CHAPTER II.

Fig. 45.—Swift-Stephenson’s Erecting Binocular.

No variation or change of any kind proposed either in the form of the instrument or the prism has proved of sufficient value or importance to bring it into use, and therefore Wenham’s instrument is scarcely likely to be superseded. It must be admitted that the improvement effected in the eye-piece form by Mr. Tolles, of Boston, U.S., is an exception to the rule laid down. It consistsin mounting the prisms in a light material, vulcanite, made to fit into the monocular microscope body, thus taking the place of the ordinary eye-piece. The image transmitted by the objective is brought to a focus on the face of the first equilateral triangular prism by the intervention of an erector-eye-piece inserted beneath it. The second set of prisms have a rack and pinion movement to adjust them to any visual angle. The illumination of both fields in this eye-piece is nearly equal in brightness. Mr. Stephenson’s erecting binocular (Fig. 45) has proved to be of some practical value. It has the advantage of being of equal use with high and low powers, and with little loss of definition. When used for dissecting purposes it gives an erect image of the object. It is equally useful as a working microscope, for arranging diatoms and botanical specimens of every kind. The sub-stage tube will receive a diaphragm or illuminating apparatus; the eye-pieces have a sliding adjustment for regulating the widths between eyes.

Fig. 46.—An early form of the Ross-Wenham Binocular; nose-piece and prism-holder detached.

Microscopes are known as simple and compound. The simple microscope may, for convenience, be divided into two classes; those used in the hand (hand magnifiers), and those provided with a stand (mounted, as it is termed) for supporting the object to be viewed, together with an adjustment for the magnifying power, and a mirror for reflecting the light through the object.

Fig. 47.—Visual Angle.

Asimple microscope, mounted, is preferable to a single lens, being usually composed of two or more lenses separated by a small distance on a common axis; the increase of the size of an object being the angle it subtends to the eye of the observer, or the angle formed by the combination drawn from the axis of vision to the extremity of the object, as inFig. 47. The lines drawn from the eye to a and r form an angle, which, when the distance is small, is nearly twice as large as the angle from the eye to o w, formed by lines drawn at twice the distance. This is called the angle of vision, or the visual angle. Now, the utility of a convex lens interposed between a near object and the eye consists in its reducing the divergence of the rays forming the several pencils issuing from it, so that they enter the eye in a state of moderate divergence, as if they were issuing from an object beyond the near point of distinct vision, and a well-defined image is thereby formed upon the retina. In the nextFig. (48), a double-convex lens illustrates the action of thesimple microscope, thesmall arrow being the object brought under view, and the large arrow the magnified image. The rays having first passed through the lens are bent into nearly parallel lines, or pencils diverging from some point within the limits of distinct vision. Thus altered, the eye receives rays precisely as if they had emanated directly from a larger arrow placed about ten inches away from it. The difference between the real and the imaginary object represents the magnifying power of the lens. The object in this case is magnified nearly in the proportion the focal distance of the lens bears to the distance of the object when viewed by the unassisted eye; and this is due to the object being more distinctly viewed so much nearer to the eye than it otherwise could be without the lens.18

Fig. 48.—Virtual Image formed by Convex Lens.

It should be remembered that the shorter the focus and the nearer the eye the magnifying lens is placed the smaller will be the diameter of the sphere of which it forms a part, and unless its aperture be proportionally reduced, the distinctness of the image will be destroyed by the spherical and chromatic aberrations of its high curvature. Nevertheless, it was by the use of lenses so constructed that the older microscopists—of whom Leeuwenhoek was the more eminent—were enabled to do so much excellent work.

The various kinds of simple pocket lenses for the most part consist of a double-convex, or a plano-convex, or a combination of both, varying in focal length from a quarter of an inch to twoinches. Sometimes they are set in pairs with a hole, a small diaphragm, cut in the piece of horn placed between them. These are extremely useful for carrying in the waistcoat pocket; to the anatomist and field botanist for examining various objects and preparations.

Fig. 49.—Wollaston’s Doublet.

