CHAPTER II.

Simple and Compound Microscope.

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.

A _simple 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 in Fig. 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 next Fig. (48), a double-convex lens illustrates the action of the _simple microscope_, the small 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]

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 two inches. 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.

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′_, in Fig. 49, being the object for a segment of the cornea of the eye, and _d d′_ the stop or diaphragm. Now, it will be seen that each pencil of light proceeding from _l l′_, the object, is rendered excentrical by the limiting aperture or the diaphragm _d d_; consequently, they pass through the lenses on opposite sides of their common axis _o p_; thus each becomes affected by opposite errors, which to some extent balance and correct each other. To take the pencil _l_, for instance, as it enters the eye at _r b_; _r b_ is 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 lenses cemented 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,[19] shown at Fig. 50: this has a somewhat larger field, and is equally balanced in all directions, as is made evident, the pencils _a b_ and _b a_ passing 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 small _holder_, Fig. 50_a_, 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.

One of the best combinations of the hand or pocket form of lens is that known as _Steinheil’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_.”

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. 52 represents a simple dissecting microscope, with a glass circular stage, 4-1/2 inches in diameter, supported on three legs--a handy and useful form of instrument for many purposes.

The Compound Microscope.

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 or _body_, and a _stand_, an arrangement for carrying the _body_, combined with which is a _stage_ for holding the object, and a _mirror_ for its illumination. To the more modern instrument has been added a substage, to carry a condenser and other accessories.

The _body_ of a microscope, which carries the system of magnifying lenses, must be placed at one particular distance from the object, termed the _focus_, in order that a clear image may be obtained. For the purpose of _focussing_ two motions are supplied, the one for _coarse adjustment_, with lower powers; the other for higher powers, termed the _fine 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 the _field-glass_, as the eye-piece of the microscope is termed.

Fig. 53 represents 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-piece _e e, f f_ the plano-convex lenses; _e e_ being the eye-glass, and _f f_ the field-glass, between which, at _b 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 object _o_; these rays, if not prevented by the lens _f f_, and the diaphragm _b b_, would form an image at _a a_; but here, as they meet with the lens _f f_ in their passage, are converged by it at _b b_, the diaphragm at _b b_ intercepting 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-glass _e e_, precisely as if it were that of the original object under examination. It will be apparent, then, that the field-lens _f f_ belongs 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.

Evolution of the Modern Achromatic Microscope.

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 drawings for 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 the body-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 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.

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.

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.

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.

The _sub-stage_ is 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-stage accessories. A fine adjustment, by screw-cone and stud, is applied by means of an extra slide.

The _stage_ is 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 3/4-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.

Ross’s Microscopes.

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 fine adjustment 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 to 1/500 of 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.

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” Microscope_ is 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.

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 1/4′′ 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 in Fig. 61, or placed in the horizontal, as in Fig. 62 or rotated from in the brass plate, a pencil of light from a fixed source shall always reach the object and pass to the objective. 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.

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

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 exactly coincident 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 where most 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.

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 various particles 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 3_s._

Messrs. Beck’s Microscopes.

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 their _Pathological Microscope_ we 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’s _Pathological 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 by spiral 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.

_Beck’s Large “Continental Model” Microscope_ is 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.

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.

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-tipped screw, 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.

When the stage is required for other purposes the object holder can be unscrewed and removed. Beneath the stage there is a cylindrical fitting for the reception of a diaphragm, a polariser, or other apparatus. The mirror, besides swinging in a rotatory semi-circle, is made to slide up or down the stem. The microscope is supported by a firm pillar on a tripod base, and the body can be inclined at any angle convenient for working. A sub-stage can be added at any time for the reception of an achromatic condenser fitted with concentric screws--a necessity for more delicate microscopical research work.

_Beck’s Star Microscope_ is in every sense a students’ or class-room instrument. It is firm and well made, with joint for inclination, large square stage, sliding coarse adjustment and fine adjustment by micrometer screw, draw-tube, iris diaphragm, double mirror on swinging crank arm, A or B eye-piece, a one-inch and quarter-inch objective, the magnifying power of which ranges from 38·5 to 183.

An early binocular microscope for dissecting purposes was devised by the late Mr. R. Beck. (Fig. 70.) This took the form of a simple instrument built up on a square mahogany base A raised about four inches upon four brass supports B B, having a large circular stage plate made to revolve on a second plate, on which the object is placed and brought under the eye for dissection. On the left hand side is a milled head rack and pinion K, which acts upon a horizontal bar I for focussing the magnifying lens. Another bar, R, carries the prism P and a pair of eye-pieces arranged on the principle of M. Nachet’s binocular microscope. Mr. Beck preferred to adopt Wenham’s method of arranging these prisms; that is, by allowing half the cone of rays to proceed to one eye without interruption, while the other half is intercepted by the prisms and transmitted to the other eye. Beneath the stage is the ordinary mirror L. The condensing lens M is supported on a separate brass holder let into one of the supports of the stand. In practice, however, this arrangement was found inconvenient, and the microscope has therefore not been brought into general use.

Messrs. Watson’s Microscopes.

