Fig. 3.—Martin’s Universal Microscope. 1782.
This stage is strictly a micrometer one, having rectangular movements and a fine adjustment, the movements being accomplished by the fine-threaded screws, the milled heads of which are graduated. The mirror, E, is a double one, and can be raised or depressed by rack and pinion; it is also capable of removal, and an apparatus for holding large opaque objects, such as minerals, can be substituted for it. The accessory instruments are very numerous, and amongst the more remarkable may be mentioned a tube, M, containing a speculum, which can take the place of the tube, R, and so form a reflecting microscope. The apparatus for holding animalcules or other live objects, which is represented at S, as well as a plate of glass six inches in diameter, with four concave wells ground in it, can be applied to the stage, so that each well may be brought in succession under the magnifying power. The lenses belonging to this microscope are twenty-four in number; they vary in focal length from four inches to one-tenth of an inch; ten of them are supplied with Lieberkuhns. A small arm, capable of carrying single lenses, can be supplied at T, and when turned over, the stage of the instrument becomes a single microscope; there are four lenses suitable for this purpose, their focal length varying from one-tenth to one-fortieth of an inch. The performance of all the lenses is excellent, and no pains appear to have been spared in their construction. There are numerous other pieces of accessory apparatus, all remarkable for the beauty of their workmanship.[4]
In addition to the movements described by Quekett, the body-tube with its support can be moved in an arc concentrically with the axis of the triangular pillar, on the top of which it is fitted with a worm-wheel and endless-screw mechanism, actuated by the screw-head, T, below. It must therefore be admitted that Martin led the way far beyond his contemporaries, both in the design and the evolution of the microscope. Furthermore, in his “New Elements of Optics,” 1759, he dealt with the principle of achromatism, by the construction of an achromatic telescope.
At a somewhat later period there lived in London a philosophical instrument maker of some repute, George Adams, who published in 1746 a quarto book, entitled “Micrographia Illustrata, or the Knowledge of the Microscope Explained.” This work fairly well describes “the nature, uses, and magnifying powers of microscopes in general, together with full directions how to prepare, apply, examine, and preserve minute objects.” Adams’ book was the first of the kind published in this country, and it contributed in no small degree to the advancement of microscopical science. Adams writes: “We owe the construction of the variable microscope to the ingenuity and generosity of a noble person. The apparatus belonging to it is more convenient, more certain, and more extensive than that of any other at present extant; consequently, the advantage and pleasure attending the observations in viewing objects through it must be as extensive in proportion.” This is believed to apply to Martin’s several microscopes, and that especially constructed for the king, afterwards improved upon by Adams. Another early form of microscope, Wilson Simple Scroll (1746), stamped on the cover of this book, and has thus become familiar to microscopists, was also made by Adams.
We now closely approach a period fertile in the improvement of the microscope, and in the discoveries made by its agency. The chief of those among the honoured names of the time we find Trembley, Ellis, Baker, Adams, Hill, Swammerdam, Lyonet, Needham, and a few others. Adams somewhat sarcastically observes “that every optician exercises his talents in improving (as he calls it) the microscope, in other words, in varying its construction and rendering it different in form from that sold by his neighbour; or at the best rendering it more complex and troublesome to manage.” There were no doubt good reasons for these and other strictures upon inventors as well as makers of microscopes, even in the Adams’ day. In the year 1787 the “Microscopical Essays” of his son were published, in which he described all the instruments in use up to that period.
Looking back, and taking a general survey of the work of nearly two centuries in the history of the microscope, it cannot be said that either in its optical or mechanical construction any great amount of progress was made. This in part may have arisen from the fact that no pressing need was felt for either delicate focussing or higher magnification. At all events, it was not until the application of achromatism to the instrument that new life was infused into its use, and a great impetus was given to its development, both optically and mechanically.