Perhaps the most important improvement effected in this form of the simple microscope was that ascribed to the celebrated Dr. Wollaston, who devised a doublet of two plano-convex lenses having their focal lengths, in the proportion of one to three, mounted with their convex side directed towards the eye of the observer, and the lens of shorter focal length next the object. The explanation given of the correction thus effected in Dr. Wollaston’s doublet will be best understood on reference to the annexed diagram,l l′, inFig. 49, being the object for a segment of the cornea of the eye, andd d′the stop or diaphragm. Now, it will beseen that each pencil of light proceeding froml l′, the object, is rendered excentrical by the limiting aperture or the diaphragmd d; consequently, they pass through the lenses on opposite sides of their common axiso p; thus each becomes affected by opposite errors, which to some extent balance and correct each other. To take the pencill, for instance, as it enters the eye atr b;r bis bent to the right at the first lens, and to the left at the second; and as each bending alters the direction of the blue ray more than the red, and as the blue ray falls nearer the margin of the second lens, where the refraction is greater than that nearer the centre, and compensates to some extent for the greater focal length of the second lens, the blue rays will emerge very nearly parallel, and colourless to the eye. At the same time, its spherical aberration has been diminished, since the side of the pencil as it proceeds through one lens passes nearer the axis, and in the other nearer the margin.

This must be taken to apply to pencils farthest from the centre of the object. Central rays, it is obvious, would pass both lenses symmetrically, the same portions of rays occupying nearly the same relative places in both lenses. The blue ray would enter the second lens nearer its axis than the red; and being thus less refracted than the red by the second lens, some amount of compensation would take place, differing in principle, and inferior in degree, to that which is found in the excentrical pencils. In the intermediate spaces the corrections are still more imperfect and uncertain; and this explains the cause of aberrations which must of necessity exist even in the best-made doublet. It is, however, infinitely superior to a single lens, and will transmit a pencil of an angle of from 35° to 50°.

The next step towards improving the simple microscope was in relation to the eye-piece, and was effected by Holland. It consisted in substituting two lenses for the first in the doublet, and placing a stop between them and the third. The first bending of the pencils of light being effected by two lenses instead of one, produces less spherical and chromatic aberration, which are more nearly balanced or corrected at the second bending, and in the opposite direction, by the third lens.

Another form of simple lens was devised by Dr. Wollaston, the “Periscopic.” This combination consists of two hemispherical lensescemented together by their plane faces, with a stop between them to limit the aperture. A similar proposal, made by Sir David Brewster in 1820, is known as the Coddington lens,19shown atFig. 50: this has a somewhat larger field, and is equally balanced in all directions, as is made evident, the pencilsa bandb apassing through under precisely the same circumstances. Its spherical form has the further advantage of rendering the position in which it is held of comparatively little consequence. It is still used as a hand magnifier, although its definition is certainly not so good as that of a well-made doublet. It is usually set in a folding case, as represented in the figure, and so contrived as to be admirably adapted for the waistcoat-pocket. It is usually sold with the smallholder,Fig. 50a, for holding and securing small objects during examination. Browning’s Platyscopic Pocket Lens is a useful form of pocket lens for the botanist and mineralogist. Its focus is nearly three times longer than that of the Coddington, and allows of opaque objects being more easily examined; it has also a magnifying power of 15, 20, and 30 diameters.

Fig. 50.—The Coddington Lens.Fig. 50a.

Fig. 50.—The Coddington Lens.

Fig. 50a.

Fig. 51.—Steinheil’s Aplanatic Lens.

One of the best combinations of the hand or pocket form of lens is that known asSteinheil’s aplanatic lens(Fig. 51); it consists of a bi-convex lens cemented between two concavo-convex lenses, giving a relatively long focal distance and a large flat field. The higher powers of this lens are much used for dissecting purposes. This handy magnifier appears to have suggested a later combination, the apochromatic of Zeiss. No hand lens can compare with Steinheil’s “loups.”

Fig. 52.—Simple Microscope.

When the magnifying power of a lens is considerable, or when its focal length is short, or it is wished to use it with greater precision and steadiness, it should be mounted on a short stand with a tubular stem, with rack-work focussing movement and mirror illumination.Fig. 52represents a simple dissecting microscope, with a glass circular stage, 4½ inches in diameter, supported on three legs—a handy and useful form of instrument for many purposes.