Among London opticians, the various microscopes manufactured by Messrs. Watson, of Holborn, are of high finish and good workmanship. Those specially designed for the use of students possess merits of their own in their mechanical construction, and also embody a provision, as indeed do all their instruments, whether for students or more pretentious work, whereby wear and tear in their frictional parts can be compensated for by the user himself. This is effected in a simple but efficient manner. The fittings are sprung, and screws set just outside the dove-tails. The very slightest turn of the screws compresses the dove-tails, and a very large amount of wear can in this way be prevented.

I am glad to notice that Messrs. Watson have adopted certain standard sizes recommended some time ago by the Royal Microscopical Society for the diameters of eye-pieces. It would be a great advantage if the same standard became generally recognised and brought into use, since it is a matter of much importance to microscopists.

_Watson’s Edinburgh Students’ Microscope_ (Fig. 71) is a thoroughly efficient one for all practical purposes, great care having been bestowed upon its smallest details, and it is not difficult to perceive the reason of its popularity among students. The tripod form of foot ensures great steadiness and firmness; the body carries the smaller 0·92 eye-piece, and with draw-tube closed is of the Continental length. The draw-tube is graduated to millimetres, and when fully extended the body measures 10 inches. The stage is provided with mechanical and rotary movements; the compound sub-stage with centring screws, rack and pinion to focus, and a means of lifting the condenser out of the optical axis when not required for use. Notwithstanding, none of the movements are at all cramped; a clear distance is maintained beneath the stage, affording plenty of room for manipulating the mirror. Both coarse and fine adjustments work with smoothness, the latter being on Watson’s latest improved principle--one revolution of the milled head moves the body 1/300 of an inch. The stage is of extra large size, to allow of the use of large culture-plates. No Continental stand of higher price compares with the Edinburgh microscope. Its height when placed in the vertical position is 11-1/2 inches.

The various sizes of oculars adopted by opticians and at present in vogue cause considerable confusion. A standard size is specially needed for students’ and small microscopes. The standard long used by Continental manufacturers is 0·92 of an inch. The adoption of this size would place the eye-piece in the same position as that of the universal screw for the objective, formulated by the Royal Microscopical Society many years ago. The desirability of using standard sizes has been fully recognised by Messrs. Watson and they are now adapted to most of their microscopes. The English diameter, 1·35 of an inch, known as the “Ross” size, is retained in all their microscopes of large size.

Watson’s Mechanical Draw-tube.

An important feature in connection with the body-tube of Watson’s Edinburgh Students’ Microscope (as, indeed, in all their fully furnished instruments) is that they are provided with two draw-tubes; one moved by rack-work, the other sliding inside the body-tube. The advantage is, that the body can be made very short or extremely long, while sufficient latitude can be given to objectives corrected for either Continental or English tube-lengths, and to adjusting the same for thickness of cover-glass by variation of tube length. Should the cover-glass be thicker than that for which the objective is corrected, a shorter tube-length is necessary; if thinner, the body must be lengthened. This is effected by means of the rackwork draw-tube. The length of the body when closed is 142 millimetres (5-5/8 inches), and when the two draw-tubes are extended, 305 millimetres (12 inches), being, therefore, shorter than the Continental and longer than the English tube lengths. Both draw-tubes are divided into millimetres, and on the rackwork draw-tube a double scale is engraved, reading continuously from the sliding draw-tube when fully drawn out, or giving the body length when the rackwork draw-tube alone is in use. The utility of this mechanical draw-tube is that it permits of quick manipulation with perfect results.

The inside top of the draw-tube is smaller than the remainder, the former making a fitting for the eye-piece about 1 inch long, permitting of the tube being blackened inside up to this fitting, thus minimising reflection. The end of the draw-tube has the universal screw for using the apertometer, &c.

_Watson’s Histological Microscope_ (Fig. 74) is a somewhat cheaper form of instrument, designed for the student; although of plainer construction it is quite as well made as the costlier model. It is provided with spiral rack and pinion coarse adjustment, and with this motion the greatest smoothness is preserved. There is no backlash, the teeth of the pinion never leaving the rack; so effective is it that a high power can be perfectly focussed by its means. It is also furnished with their universal pattern of fine adjustment. This can be had for £3 3_s._

Messrs. Watson have among other accessories of value introduced in connection with their several microscopes a semi-mechanical stage, whereby they are enabled to reduce the cost of manufacture. Fig. 75 is an outline sketch of the same.

This stage is of the horse-shoe shape, with cut-out centre, constructed of 1/4-inch brass plate, and measures over all 5-1/4 inches wide by 4 inches deep. Fitting on the edges of the main stage is a frame which is actuated vertically by means of a double rack and pinion from beneath, giving 3/4-inch of movement, having controlling heads on either side of the stage; on the edges of this mechanical frame a sliding bar is fitted, consequently movement may be imparted either by rackwork or by hand. The mechanical movement, however is in one direction only; but as the bar carries the object, the worker can easily move the object out horizontally with the finger. The advantage of this stage is that the whole surface is perfectly flush, and the pinion heads are below its level, so that culture plates or continuous sections may be conveniently examined.

Another addition of considerable value is the centring underfitting for students’ microscopes.

This fitting places in the hands of student workers a means of accurately centring the sub-stage condenser, at a low cost. It consists of the usual underfitting tube, having a flange at the top which is fitted in a box between two plates. The centring is effected by means of two screws, which press the flange against a spring, as in the ordinary sub-stage centring movement. The fitting can be adapted to any form of Messrs. Watson’s and most other makers of students’ microscopes.

