III. THE MICROSCOPE.

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

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:

1. The Base or Foot (Fig. 30, a).-Two elementary forms-the tripod (Fig. 31, a) and the vertical column

a

Fig. 31.-Foot, three types.

set into a plate (Fig. 31, 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. 31, c), is based upon the same principle, and on the score of appearance has much to recommend it.

2. The body tube (Fig. 30, 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-class 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

Ao

Fig. 32. Coarse adjustment.

Fig. 33.-Fine adjustment.

than the English, thus permitting the use of either form of objective.

(Optical tube length distance from the back lens of the objective to the field glass of the ocular.

Mechanical tube length

distance from the end of the

nosepiece to the eyeglass of the ocular.)

3. The coarse adjustment (Fig. 30, c) should be a rack-and-pinion movement, steadiness and smoothness of action being secured by means of deeply bevelled edges and careful countersinking (Fig. 32).

4. The fine adjustment (Fig. 30, d) should on no account depend upon the action of springs, but should be of the lever pattern, preferably the Nelson (Fig. 33). 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.

5. The stage (Fig. 30, 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 I glass slides.

A mechanical stage must be classed 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 ordinary stage (Fig. 34) rather than one which needs to be clamped on every time it is required. The mechanical stage should be fitted with three (removable) screw studs, so that if necessary a Vernier finder (Fig. 34, D), such as is usually fitted to this class of stage, or a Maltwood finder, may be used.

6. Diaphragm.-Separate single diaphragms must be avoided; a revolving plate pierced with differentsized apertures and secured below the stage is preferable, but undoubtedly the best form is the "iris" diaphragm (Fig. 35).

7. The substage condenser is a necessary part of the optical outfit. Its purpose is to collect the rays of light reflected by the 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 Abbé (Fig. 36) and consists of a plano-convex lens mounted above a biconvex lens. This combination is carried in a screw-centering collar below the stage of