The compound microscope differs from the simple, inasmuch as the image is formed by an object-glass, and further magnified by one or more lenses forming an eye-glass. For a microscope to be a compound one, its essential qualification is that it should have an object-glass or objective, and an eye-glass or eye-piece, so called because they are respectively near the object and the eye of the observer when the instrument is in use. The microscope consists of a tube orbody, and astand, an arrangement for carrying thebody, combined with which is astagefor holding the object, and amirrorfor its illumination. To the more modern instrument has been added a substage, to carry a condenser and other accessories.

Thebodyof a microscope, which carries the system of magnifying lenses, must be placed at one particular distance from the object, termed thefocus, in order that a clear image may be obtained. For the purpose offocussingtwo motions are supplied, the one forcoarse adjustment, with lower powers; the other for higher powers, termed thefine adjustment. It is in this wise that the magnifying power of the compound microscope is turned to good account.

There are, however, limits to the use to which lenses can be put with advantage in the direction of magnifying the object, just as there are in varying the magnifying power of the eye-glass. Defects in either, although not first seen, that is, when the image is but moderately enlarged, are brought into prominence by greater amplification. In practice, therefore, it is found to be of advantage to vary the power by employing object-glasses of different values (foci). In whatever way increase of amplification is brought about, two things will always result from the change: the proportion of surface of the object of which an image can be formed must be diminished, and the amount of light spread over the image proportionally lessened.

In addition to the two lenses mentioned, it was found to be of considerable advantage to introduce a third lens between the object-glass and the image formed by it at eye-piece, the purport of which is to change the course of the rays (bend in the pencil)so that the image may not be found of too great a dimension for the whole to be brought within the circumference of the eye-glass. This, it will be readily seen, allows more of the object to be viewed at the same time by thefield-glass, as the eye-piece of the microscope is termed.

Fig. 53.

Fig. 53represents the body of an ordinary compound microscope with its triplet object-glasses; o is an object, above it is the triple achromatic object-glass, in connection with the eye-piecee e, f fthe plano-convex lenses;e ebeing the eye-glass, andf fthe field-glass, between which, atb b, the arrow represents the diaphragm. The course of the light is shown by three rays drawn from the centre, and three from each end of the objecto; these rays, if not prevented by the lensf f, and the diaphragmb b, would form an image ata a; but here, as they meet with the lensf fin their passage, are converged by it atb b, the diaphragm atb bintercepting a portion of peripheral rays, permitting only those to pass that are necessary for the formation of the image, the further magnification of which is, however, here brought about by the eye-glasse e, precisely as if it were that of the original object under examination. It will be apparent, then, that the field-lensf fbelongs in principle to the object-glass, or objective, taking a share in the image-forming rays, although this is taken to be a part of the eye-piece.

The great advances made in the optical arrangements of the modern microscope necessitated important changes and improvements in its several mechanical parts. Indeed, as the apertures of objectives became increased, and focal planes became correspondingly shallower, it was absolutely necessary to apply a more sensitive system of focussing than that for many years past commonly in use.The leading manufacturers at once grasped the situation, and in a short space of time the older model microscopes were discarded, and replaced by instruments better in workmanship and finish, and in every way more suitable for the student and the promotion of original scientific research.

From an early period English amateurs appear to have bestowed greater attention on the improvement of the microscope than those of any other country. Between 1820 and 1835 Tully, Pritchard, Dolland, James Smith, Andrew Ross, and Hugh Powell, encouraged by Wollaston, Brewster, Goring, Herschel, and Lister, worked out innumerable combinations of single and compound lenses to be employed as simple microscopes, explained in a previous chapter.

The theories propounded about this time for the improvement of lenses and the various combinations for amateurs were not of lasting value. Nevertheless, they were not wholly made in vain, as during the last twenty years they have indirectly borne good fruit, inasmuch as by working in another direction Professor Abbe was led to the discovery of new and better kinds of glass, by which the secondary spectrum has been so nearly eliminated, and the optical parts of the microscope so materially improved. In pursuing this subject I would not have it supposed that Continental opticians were either idle or supine. On the contrary, Oberhäuser, Fraunhofer, Chevalier, Nachet, Hartnach, and others took an active part in the work.