_Watson’s Bacteriological Improved Van Heurck’s Microscope_ (Fig. 77) is in every way a superior instrument, and it at once conveys a favourable impression to the practical worker. When set up for use its many convenient points--its excellence of workmanship and the precision of its movements--seem to imply its special adaptation for the bacteriological laboratory and for other high-class work where absolute reliance has to be placed in the results obtained. Every detail of the instrument is carried out in the best possible manner. The coarse adjustment is effected by means of a diagonal rack and spiral pinion, which ensures the smoothest possible motion; while the fine, the most important movement in the instrument, is made with an extra long lever, a specialty of Messrs. Watson’s, and which imparts an extremely slow action: this is now one of the most delicate and reliable forms of fine adjustment. By its means the entire body is raised or lowered by means of a milled head fixed to a screw having a hardened steel point acting on a lever against a point attached to the body slide, in a dove-tailed fitting about 2-1/2 inches long. Owing to the position of the controlling milled head on the limb, it can be worked with either hand. Another feature of importance is that, in using the fine adjustment the distance between the eye-piece and objective remains unaltered. All the frictional parts of the microscope have spring slots to the dove-tailed fittings, in which compensating screws are fitted. These are some few of the more important points, to which much thought and attention have been given. The body permits also of the use of objectives of any other optician, since its total length when the draw tubes are closed up is only 143 mm.; when extended, a total length of 320 mm. is available. By this means an ample margin is left for the correction for cover-glass thickness, whether the objective used be intended for the 160 mm. or 250 mm. tube length. The height of the microscope when placed in the vertical position is 13-1/8 inches.

_The Stage._--A somewhat new design has been used in building this up so as to reduce vibration to a minimum. The bracket carrying the stage, instead of being screwed on to the front of the limb, as is usually done, is made in a solid casting, taking the sub-stage beneath, and passing into the joint at the top of the foot. The joint bolt goes through the whole (limb and stage bracket), rendering the limb stage and sub-stage as firm as if it were one piece; a point of considerable importance.

The mirrors, which are plain and concave, are mounted on a swing arm, so that they may be turned aside when direct illumination of the object is required. On the right hand side also there is a steel clamping bar for fixing the microscope at any angle of inclination. The tripod foot, which has superseded most other forms, is adopted. At the points of contact with the table the feet are provided with cork pads, which give increased firmness and prevent vibration to some extent.

The sub-stage is provided with a fine adjustment of similar design to that employed for the focussing of the objective. It has become needful to embody such a refinement, in order that sub-stage condensers of large aperture, such as are in constant use for critical high-power work, may be adjusted with the same facility and precision as the objective--they, in fact, require it if the best work is to be got out of them. No pains have been spared by Messrs. Watson to render it absolutely perfect.

_Watson’s Portable Microscope._--This instrument is similar in general detail to the Histological Microscope, but the foot, mirror stem, &c., are made to fold up in exceedingly compact form, and when set up for use the stand is perfectly rigid. Portable microscopes are, as a rule, but makeshifts. This, however, is a thoroughly sound, practical instrument and capable of best work with the highest power objectives, having good adjustments and universal size fittings throughout, so that the objectives and apparatus made for the larger instruments can be employed with it.

_Watson’s Petrological Microscope_ (Fig. 79) is a modification of their Edinburgh Students’ pattern, and designed specially for petrological and mineralogical work.

A polariscope having prisms of large size is supplied with it, the analyser being fitted in the body, and the polariser in the under-stage fitting. The latter has a divided circle and a spring catch at every quarter circle. By removing the polariser and withdrawing the analyser, for which provision is made, the microscope can be used for purposes of ordinary research. A Klein’s quartz plate is fitted beneath the analyser, also in the body of the microscope.

The stage, which has a glass surface, rotates concentrically, and has a divided circumferential edge reading by the verniers. The eye-piece has cross webs to the diaphragm, and when it is desired, an analyser, having a divided circle fitted with a calc-spar plate, can be used above the eye-piece, and condenser lenses attached to the polariser for stereoscopic purposes. All the fittings have the universal thread, and are interchangeable.

Messrs. Swift’s Microscopes.

_Messrs. Swift’s Microscopes_ have a well-established reputation for quality and good workmanship, and therefore can in no way suffer by comparison when placed beside those of other opticians. One of the characteristics of Messrs. Swift’s microscopes--and this runs through the whole series--is that they are all made to a _standard_ gauge, so that the several parts of the instruments, as well as their accessories, are interchangeable; the cheaper forms, with those of the first quality and finish. Should the student, then, start with a No. 1 model, he can at any time build it up, as it were, with the accessories designed for a No. 3 or 4, that is, for an instrument of double the price he started with. The optical centre is preserved throughout the whole series of microscopes.

The tripod foot has, it appears, taken the place of some of their other forms of instruments, while their four-legged _tripod_, if it can be so designated, is a novelty of quite an unusual character.

The swing leg is attached to the framework of the tripod by the screw (Fig. A), which is provided with a powerful steel spiral spring, compressed between two steel collets when the screw is driven home, as shown in Fig. B.

The expansion of this spring will obviously take up and compensate automatically any wear and tear that is likely to occur between the bearing surfaces, and it is therefore impossible for the fitting to get loose.

