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The Elements of Bacteriological Technique, second edition, is a practical laboratory manual aimed at medical, dental and technical students who need to master the hands‑on methods of microbiology. The opening pages plunge directly into detailed instructions for measuring microscopic objects, beginning with step‑by‑step guidance on attaching a camera lucida, calibrating stage and eyepiece micrometers, and recording magnification constants for every ocular‑objective combination. The text proceeds to describe a range of measuring devices, including the eyepiece micrometer, Wright’s eikonometer, and Ramsden’s filar micrometer, each accompanied by precise procedural lists, diagrams, and formulas for converting divisions to micra. After the measurement section, the book shifts to the practicalities of microscope illumination and then enumerates the essential apparatus and reagents for routine bacterial work, from change‑mats and disinfectant jars to specially fashioned platinum‑handled loops and staining racks.
Written in the clear, instructional style of an early twentieth‑century scientific textbook, the author’s voice is methodical and unembellished, favoring numbered steps, occasional footnotes, and occasional quotations from contemporary authorities such as Professor Abbé. The language reflects the period’s emphasis on exactness and laboratory discipline, with frequent references to specific manufacturers (Zeiss, Wright) and to the materials then in common use (lysol, Canada balsam). Readers who appreciate a systematic, detail‑driven approach to laboratory technique, particularly those studying microbiology, histology, or related health sciences, will find this guide both a useful reference and a window into the pedagogical practices of its era.
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Where x = the number of included divisions of the eyepiece micrometer.
y = the number of included divisions of the stage micrometer.
5. Note the optical combination employed in this experiment and record it with the calculated micrometer value.
Repeat this process for each of the other combinations. Carefully record the results.
To measure an object by this method read off the number of divisions of the eyepiece micrometer it occupies and express the result in micra by a reference to the standard value for the particular optical combination employed.
Zeiss prepares a compensating eyepiece micrometer for use with his apochromatic objectives, the divisions of which are so computed that (with a tube length of 160 mm.) the value of each is equivalent to as many micra as there are millimetres in the focal length of the objective employed.
Wright's Eikonometer is really a modification of the eyepiece micrometer for rapidly measuring microscopical objects by direct inspection, having previously determined the magnifying power of the particular optical combination employed. It is a small piece of apparatus resembling an eyepiece, with a sliding eye lens, which can be accurately focussed on a micrometer scale fixed within the instrument. When placed over the microscope ocular the divisions of this scale measure the actual size of the virtual image in millimetres.
In order to use this instrument for direct measurement, it is first necessary to determine the magnifying power of each combination of ocular, tube length and objective.
Place a stage micrometer divided into hundredths of a millimetre on the microscope stage and focus accurately.
Rest the eikonometer on the eyepiece. Observation through the eikonometer shows its micrometer scale superposed on the image of the stage micrometer.
Rotate the eikonometer until the lines on the two scales are parallel, and make the various adjustments to ensure that two lines on the eikonometer scale coincide with two lines on the stage micrometer.
For the sake of illustration it may be assumed that five of the divisions on the stage micrometer accurately fill one of the divisions of the eikonometer scale; this indicates a magnifying power of 500 as the constant for that particular optical combination, and a record should be made of the fact.
The magnification constants of the various other optical combinations should be similarly made and recorded.
To measure any object subsequently it should be first focussed carefully in the ordinary way.
The eikonometer should then be applied to the eyepiece and the size of the object read off on the eikonometer scale as millimetres, and the actual size calculated by dividing the observed size by the magnification constant for the particular optical combination employed in the observation.
(c) By means of the filar micrometer.
The ~Filar~ or cobweb Micrometer (Ramsden's micrometer) eyepiece (Fig. 58) consists of an ocular having a fine "fixed" wire stretching horizontally across the field (Fig. 59), a vertical reference wire--fixed--adjusted at right angles to the first; and a fine wire, parallel to the reference wire, which can be moved across the field by the action of a micrometer screw; the drum head is divided into one hundred parts, which successively pass a fixed index as the head is turned. In the lower part of the field is a comb with the intervals between its teeth corresponding to one complete revolution of this screw-head.