The compound microscope made for anatomists by the first-named optician about 1825 has not been entirely superseded. He was the first to make a rotating stage, to apply mechanism to focussing, and to introduce the system of direct push or pull of the condenser tube within the sub-stage socket. Nachet made other improvements on the Oberhäuser microscope by applying under the stage a tail-piece having a dove-tailed groove in which a slide carrying the sub-stage was moved by a stud-pin. More recently the lever movement was superseded by American opticians, who made other changes. Hartnach ultimately very much improved Oberhäuser’s model, and this remains with us.

The English modern compound microscope, together with the achromatic objective, we owe to a mind teeming with scientific inventions, Joseph Jackson Lister, F.R.S., who in 1826 supplied Mr. Tully, a well-known London optician of that period, with original drawingsfor the important improvements in its mechanical details and accessory apparatus which followed so soon afterwards.

Among the many ingenious novelties enumerated in his published papers we find the graduated lengthening of the body-tube of the microscope; a stage-fitting for clamping and rotating the object; a subsidiary stage; a dark-well, and a large disc to incline and rotate opaque objects; a ground-glass light moderator; a live-box with bevelled flat-glass plate; an erector-eye-piece; an adapter for using Wollaston’s camera lucida for microscopical drawing; and, above all, a combination of lenses to act as a condenser under the object (evidently the first approach to the present achromatic sub-stage condenser). The value of the erector-eye-piece for facilitating dissections under the microscope is not even yet sufficiently appreciated. Tully published a descriptive account of Lister’s microscope, the first one of which he made, and acknowledged his indebtedness to “Mr. Lister’s ingenuity and skill.” Shortly afterwards Lister made known his discovery of the two aplanatic foci in a double achromatic object glass, and gave verbal directions to the three principal makers of microscopes in London, James Smith, Andrew Ross, and Hugh Powell, for the future construction of the achromatic objective, all of whom were intent on the improvement of their several models. To the latter the Society of Arts awarded, in 1832, a medal for his improved mechanical stage movements, on the “Turrell system,” which Powell first constructed for Edmund Turrell. This stage was made to rotate completely on its optic axis by means of an obliquely-placed pinion acting on a bevelled rack on the inner face of the stage-ring supporting the mechanism. In 1834 Powell once more received a Society of Arts medal, “the Iris,” for improvements in the application of a new form of fine adjustment.

About the same date (1835) Andrew Ross introduced the socket-carrier of the body-tube of the microscope on a strong stem, with rack bent in the middle, thus affording space for a larger stage. He likewise devised the hollow cross-bar, placed at right angles to the rack-stem, whereby he was enabled to use a new system of fine adjustment, consisting of a delicate screw with large milled head, acting by a point on the long arm of a lever, the short arm of which ends in a fork in contact with a stud placed on either side of a cylindrical sliding tube forming the nose-piece of thebody-tube, and into which the objective is screwed. A spiral spring presses down the nose-piece, and against this the screw and lever act.

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 inFig. 54in 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—defectsstill 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 of10 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 of½ 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.

Fig. 57.—Powell’s larger No. 2 Instrument.

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.

The cross arm encloses the lever mechanism for the fine adjustment, as originally devised by Andrew Ross. This cross arm is longer than that used by Ross, and carries the body more forward, so as to provide radial space for the complete rotation of the stage and the optic axis, and at the same time the lever of the adjustment is lengthened, and delicacy of motion secured. The stage retains the mechanical movements invented by E. Turrell, and first applied by Hugh Powell. It also rotates completely by means of an obliquely placed pinion acting on a bevelled rack on the inner face of the stage-ring supporting the mechanism. Finders are engraved on the plates, and the main support of the stage-ring is graduated for angle measuring, a pointer on the ring marking the unit of motion in arc.

Thesub-stageis carried by rack-work, and has rectangular centring movements, supporting an inner socket that can be rotated by rack and pinion, and which carries the several sub-stageaccessories. A fine adjustment, by screw-cone and stud, is applied by means of an extra slide.