_Swift’s Four-legged Microscope_ (Fig. 80) is one possessing great stability in whatever position it may be placed; the body being supported on a horse-shoe platform, from which its four legs spring, the two front legs being fixed, while the hind legs are pivoted to the platform. This arrangement of pivoting the hind legs enables the microscope to adapt itself to any uneven surface, thus keeping it always in a steady position, while it also reduces the danger of being upset by any lateral movement of an accidental nature. The feet are studded with corks, an additional aid to steadiness and fixity for microphotography. The length of the body from the ocular to the nose-piece is 6-1/2 inches, and can be extended to 9 or 10 inches by means of the draw-tube, which has a millimetre graduation. The stage, which is of horse-shoe shape, is provided with spring clips, to which a movable mechanical stage can at any time be attached. The sub-stage partakes of two forms, one being an ordinary fitting, taking an ordinary condenser; the other, the regular rack and pinion achromatic condenser with centring adjustments. It has a diagonal rack and pinion coarse adjustment, the fine adjustment being made by micrometer screw of the finest character.

Fig. 82 is intended to illustrate the advantage of the spiral rack and pinion which Messrs. Swift fit to their microscopes, in place of the ordinary conventional horizontal rack and pinion movement. The advantage will at once be seen, since there is more gearing contact between rack and pinion, thus ensuring durability and reducing loss of time or back lash to a minimum, with less wear and tear. The leaves of the pinion also roll into the teeth of the rack by degrees, ensuring a very much smoother action, which, if properly made and fitted, prevents the gearing of the two being felt by the hand whilst focussing.

Fig. 83 is a supplementary draw-tube with rack and pinion movement, which can be adapted to any of Swift’s microscopes in place of the ordinary draw-tube, the size of the thread being of the same diameter, so as to render all draw-tubes, as well as other parts of these instruments, interchangeable. The draw-tube being divided into millimetres can be extended from 160 to 250 millimetres. One advantage of this arrangement is that the correct adjustment of any objective with each eye-piece is easily found and recorded for future observations with the same combination.

Messrs. Swift’s Three-legged Tripod Microscope (Fig. 86). In most respects the description already given of the four-legged instrument is applicable to this stand. Although of an apparently different form, it can be built up, as already explained, into one of a higher class. It is suitable in every way for histological investigations. The horse-shoe platform in this, as in the preceding stand, is extremely serviceable, as it allows the pillar of the instrument to rest firmly upon it, thus rendering the stand very rigid.

_Swift’s Bacteriological Microscope_ (Fig. 84), designed by Professor Wright, of the Army Medical School, Netley, a sufficient warranty of its excellency and perfect adaptation for bacteriological high-class work. One of the advantages connected with this microscope is the facility with which it can be adapted for either high or low power investigation, without the necessity of adding or detaching any part. The objectives, arranged on a triple nose-piece, are approximately in focus when revolved into position for immediate use, thus effecting a saving of time in changing the objective. Moreover, the nose-piece carrying the objectives is of new construction, and fitted in such a way that the entry of dust is rendered impossible.

The Abbe condenser, fitted with an iris diaphragm, is mounted on an eccentric arm, so that it can readily be thrown out of the axis of the microscope when not required, without having to re-arrange the focus when again brought into position. The condenser must be turned aside when plate cultivations and preparations of unstained bacteria are being looked over for selection of colonies for mounting, in which case an arm carrying a quadrant with three apertures is brought into position in place of the condenser, the apertures being severally centred by a spring catch and used with oblique light. This arrangement, shown in Fig. 85, is seen from the under surface of the stage. The stage is sufficiently large, so that when Petrie plates are being examined at the extreme edges there is little fear of their overbalancing.

The fine adjustment is the Swift’s Patent Campbell Differential Screw, which offers great facilities for delicate focussing with the highest power objectives. The stand is of the most substantial and rigid form, and thus ensures the microscope from vibration.

The under-stage of microscope (Fig. 85) is seen to be of the most approved form.

_Swift’s Advanced Students’ Microscope._--In this microscope (Fig. 87) we have a superior instrument for the use of the advanced student, which may be described as of high mechanical excellence, well suited for every requirement of work. The stand is the well-known tripod form of their Challenger Microscope, and admits of the instrument being placed at any angle of inclination; the body is short enough to work with objectives of Continental makers, and is provided with a draw-tube, to elongate it to the standard of 10 inches, with a diameter of 1-3/16 inch to take the same eye-pieces as the larger stands. The coarse adjustment is by spiral rack and pinion; the fine, by a carefully made differential screw motion for delicate focussing. The stage is of the horse-shoe pattern, to which a mechanical stage can at any time be adapted, as well as an achromatic condenser to the sub-stage seen beneath. Here the student will find the foundation for a superior instrument.

Messrs. Baker’s Microscopes.

