Chapter II: Simple and Compound Microscope (2)

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.

Fig. 88.--Baker’s Improved “Nelson Model” Microscope. Dimensions.--Height when in vertical position and body racked down, 11′′; Height of stage, 4-1/8′′; Height of optic axis when in horizontal position, 8-1/2′′; Spread of tripod foot, 8 × 8-1/2′′; Diameter of mirrors, 2-3/8′′; Internal diameter of draw-tube, 1-3/10′′.]

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.

Fig. 89.--Baker’s Advanced Students’ Microscope. Dimensions.--Height when in vertical position and body racked down, 11-1/4′′; Height of stage, 4-3/4′′; Width of stage, 4′′; Height of optic axis when in horizontal position, 6-1/2′′; Spread of foot, 6′′ × 6′′; Diameter of mirrors, 1-3/4′′; Internal diameter of draw-tube, 11/12′′.

Explanatory lettering of instrument: A, Huyghenian eye-piece; B, draw-tube graduated in millimetres; C, nose-piece; D, coarse adjustment; E, fine adjustment with millimetre screw; F, horse-shoe sliding stage, graduated with sliding bar in vertical and horizontal directions _for use as finder_; G, sub-stage rack and pinion screw; H and I, centring screws to sub-stage; J, carrier for condenser; K, mirror with movable arm supported on solid tripod foot.]

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.

Fig. 91.--Baker’s Model Histological Microscope. Dimensions.--Height when in vertical position and body racked down, 10-1/2′′; Height of stage, 4′′; Width of stage, 3-1/2′′; Height of optic axis when in horizontal position, 5-1/4′′; Spread of foot, 5-1/4′′; Diameter of mirrors, 1-5/8′′; Internal diameter of draw-tube, 11/12′′.]

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.