Thestageis attached to the sheath of the stem by a special arrangement of screws, by which the rotation in the optic axis can be centred; sliding spring clips and a movable and a removable and adjustable angle-piece to hold the slides are applied on the upper surface. The body-tube is pivoted to move laterally on the top of the stem, and an adjustable steel stud beneath serves to stop the movement in the axis. Such is Powell’s present instrument, and it represents the results of sixty years’ steady devotion to secure perfection, and at the same time embody the best ideas of mechanical design by Andrew Ross.

A cheaper form of students’ microscope is furnished by Powell and Lealand, with ¾-inch stage movement, coarse and fine adjustments to body, plane and concave mirrors, revolving diaphragm, two eye-pieces, and Lister’s dark wells. These makers also adopt a gauge of tubing, the size being such that it will take in a binocular body, a Huyghenian 2 inch eye-piece having the largest field-glass possible. The tube of the sub-stage is the same size, so as to secure one gauge of tubing throughout. This allows of a Kellner or other eye-piece to be used as a condenser.

Messrs. Ross have more recently introduced several changes and modifications in the Zentmayer stand, all tending to improve it, so that the Ross-Zentmayer model takes its place as a first-class microscope.

Messrs. Ross have lately manufactured other forms of microscopes; one especially designed for those commencing the study of bacteriology (Fig. 59). This instrument is one of the steadiest among those lately constructed for high-class work. The circular foot and short stout pillar support the whole instrument, and a substantial knee-joint sustains the full weight in the upright or inclined positions, while the centre of gravity is by no means disturbed, and absolute steadiness secured. The stage is of the horse-shoe form, which affords convenient space for the fingers to lift the slide up while the oil is placed in contact with the objective. The fineadjustment is extremely sensitive, working smoothly and direct; this is entirely covered, to prevent injury by dust. The micrometer screw works directly in the centre of its fittings, the milled head being divided to read to1⁄500of an inch. The sub-stage is fitted with a new centring coarse and fine adjustment, so that when using high powers with the Abbe condenser accurate focus can be secured with the least amount of trouble.

Fig. 59.—Ross’s “Bacteriological and Histological” Microscope.

The amount of activity shown during the last few years by opticians in the manufacture of new forms of microscopes renders it somewhat difficult to keep pace with improvements, some of which are novel. A further source of congratulation is that economy has all along been studied; so much so, that the instruments in question are within the reach of persons of moderate means. Messrs. Ross and Co. have taken a new departure in this respect, and their“Eclipse” Microscopeis an entirely new form of stand with a ring foot. This microscope has been produced for the especial use of students, and can be purchased for a moderate sum. It will be seen at a glance(Fig. 60) how steady this form of stand must necessarily be, since the centre of gravity is secured in every direction and inclination. The body-tube carries eye-pieces, numbered, of the Continental size and optical tube-length (160 mm.), for which the object glasses are adjusted, and a draw-tube extending to eight inches.

Fig. 60.—Ross’s Rigid Pattern “Eclipse” Microscope.

The fine adjustment is independent of set screws, and not subject to derangement. It is extremely sensitive and direct in action, and from its construction is equal in perfection of working to the best that can be made. Its fitting, by a new contrivance, is completely covered at all points, being thus preserved from disturbance or injury by dust.

The Eclipse is furnished with two eye-pieces, 1′′ and ¼′′ object glasses of highest excellence and large angular aperture, both adjusted to a double nose-piece, so that they focus in the same plane; and a swinging mirror and stage iris diaphragm.

In “Wenham’s Radial” Microscope the chief aim has been directed towards providing a very considerable range of effects, both in altitude and azimuth. The leading principle followed throughout in the construction of this form of stand is that of facilitating the work of the microscopist and of obtaining the maximum range of oblique illumination in all directions. This is fairly well attained by causing all the movements of inclination and rotation to radiate from the object as a common centre. Thus it has been found possible to combine seven radial motions, so that when the instrument is inclined backwards, as inFig. 61, or placed in the horizontal, as inFig. 62or rotated from in the brass plate, a pencil of light from a fixed source shall always reach the object and pass to theobjective. The stage is made to rotate completely, and its rectangular motions are effected by milled heads acting entirely within the circumference. The sub-stage is mounted on the Zentmayer system, with two centring screws, by means of which the optic axis is secured. It is also provided with rectangular and rotating motions. The coarse adjustment is that of the Ross-Jackson form—a spiral pinion and diagonal rackwork, while the fine is on an entirely new principle designed by Dr. H. Schrœder.