Of Messrs. Baker’s larger stands, the Improved “Nelson Model,” No. 2 (Fig. 88) stand is selected in preference to their more elaborate No. 1, and their simpler form, No. 3, as a high-class instrument, and one well suited for fine critical work; the former being somewhat better, only from having extra adjustments; the latter possessing no superior advantage over the “Advanced Students’” Microscope. This microscope is mounted on a solid tripod foot, which insures stability, whether placed in a vertical, horizontal, or inclined position; the front toes are slotted, so that they may be clamped to the base plate of a photo-micrographic apparatus, first introduced for photo-micrographic work, and will also be found convenient in ordinary work; as the fine adjustment milled head is placed at the bottom of the pillar, instead of at the top, the more usual place. For photo-micrographic work the advantage is that the strain of the pulley in such apparatus actuates the fine adjustment, and is less liable to cause vibration of the instrument. The advantage when the instrument is used for ordinary work lies in the fact that the weight of the hand is rested on the top of the tripod, thus admitting of steadier movement of the milled head. The fine adjustment is obtained by a “Campbell” differential screw, each revolution of which is equal to 1/200 m.m. The draw-tubes being graduated in m.m., allow of either short or long tube objectives being used, closing up to 150 m.m. and extending to 280 m.m., the rack and pinion adjustment to the lower tube affording a ready means of correction for cover-glass thicknesses. The eye-piece gauge, as will be seen from its dimensions, is of large size, being the same as that adopted by Zeiss for his long tube compensating oculars; smaller eye-pieces can, however, be adapted at any time.

The mechanical rotating stage is divided on brass to 1/100 inch, with clamping bars and stop, by which a specimen can always be brought back to a certain position for registration. The sub-stage has rack-work focussing adjustment, and centring screws; a fine adjustment is added, if desired. On the whole, the instrument is suitable for special critical work, and is equally well suited for photo-micrography.

The points of difference between this stand and the No. 1 model are that in the latter the fine adjustment carries the body only, and not the rack adjustment; the limb carrying both the body and the sub-stage is in one piece, giving, if possible, still greater rigidity; the rotation of the mechanical stage, which is divided on silver, is complete, and can be actuated by hand or rack work; it has a clamping screw and fine adjustment to sub-stage.

_Baker’s Advanced Students’ Microscope_ (Fig. 89) may be described as a typical instrument, equally suitable for histological work and that of the advanced student. The intention of the maker in simplifying the adjustments and reducing the instrument in size, was to furnish a well-finished portable instrument at a moderate cost. This object has not been attained by supplying adjustments of second-rate quality, but by reducing their number to a minimum.

The tripod foot of the “Nelson Model” is replaced by a claw foot, which is in effect a tripod, as it rests on three points; it has not the same wide spread, but this, far from being a disadvantage, renders the instrument more portable. It has rack and pinion coarse and Campbell differential screw fine adjustments, draw-tube graduated in m.m., extending to 180 m.m., eye-piece gauge the same as the Continental size, large square open stage to afford the greater freedom of manipulation; sliding bar with graduations on bar and stage, which suffice for registering any given field under a low power; holes are also drilled in the stage ready to receive an attachable mechanical stage should it be thought advisable to add one at a later date. The sub-stage is of the universal size with rack-work focussing, adjustment, and centring screws.

Messrs. Baker have recently introduced a similar instrument with swing-out sub-stage and adjustments for compensating for wear and tear of rack. The stage is also somewhat larger from back to front.

These stands are very suitable for bacteriological research, and for amateurs wishing to obtain a stand which will carry all the apparatus they are likely to need, without going to the expense of the larger models, no better instrument could be desired.

Their “Removable Mechanical Stage” (Fig. 90) is a modification of the pattern designed by the late Mr. J. Mayall. The vertical movement is by rack and pinion, giving a range of 1-1/8 inch. The horizontal motion of 1-1/2 inch is accomplished by means of a quick-acting screw. The object is pressed tightly to the stage of the microscope by means of three points, and the whole of the mechanical part is firmly clamped by two thumb screws which can be readily removed. The stage is made to carry slides of any size less than 1-3/4 inch wide.

_Baker’s Histological Microscope_ (Fig. 91) is of a different type to the preceding, and is intended to represent one of medium power, affording magnification of about × 400 as a maximum. It is supplied with a diaphragm beneath the stage, without other illuminating apparatus than that of the mirror. But if the adjustments of such a stand are good, there is no reason why some form of sub-stage condenser should not be added, to make the instrument somewhat more serviceable. There is, however, a rather too limited space beneath the stage of an instrument of this kind to admit of a sub-stage condenser, consequently it cannot be said to be suitable for critical work. For all ordinary students’ work this microscope is certainly available.

The stand of the Model Histological Microscope has the same form of foot as the more advanced student’s stand. It is somewhat lighter, and more portable, a matter of consideration in a student’s microscope, which often has to be carried to and from a class-room. It is provided with rack and pinion coarse adjustment, and a Campbell differential screw fine adjustment, draw-tube, and diaphragm; the diaphragm carrier being of the universal size, so that it can be replaced by an Abbe condenser at any time. With the additions suggested, this instrument can be made equal to those of a higher standard.

Rousselet’s Tank Microscope (Fig. 92), for rapidly looking over pond water and weeds, consists of a jointed arm moving parallel to the side of the tank to carry an aplanatic lens; the arm is focussed by means of rack and pinion fixed to the upright of a mahogany stand, upon which the tank can be placed, or it can be clamped directly to the tank by means of a screw. This handy form of pond microscope is made by Messrs. Baker.

Pillischer’s Microscopes.

