IV. THE MICROSCOPE.

The essentials of a microscope for bacteriological work may be briefly summed up as follows:

[Ill.u.s.tration: FIG. 40.--Microscope stand.]

The instrument, of the monocular type, must be of good workmanship and well finished, rigid, firm, and free from vibration, not only when upright, but also when inclined to an angle or in the horizontal position. The various joints and movements must work smoothly and precisely, equally free from the defects of "loss of time" and "slipping." All screws, etc., should conform to the Royal Microscopical Society's standard. It must also be provided with good lenses and a sufficiently large stage. The details of its component parts, to which attention must be specially directed, are as follows:

[Ill.u.s.tration: FIG. 41.--Foot, three types.]

~1. The Base or Foot~ (Fig. 40, a).--Two elementary forms--the tripod (Fig. 41, a) and the vertical column set into a plate known as the "horse-shoe" (Fig. 41, b)--serve as the patterns for countless modifications in shape and size of this portion of the stand. The chief desiderata--stability and ease of manipulation--are attained in the first by means of the "spread" of the three feet, which are usually shod with cork; in the second, by the dead weight of the foot-plate. The tripod is mechanically the more correct form, and for practical use is much to be preferred. Its chief rival, the Jackson foot (Fig. 41, c), is based upon the same principle, and on the score of appearance has much to recommend it.

~2.~ The ~body tube~ (Fig. 40, b) may be either that known as the "long"

or "English" (length 250 mm.), or the "short" or "Continental" (length 160 mm.). Neither length appears to possess any material advantage over the other, but it is absolutely necessary to secure objectives which have been manufactured for the particular tube length chosen. In the high-cla.s.s microscope of the present day the body tube is usually shorter than the Continental, but is provided with a draw tube which, when fully extended, gives a tube length greater than the English, thus permitting the use of either form of objective.

[Ill.u.s.tration: FIG. 42.--Coa.r.s.e adjustment.]

[Ill.u.s.tration: FIG. 43.--Fine adjustment.]

For practical purposes the tube length = distance from the end of the nosepiece to the eyegla.s.s of the ocular. This is the measurement referred to in speaking of "long" or "short"

tube.

~3.~ The ~coa.r.s.e adjustment~ (Fig. 40, c) should be a rack-and-pinion movement, steadiness and smoothness of action being secured by means of accurately fitting dovetailed bearings and perfect correspondence between the teeth of the rack and the leaves of the pinion (Fig. 42).

Also provision should be made for taking up the "slack" (as by the screws _AA_, Fig. 42).

~4.~ The ~fine adjustment~ (Fig. 40, d) should on no account depend upon the direct action of springs, but should be of the lever pattern, preferably the Nelson (Fig. 43). In this form the unequal length of the arms of the lever secures very delicate movement, and, moreover, only a small portion of the weight of the body tube is transmitted to the thread of the vertical screw actuating the movement.

[Ill.u.s.tration: FIG. 44.--Spindle head to fine adjustment.]

A spindle milled head (Fig. 44) will be found a very useful device to have fitted in place of the ordinary milled head controlling the fine adjustment. In this contrivance the axis of the milled head is prolonged upward in a short column, the diameter of which is one-sixth of that of the head. The spindle can be rapidly rotated between the fingers for medium power adjustments while the larger milled head can be slowly moved when focussing high powers.

~5.~ The ~stage~ (Fig. 40, e) should be square in shape and large in area--at least 12 cm.--flat and rigid, in order to afford a safe support for the Petri dish used for plate cultivations; and should be supplied with spring clips (removable at will) to secure the 3 by 1 gla.s.s slides.

A mechanical stage must be cla.s.sed as a necessity rather than a luxury so far as the bacteriologist is concerned, as when working with high powers, and especially when examining hanging-drop specimens, it is almost impossible to execute sufficiently delicate movements with the fingers. In selecting a mechanical stage, preference should be given to one which forms an integral part of the instrument (Fig. 45) rather than one which needs to be clamped on to an ordinary plain stage every time it is required, and its traversing movements should be controlled by stationary milled heads (Fig. 45, _AA'_). The shape of the aperture is a not unimportant point; it should be square to allow of free movement over the substage condenser. The mechanical stage should be tapped for three (removable) screw studs to be used in place of the sliding bar, so that if desired the Vernier finder (Fig. 45, _BB'_), such as is usually fitted to this cla.s.s of stage, or a Maltwood finder, may be employed.

[Ill.u.s.tration: FIG. 45.--Mechanical stage.]

[Ill.u.s.tration: FIG. 46.--Iris diaphragm.]

~6. Diaphragm.~--Separate single diaphragms must be avoided; a revolving plate pierced with different sized apertures and secured below the stage is preferable, but undoubtedly the best form is the "iris" diaphragm (Fig. 46) which enters into the construction of the substage condenser.

~7.~ The ~substage condenser~ is a necessary part of the optical outfit.

Its purpose is to collect the beam of parallel rays of light reflected by the plane mirror, by virtue of a short focus system of lenses, into a cone of large aperture (reducible at will by means of an iris diaphragm mounted as a part of the condenser), which can be accurately focussed on the plane of the object. This focussing must be performed anew for each object, on account of the variation in the thickness of the slides.

The form in most general use is that known as the Abbe (Fig. 47) and consists of a plano-convex lens mounted above a biconvex lens. This combination is carried in a screw-centering holder known as the substage below the stage of the microscope (Fig. 40 f), and must be accurately adjusted so that its optical axis coincides with that of the objective.

Vertical movement of the entire substage apparatus effected by means of a rack and pinion is a decided advantage, and some means should be provided for temporarily removing the condenser from the optical axis of the microscope.