Fig. 61.—Ross’s Wenham Radial Microscope.

The “Ross-Zentmayer Microscope” is a thoroughly substantial and practical instrument, combining elegance of appearance with strength and firmness.

Fig. 62.—The Ross-Wenham Radial Microscope.

It is a true tripod model, consisting of a triangular base with two pillars rising from a cross-piece, which carries the trunnions. The slow movement is obtained by a second slide close behind the first; but to avoid the friction of rubbing surfaces, hardened steel rollers are inserted between them, which give a frictionless fine motion, amenable to the slightest touch of the milled-head screw situated conveniently at the back of the limb, through which a steel lever passes which actuates the slow motion slide. The body of the instrument is therefore not touched during the fine focussing, so that all lateral movement is avoided. The mechanical stage rotates axially, and the outer edge of the lower plate is divided into degrees, in order to register the angles; a simple mode of adjustment is provided for setting the centre of rotation exactlycoincident with the focal point of the objective. As the plates of the stage have no screw or rackwork between them (these are placed externally), they are brought close together, thus affording the advantage of a thin substantial stage, and ensuring rigidity wheremost required; phosphor-bronze being used in its construction. The stage is attached to the limb by a conical stem, with a screw and clamp nut at the back, so that it can be easily removed for the substitution of a simple plate or other stage; by turning the stem in the socket the stage may be tilted sideways at any angle required. A feature in the Ross-Zentmayer stand is the swinging sub-stage and bar carrying the mirror, having its axis of rotation situated from an axial point in the plane of the object, which consequently receives the light without requiring alteration of focus in any position of the bar; by this means facilities are afforded for the resolution of objects requiring oblique light and for the development of their structure. Rays are thus obtained from any angle and indicated by the graduated circle round the top of the swing-bar, and many troublesome and expensive pieces of sub-stage apparatus dispensed with. The value of this arrangement was long ago recognised in Grubb’s “Sector Stand,” the movement of which was obtained in a far less efficient manner.

Fig. 63.—The Improved Ross-Zentmayer Model.

The base or foot of the Ross-Zentmayer instrument is made in one piece. Preference must be given to the double pillar support, as this is firmer, and allows the sub-stage to swing free while the microscope is in a vertical position, as in working with fluid preparations. The sub-stage is provided with screws for centring, and, when determined, secured by a clamping screw.

The sub-stage, with its apparatus in place, can be instantly removed, by being drawn out sideways, so as to use the mirror alone, which is a great convenience.

The mechanical movements of this instrument are perfect, and well adapted to their purpose.

Messrs. Ross have other typical forms of microscopes. Their“New Industrial” Microscope, for the use of farmers, horticulturists, textile and other trades, for the examination of produce and raw materials, is a surprisingly cheap one, and deserving of commendation. The great utility of microscopical research to purposes of advanced agriculture is fully recognised, and a less costly instrument than that usually supplied for more complex investigations was much needed. It is provided with a broad square stage for the purpose of receiving a glass dish to contain liquids or manifold objects, and which may be moved on the stage to bring the variousparticles under observation. A fitting beneath the stage carries a plate with diaphragm apertures for modifying the light, and as seeds, textile fibres, and other opaque objects form a large portion of those to be examined, this sub-stage plate has a space between the perforations which, when brought into position, provides a dark ground by preventing the passage of light from underneath. A condensing lens is, however, provided for the better lighting of opaque objects. Here we have a microscope which combines efficiency with stability, while its very simplification allows of a really good and effective instrument for the small sum of £3 3s.

Fig. 64.—Ross’s “New Industrial” Microscope.