Mr. Pillischer (New Bond Street) is favourably known for the excellency of his instruments. He has lately brought out several microscopes of an improved form. His larger model, the “New International,” consists of a solid, well-built, firm tripod stand of the Ross-Jackson pattern, which appears to be quite in the ascendant among London opticians; rack and pinion coarse adjustment, and a superior micrometer fine adjustment; sub-stage with centring screws and rack and pinion focussing adjustment; a new form of sliding pin-hole diaphragm and iris diaphragm; B and C eye-pieces; 5/8 and 1/7 objectives; Abbe condenser, N.A. 1·20; in every respect a perfect model, neatly packed in a mahogany case, for a very moderate sum. Mr. Pillischer’s No. 2 (Fig. 93) “International” Microscope, being the _Army pattern_ as well as the _student’s_, is well adapted for clinical work. A firm tripod stand supports two dark bronze uprights, with rack and pinion coarse adjustment, _e_, and fine adjustment, _d_, the stage, _i_, is wide and suitable for clinical work, and large enough for dissecting upon. The whole instrument is well made; the coarse adjustment is so good that the one-eighth inch can be focussed with ease, and without using the fine adjustment.

For a few shillings extra, a mechanical stage can be added, consisting of levers, having an action similar to the movements of a parallel ruler, which is so easy of adjustment that it can be worked under the eighth-inch objective with the hands--an advantage in a clinical microscope.

The following reference letters serve to explain the general construction of the microscope (Fig. 93):--_a_, the eye-piece; _b_, the draw-tube; _c_, the sliding-tube; _d_, micrometer or fine adjustment; _e e_, the coarse adjustment; _g_, the mirror arm and mirror; _h_, sub-stage carrying Abbe condenser; _i_, the stage with spring-clips; _j_, objectives screwed into place and double nose-piece.

The “Kosmos” is Pillischer’s cheaper model. The stand of this somewhat novel and original microscope is framed entirely of brass and gun-metal. The fine adjustment is very sensitive and perfectly steady, admitting of the highest immersion objectives being used. The optical parts are constructed upon principles consistent with the latest improvements. It has a claw-foot stand with a semi-circular arm, which carries the body, with sliding-tube coarse adjustment, and micrometer screw fine adjustment, with a large square stage diaphragm and mirror. The instrument is neatly packed in a mahogany box, together with the A or B eye-piece, 1-inch and 1/5-inch objectives of good defining and penetrating power, magnifying from 30 to 380 diameters, in mahogany cabinet, for the moderate sum of £5.

Pillischer’s Binocular Microscope (Fig. 94) is constructed on a plan somewhat intermediate between that of Beck’s and Ross’s well-known patterns, and in point of finish is equal to any student’s microscope in use. The semi-circular form given to the arm carrying the body increases the strength and solidity of the instrument, although it is doubtful whether it adds to its steadiness when placed in the horizontal position. The straight body rests for a great part of its length upon a parallel bar of solid brass ploughed into which is a groove for the reception of the rack attached to the body, the groove being of such a form that the rack is held firmly while the pinion glides smoothly through it. A steady, uniform motion is thus obtained, which almost renders the fine adjustment unnecessary. The binocular bodies are inclined at a smaller angle to one another than in most instruments; nevertheless, the range of motion given to the eye-pieces by the rack and pinion enables those whose eyes are widely separated to use the instrument with comfort. The prism is so well set that it illuminates both fields with equal intensity. The stage is provided with rectangular traversing movements to the extent of an inch and a quarter in each direction. The milled heads which effect these are placed on the same axis, instead of side by side, one of them--the vertical one--being repeated on the left of the stage, so that the movements may be communicated either by the right hand alone or by both hands acting in concert. The stage-plate has the ordinary vertical and rotatory motions, but to a much greater extent than usual; and the platform which carries the object is provided with a spring clip to secure the object when the stage is placed in the vertical position. A new form of sub-stage with centring screws is made to carry the Abbe achromatic condenser, diaphragm, polarising and other apparatus.

Continental Microscopes.

_Continental Microscopes._--The better known among continental opticians are Zeiss, Leitz, Seibert, Reichert and Hartnack. All seem to have vied with each other in the attainment of perfection in the manufacture of the most useful forms of microscopes. The late Carl Zeiss did more for the modern microscope than either of the opticians referred to above. I therefore take a medium typical model of his from a long series of highly-finished instruments for my illustration. Zeiss’s successors have of late endeavoured to perfect the mechanical details of their instruments in three or four directions, i.e., fundamental features of the stand, stage arrangements, means of focussing, and illumination.

_The Stand._--The general form of the stand still partakes too much of the original sameness of type introduced by Oberhäuser, and modified and improved by Hartnack; the “Babuchin” stand being still in favour with some few makers. The greater firmness and steadiness of Zeiss’s stand (Fig. 95) is secured by the horse-shoe form of foot, which, for the most part, is massive and well adapted to carry the stout uprights, which support a well-balanced, substantial body-tube and a graduated draw-tube, circular stage with a vulcanite disc, 4 inches in diameter; a sub-stage with centring arrangement for Abbe’s illuminating apparatus, and iris diaphragm and other diaphragms for use when the condenser is thrown aside. The mirror is full-sized, plane and concave. The coarse adjustment is regulated by a rack and pillion movement so perfect that objectives of medium power can be focussed by it alone. The fine adjustment is made by micrometer screw, the force exercised by which is transferred to the movable body by a single contact between two hardened steel surfaces. This ensures extremely delicate and uniform motion of the body which carries the tube.