As in the previous method, the value of each division of the micrometer scale (i. e., the comb) must first be determined for each optical combination. This is effected as follows:
1. Place the filar micrometer and the stage micrometer in their respective positions.
2. Rotate the screw of the filar micrometer until the movable wire coincides with the fixed one, and the index marks zero on the drum head. (If when the drum head is at zero the two wires do not exactly coincide they must be adjusted by loosening the drum screw and resetting the drum.)
3. Focus the scale of each micrometer accurately, and make the lines on them parallel.
4. Rotate the head of the micrometer screw until the movable line has transversed one division of the stage micrometer. Note the number of complete revolutions (by means of the recording comb) and the fractions of a revolution (by means of scale on the head of the micrometer screw), which are required to measure the 0.01 mm.
5. Make several such estimations and average the results.
6. Note the optical combination employed in this experiment and record it carefully, together with the micrometer value in terms of mu.
7. Repeat this process for each of the different optical combinations and record the results.
To measure an object by this method, simply note the number of revolutions and fractions of a revolution of the screw-head required to traverse such object from edge to edge, and express the result as micra by reference to the recorded values for that particular optical combination.
_Microscope Illuminant._--In tropical and subtropical regions diffuse daylight is the best illuminant. In temperate climes however daylight of the desirable quantity is not always available, and recourse must be had to oil lamps, gas lamps--preferably those with incandescent mantles--and electricity; and of these the last is undoubtedly the best. A handy lamp holder which can be manufactured in the laboratory is shown in Fig. 60. It consists of a base board weighted with lead to which is attached the ordinary domestic lamp holder, and behind this is fastened a curved sheet-iron reflector. An obscured metal filament lamp of about 16 candle power gives the most suitable light, and if monochromatic light is needed, the blue grease pencil is streaked over the side of the lamp nearest the microscope; the current is switched on and when the glass bulb is warm, rubbing with a wad of cotton-wool will readily distribute the blue greasy material in an even film over the ground glass.
[1] Its importance will be realised, however, when it is stated in the words of the late Professor Abbe: "The numerical aperture of a lens determines all its essential qualities; the brightness of the image increases with a given magnification and other things being equal, as the square of the aperture; the resolving and defining powers are directly related to it, the focal depth of differentiation of depths varies inversely as the aperture, and so forth."
[2] Made by Mr. Otto Baumbach, 10, Lime Grove, Manchester.
V. MICROSCOPICAL EXAMINATION OF BACTERIA AND OTHER MICRO-FUNGI.
APPARATUS AND REAGENTS USED IN ORDINARY MICROSCOPICAL EXAMINATION.
The following comprises the essential apparatus and reagents for routine work with which each student should be provided.
1. India-rubber "change-mat" upon which cover-glasses may be rested during the process of staining.
2. Squares of blotting paper about 10 cm., for drying cover-slips and slides.
(The filter paper known as "German lined"--a highly absorbent, closely woven paper, having an even surface and no loose "fluff" to adhere to the specimens--is the most useful for this purpose.)
3. Glass jar filled with 2 per cent. lysol solution for the reception of infected cover-glasses and infected pipettes, etc.
4. A square glazed earthenware box with a loose lining containing 2 per cent. lysol solution for the reception of infected material and used slides. The bottom of the lining is perforated so that when full the lining and its contents can be lifted bodily out of the box, when the disinfectant solution drains away and the slides, etc., can easily be emptied out. The empty lining is then returned to the box with its disinfectant solution (Fig. 61).
5. Bunsen burner provided with "peep-flame" by-pass.
6. Porcelain trough holding five or six hanging-drop slides (Fig. 62).
The best form of hanging-drop slide is a modification of Boettcher's glass ring slide, and is prepared by cementing a circular cell of tin, 13 to 15 mm. diameter, and 1 to 2 mm. in height, to the centre of a 3 by 1 slip by means of Canada balsam. It is often extremely convenient to have two of these cells cemented close together on one slide (Fig. 62, a).
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