Messrs. Beck have adopted what may be termed a rival system of fine adjustment in their modern microscopes. The short lever and screw applied externally to the body tube is peculiar, I may say, to the Ross-Jackson system, and was originally devised to allow of the body tube being supported somewhat more firmly on the limb. This change had its merits fully realised in the early microscopes of Smith and Beck. To their successors, R. & J. Beck, the microscope owes much, and very many important improvements, while all their instruments and accessories are excellent examples of good workmanship and finish. In theirPathological Microscopewe have a movement originally found in Tolles’ microscopes: a vertical disc, by which the centre can be raised or depressed to correspond with the thickness of the slide. The stage can also be brought into an inverted position by rack and pinion. Their fine adjustment has been greatly improved, as we shall presently see, whereby it has been made more sensitive and delicate of adjustment. The general construction of their microscopes as a rule possess the following advantages: the stands are strong, firm, and yet not too light or too heavy, the instruments cannot alter from the position in which they are placed, as, unfortunately, will occasionally happen when joints work loose; in every position the heavier part of the stand maintains the centre of gravity.

Beck’sPathological Microscope(Fig. 65) is a nearly perfect instrument, furnished with a firm triangular foot, which ensures great steadiness in any position. It has a well adapted joint for placing the instrument at any angle of inclination; coarse adjustment byspiral rack and pinion; fine adjustment by delicate lever and micrometer screw motion; rack and pinion focussing and screw centring sub-stage, made to carry all condensers and other sub-stage apparatus; mechanical stage with horizontal and vertical traversing motions. The stage is attached to the instrument by two screws and can therefore be removed at pleasure, leaving a large square flat glass stage for the culture-plate. It is likewise provided with finder divisions, and as it always fits on to the same place, any particular portion of the object can be recorded and found at any moment. The triple nose-piece is a convenient addition, and a very acceptable one to the student while diligently engaged in histological research.

Fig. 65.—Beck’s Pathological Microscope, with square and removable stage.

Fig. 66.—Beck’s Large “Continental Model” Microscope.

Beck’s Large “Continental Model” Microscopeis of superior finish. It is provided with a substantial horse-shoe foot, which gives support to the strong, well-balanced body, jointed for giving the microscope any angle of inclination. The body is provided with a draw-tube which can be racked down to the Continental measurement. It has a spiral rack and pinion coarse adjustment, and a fine adjustment of the most perfect workmanship, which will be described in detail presently. It has a large square stage with vulcanite top plate to receive culture preparations. The sub-stage is of the most approved form for centring, and carries an achromatic or Abbe condenser, iris diaphragm, &c. The double mirror can be swung out of place for direct illumination and micro-photography. Altogether, this instrument is in every way fitted for critical or class-room work.

Fig. 67.—Beck’s “New Fine Adjustment.”

To return to the fine adjustment of this, as of other forms of Messrs. Beck’s microscopes, the applied mechanism of which is believed to be one of the most sensitive and delicate character yet contrived. It is constructed as shown in the accompanying figure. The body of the instrument is supported upon the barrel D D; this barrel is accurately and smoothly fitted to the triangular core E E. At the top of barrel D D is screwed the cap G, to which is attached the rod C; this rod passes through the centre of the core E E and connects with the lever arm A at B. The action of the spring J, which is wrapped spirally around the rod C, raises the body of the microscope and holds the lever arm A tightly against the screw arm F. The slightest motion, therefore, of the screw F is communicated through the lever A and the rod C to the body of the microscope.

The great delicacy of this arrangement will be appreciated when it is noticed that the distance from I H is double the distance of I B, therefore any motion at B is only half that at H. This adjustment is one of the most delicate made for use with high powers.

Fig. 68.—Beck’s National Binocular Microscope.

In the construction of Beck’s Binocular National Microscope, the body is held in a sliding fitting in the limb, and is moved up or down by means of a rack and pinion motion. This constitutes the coarse focussing adjustment. The fine adjustment is effected by the milled head, which acts upon the body by means of a lever inside the limb. The upper circular surface of the stage is made of glass, and carries the object holder, which is provided with a ledge and spring to hold the object by means of the pressure of an ivory-tippedscrew, so that it can be moved about readily and smoothly. The pressure of the screw is adjusted by the milled head, which permits of more or less pressure being made upon the edge of the object.

Back to Index Next