The divisions in the milled head of the screw furnish a means for the registration of the vertical movements of the tube. In the latest stands, each division corresponds to an elevation or depression of the tube in the direction of the optic axis of 0·01 mm. By this means measurements of thicknesses may be made with a considerable degree of accuracy, the upper and lower surfaces of the object being successively focussed, and the amount read off on the milled-head, by the fixed index. In doing this, care must be taken to make both adjustments by a rotation of the screw in the same direction. The thickness of an object in air is then equal to the difference between the two readings. By this means the thickness of any other substance may be measured--that, for instance, of the cover-glass of the object.

The medium tube-length of the microscope is 160 mm. from the attachment of the objective to the eye-piece end. The draw-tube admits of the length being increased or diminished, and this may be read off by means of the millimetre scale engraved on the tube. My description of this model also applies to the higher class microscopes, which will be found in every way well finished and adapted to biological and scientific research.

_E. Leitz’s of Wetzlar Microscopes._--This optician publishes a series of twelve high-class forms of instruments. By preference, the horse-shoe form of stand (Fig. 96) is adopted in the whole of this maker’s models, the body being supported on a hinge joint and clamped over, and fitted with a circular revolving centred mechanical stage, attached to the ordinary stage by means of a set pin, which fixes the stage in position. By removing the screw, the stage can be detached; in this way, the stage serves for searching over large surfaces and registering the results.

The coarse adjustment is made by rack and pinion, and the fine adjustment by micrometer screw, the head of which is provided with a scale reading 1/100 mm. The draw-tube is also cut and ruled to millimetre scale. The sub-stage has rack and pinion movement, and is arranged for the Abbe condenser and iris diaphragm. This is attached to the upper stage by means of a set pin, which fixes and retains it in position after perfect centring. By removing the pin, the sub-stage can be either detached or swung aside by pressing a button. In short, this microscope is in all respects well furnished and fitted with the requisite complex mechanism necessitated by modern high-class technicological work.

Leitz’s students’ microscope, with sliding body, micrometer screw fine adjustment, concave mirror, two eye-pieces and two objectives, 3/4 inch and 1/8 inch, in mahogany case, costs £3 10_s._ Leitz’s dissecting microscope, with a heavy foot and rests, is fitted with two aplanatic lenses, magnifying × 10, × 20 diameters.

_Reichert and Seibert_ adhere to the same model as that of Zeiss, and therefore require only a brief notice. Their microscopes are characterised by substantial workmanship, suitable construction, and exact centring. The coarse adjustment is obtained in the usual way by rack and pinion, the fine by micrometer screws, which work easily, and are protected against wear and tear by having their working surfaces hardened. The stands of the better class instruments have micrometer screws graduated, and draw-tubes cut to millimetre scale. Their mechanical stages and sub-stages and accessories are in every way well finished; stage forceps, tests, and an assortment of cover glasses and slides being added. Their first-class microscopes are sent out in mahogany boxes.

On going through the continental makers’ catalogues, it will be noticed that their well-equipped microscopes are rather more costly than that of their English _confreres_. It is understood Messrs. Baker and Watson are the constituted agents for these opticians.

_Nachet’s Microscope_, a new form of which was first seen at the Antwerp Exhibition 1892, is very solidly built, and has all the qualities necessary for histological work. The stage rotates about the optic axis, and carries a movable slide holder. The coarse adjustment is by rack and pinion movement, the fine by the new system of micrometer screw (described in the journal of the Royal Microscopical Society of 1886), with divided head indicating the 1/400 part of a mm. The plane and convex mirror is mounted on a jointed arm. The draw-tube is divided into millimetres. The illuminating system, consisting of a wide-angled Abbe condenser (N.A. 1·40) with iris diaphragm, is raised or lowered by rack and pinion screws. The iris diaphragm, being mounted on a wheel, is worked by a tangent screw, which by a very slight movement causes the aperture of the diaphragm to pass from the centre to the periphery of the condenser. Altogether the arrangement of the sub-stage is novel, and the instrument is extremely well arranged and adapted to modern requirements.

Nachet and Hartnack, of Paris, hold an almost equal rank as makers of first-class microscopes, and in point of excellence of workmanship fairy rival those of our English makers.

There are very many other London and Continental makers of microscopes besides those especially mentioned, who have well-sustained reputations as opticians, and who, from want of space, I have been obliged to pass over. Messrs. Newton’s Students’ Microscope must be mentioned with respect. It is a good and useful instrument, has a firm stand with a reversible (rotatory) body movement, which seems to ensure steadiness when brought into the horizontal position for micro-photographic purposes. There are other opticians whose microscopes have stood the test of time--Messrs. Collins, Crouch, &c. It may, however, be taken as a well-established fact that those opticians known to manufacture the more highly-finished models also produce the more serviceable forms of students’ class-room, and other microscopes.

The Bacteriological Microscope.

The microscope required for bacteriological studies should be perfect in all its parts. With regard to the choice of an instrument, it is very much a matter of price, since the most perfect is usually the most costly; I shall therefore proceed to give a typical example of the instrument in use in a bacteriological laboratory. The microscope should possess the following qualifications, all of which are absolutely necessary for the study of such minute objects as bacteria and other micro-organisms.

“The typical bacteriological microscope should be well equipped with objectives of sufficiently high magnifying power, and with a special form of illuminating apparatus; while the mechanical arrangements for focussing should act with the greatest smoothness and precision; the stage, also, should be wide enough to admit of the examination of plate cultivations.”