[Ill.u.s.tration: FIG. 47--Optical part of Abbe illuminator.]

With the oil immersion objective, however, an ~achromatic condenser~, giving an illuminating cone of about 0.9, should be used if the full value of the lens is to be obtained. It is generally a.s.sumed that a good objective requires an illuminating cone equivalent to two-thirds of its numerical aperture. The best Abbe condenser transmits a cone of about .45 whilst the aperture of the 1/12 inch immersion lenses of different makers varies from 1.0 to 1.4, hence, the efficiency of these lenses is much curtailed if the condenser is merely the Abbe. These improved condensers must be absolutely centered to the objective and capable of very accurate focussing otherwise much of their value is lost.

~8. Mirrors.~--Below the substage condenser is attached a gymbal carrying a reversible circular frame with a plane mirror on one side and a concave mirror on the other (Fig. 40, g). The plane mirror is that usually employed, but occasionally, as for example when using low powers and with the condenser racked down and thrown out of the optical axis, the concave mirror is used.

~9. Oculars, or Eyepieces.~--Those known as the Huyghenian oculars (Fig.

48) will be sufficient for all ordinary work without resorting to the more expensive "compensation" oculars. Two or three, magnifying the "real" image (formed by the objective) four, six, or eight times respectively, form a useful equipment.

As an accessory ~Ehrlich's Eyepiece~ is a very useful piece of apparatus when the enumeration of cells or bacteria has to be carried out. This is an ordinary eyepiece fitted with an adjustable square diaphragm operated by a lever projecting from the side of the mount. Three notches are made in one of the sides of the square and by moving the lever square aperture can be reduced to three-quarters, one-half or one-quarter of the original size.

~10. Objectives.~--Three objectives are necessary: one for low-power work--e. g., 1 inch, 2/3 inch, or 1/2 inch; one for high-power work--e. g., 1/12 inch oil immersion lens; and an intermediate "medium-power" lens--e. g., 1/6 inch or 1/8 inch (dry). These lenses must be carefully selected, especial attention being paid to the following points:

(a) _Correction of Spherical Aberration._--Spherical aberration gives rise to an ill-defined image, due to the central and peripheral rays focussing at different points.

(b) _Correction of Chromatic Aberration._--Chromatic aberration gives rise to a coloured fringe around the edges of objects due to the fact that the different-coloured rays of the spectrum possess varying refrangibilities and that a simple lens acts toward them as a prism.

(c) _Flatness of Field._--The ideal visual field would be large and, above all, _flat_; in other words, objects at the periphery of the field would be as distinctly "in focus" as those in the centre. Unfortunately, however, this is an optical impossibility and the field is always spherical in shape. Some makers succeed in giving a larger central area that is in focus at one time than others, and although this may theoretically cause an infinitesimal sacrifice of other qualities, it should always be sought for. Successive zones and the entire peripheral ring should come into focus with the alteration of the fine adjustment.

This simultaneous sharpness of the entire circle is an indication of the perfect centering of the whole of the lenses in the objective.

[Ill.u.s.tration: FIG. 48.--Huyghenian eyepiece.]

(d) _Good Definition._--Actual magnification is, within limits, of course, of less value than clear definition and high resolving power, for it is upon these properties we depend for our knowledge of the detailed structure of the objects examined.

(e) _Numerical Aperture_ (_N. A._).--The numerical aperture may be defined, in general terms, as the ratio of the _effective_ diameter of the back lens of the objective to its equivalent focal length. The determination of this point is a process requiring considerable technical skill and mathematical ability, and is completely beyond the powers of the average microscopist.[1]

Although with the increase in power it is correspondingly difficult to combine all these corrections in one objective, they are brought to a high pitch of excellence in the present-day "achromatic" objectives, and so remove the necessity for the use of the higher priced and less durable apochromatic lenses.

In selecting objectives the best "test" objects to employ are:

1. A thin (one cell layer), even } { 1", 2/3", 1/2": "blood film," stained with Jenner's } for { 1/6", 1/8"

or Romanowsky's stain. } { 1/12" oil

2. A thin cover-slip preparation } of a young cultivation of } { 1/8" dry _B. diphtheriae_ (showing } for { segmentation) stained with } { 1/12" oil methylene-blue. }

~Accessories.~--_Eye Shade_ (Fig. 49).--This piece of apparatus consists of a pear-shaped piece of blackened metal or ebonite, hinged to a collar which rotates on the upper part of the body tube of the microscope. It can be used to shut out the image of surrounding objects from the unoccupied eye, and when carrying out prolonged observations will be found of real service.

_Nosepiece._--Perhaps the most useful accessory is a nosepiece to carry two of the objectives (Fig. 50), or, better still, all three (Fig. 51).

This nosepiece, preferably constructed of aluminium, must be of the covered-in type, consisting of a curved plate attached to the lower end of the body tube--a circular aperture being cut to correspond to the lumen of that tube. To the under surface of this plate is pivoted a similarly curved plate, fitted with three tubulures, each of which carries an objective. By rotating the lower plate each of the objectives can be brought successively in to the optical axis of the microscope.

[Ill.u.s.tration: FIG. 49.--Eye shade.]

For critical work and particularly for photo-micrography, however, the interchangeable nosepiece is by no means perfect as it is next to impossible to secure accurate centreing of each lens in the optical axis. For special purposes, therefore, it is necessary to employ a special nosepiece such as that made by Zeiss or Leitz into which each objective slides on its own carrier and upon which it is accurately centred.

[Ill.u.s.tration: FIG. 50.--Double nosepiece.]

[Ill.u.s.tration: FIG. 51.--Triple nosepiece.]