We will consider these several points and recommendations _seriatim_, commencing with the stand.

Messrs. Watson & Sons’ Van Heurck model stand so well answers the several conditions laid down by an experienced teacher of bacteriology, that I have no hesitation in presenting it to my readers as a typical instrument, one in every way worthy of the high praise it has already received from those who have worked with it, and whose judgment may be relied upon in every way. The microscope is fully described among Messrs. Watson’s instruments, page 108.

_The Stand._--A good firm stand is undoubtedly of the first importance for all high-class work. The steadiness of the instrument and its entire freedom from vibration depends largely upon the form of the stand. I am glad to find Dr. Crookshank in accord with me as to the Ross-Jackson model, one which, in my opinion, has not been entirely superseded by models of a more recent date. Indeed, the latest improvement effected in the Ross-Jackson form, in which attention has been given to the spreading-out of the feet, has converted it into as solid and firm a stand as Powell’s; it is equally free from vibration when placed in the horizontal position.

There are, however, four different forms of stands--the tripod; the plate with double columns; the single column ending in a plate or a bent claw; and the horse shoe. The tripod stand, with cork feet, is by far the steadiest form of model. The single upright pillar support should unquestionably be condemned, as it admits of considerable vibration, and is most inconvenient for laboratory work. The heavy horse-shoe form is compact and firm, and the weight of it can hardly be considered an objection.

_The Tubular Body_ is from eight to ten inches in length, to which is added a draw-tube with an engraved millimetre scale. By extending the draw-tube greater magnification is obtained, but since this is at the cost of definition it should hardly ever be employed in the examination of bacteria. _A Triple Nose-piece_ is doubtless a convenience, saving time which is otherwise spent in replacing objectives of different magnifying powers; there is also less risk of injuring them. _Focus_ should be obtained by means of a rack and pinion coarse adjustment, together with the most approved kind of fine adjustment. The sliding tube cannot be recommended, as the motion may be stiff, encouraging the use of force, which in turn may result in the objective being brought violently into contact with the specimen, thus doing injury to the lens or damage to the preparation; or it may get too loose and readily slip out of focus.

_The Stage_ should be flat and rigid, either rectangular or circular, so long as it is sufficiently large to accommodate plate cultivation. A removable mechanical stage is of great advantage for working with high powers, as a motile bacterium can be constantly kept in view, while one hand is engaged in working the fine adjustment; it may also be employed as a finder, if engraved with a longitudinal and vertical scale, and provided with a stop. The mechanical stage must be removable, so that the stage proper may be free from any attachments when required for the examination of cultures.

_Diaphragms._--The plan of using a series of separate discs of different sizes should be avoided, as they are easily lost, and bacteriological investigations may have to be made under conditions in which it is difficult to replace them. A better plan is a revolving plate with apertures of different sizes, but the most convenient form is the _iris diaphragm_.

_The Sub-stage Condenser_ is as necessary in biological work as in the objective--in fact, the condenser and the objective should be considered as forming one piece of optical apparatus; the microscope must be regarded as incomplete without it.

It is by the _sub-stage condenser_ that the rays of light are concentrated at one point, or on one particular bacterium; for the best definition it is essential that there should be mechanical arrangements for accurately centring and focussing the condenser. All this will be explained and enlarged upon under “Practical Optics.”

In the historical review presented to my readers on the evolution of the modern microscope, I have for the most part relied upon my long and close association, extending over a period of upwards of half a century, with microscopy. I need hardly say I could have very much extended my remarks with pleasure and profit had space permitted, and thereby much increased the number of names of manufacturers, who have well-established reputations for the quality of their work, and whose instruments, more or less complete in design, realise the wants of students and of that large class of present-day workers engaged in microscopical pursuits to whom economy of outlay is almost a first consideration. No valid reason, however, can be assigned for splitting up, as some writers do, the several forms of microscopes into some six different classes, which implies inferiority in mechanical details or finish, whereas the difference wholly consists in luxurious appliances to save time, and in accessories for special work or original research. Before bringing these remarks to a close, it is my wish to direct the student’s attention to one or two points of importance in connection with the use of the instrument, viz.: variations in body-lengths of microscopes, especially between those of English and of Continental manufacture. The _optical-standard_ measurement adopted in this country for the body-tube-length is 10 inches; and for its _mechanical_, 8-3/4 inches. That of Continental opticians is, optical-tube-length 7·08 inches, or 180 mm.; the mechanical, 6·3 inches = to 168 mm.

Professor Abbe constructed an apochromatic immersion objective especially for the English optical tube-length of 10·6 inches (= to 270 m.m.), and mechanical tube-length somewhat less in measurement. This may be taken to mean a slight increase in the standard value of the tube, and therefore the addition of the rack-and-pinion to the draw-tube, now generally made a part of the microscope, is certainly of some practical value. This difference, however, when working with the English body-tube of 10 inches, may be discarded; it is, in fact, only where the shorter Continental body is in use, that so small a difference of tube-length exercises a disturbing effect over adjustment. Moreover, an object placed on the stage of the shorter body microscope will not be seen with the same distinctness by the draughtsman should he wish to make use of the _camera lucida_.

The _optical_ tube-length of the body is measured from the back lens of the objective to the front lens or principal focus of the eye-piece; the _mechanical_ tube-length from the end of nose-piece of objective to the top lens of the eye-piece.